Homeostasis and the kidney: WJEC A level Biology notes

Lesson 1 of 17

What is homeostasis and why does it matter?

These notes cover topic 3.7, and the kidney, in WJEC A level Biology Unit 3: Energy, and the Environment. Each lesson follows the WJEC specification, and the exam tips are based on BioWelsh analysis of WJEC past papers.

Your cells are bathed in tissue fluid, which forms from blood plasma. Together these make up the . is keeping conditions in this within narrow limits, even when conditions outside the body change a lot.

Core body temperature, kept close to 37 °C.
Blood glucose concentration.
of the blood, controlled by the kidneys.
pH, and the concentration of ions such as sodium and potassium.
Concentrations of carbon dioxide and oxygen in the blood.

These factors matter because work best within a narrow of temperature and pH, and cells swell or shrink by if the of the tissue fluid changes. Keeping conditions steady lets cells work efficiently and makes the organism more independent of its surroundings.

Each factor has a , the value the body tries to maintain. The graph below shows how core body temperature behaves over a few hours.

Line graph of core body temperature in degrees Celsius, from 36.4 to 37.6, against time from 0 to 12 hours. A red wavy line rises and falls regularly between about 36.7 and 37.3 around a dashed horizontal line at 37 labelled set point. Text near the top reads too high: corrective mechanism cools the body, and near the bottom too low: corrective mechanism warms the body. — Core body temperature fluctuating around its set point of 37 °C. Original BioWelsh graph.

The dashed line is the , 37 °C.
The temperature never stays exactly at the . It rises slightly above it and falls slightly below it.
Each time it moves away, a corrective mechanism brings it back, so it stays within a narrow of about 36.7 to 37.3 °C.
This constant small fluctuation around a is called .

Lesson 2 of 17

How do negative and positive feedback work?

Every homeostatic system has the same three parts. A (detector) detects a change. A , such as the , compares it with the and sends instructions by impulses or hormones. An , a muscle or gland, carries out the response.

The diagram below shows these parts in a .

Flow diagram of five boxes. A factor changes from its set point, the stimulus, leads by an arrow to receptor detects the change, then to coordinator compares with the set point and sends instructions, then to effector, muscle or gland, makes a corrective response, then to the factor returns to its set point. A dashed blue arrow runs from the last box back to the receptor, labelled receptor keeps monitoring: information is fed back. Text in the centre reads negative feedback reverses the change. — A negative feedback loop. Original BioWelsh diagram.

A factor changes from its . This change is the .
The detects it and passes information to the .
The sends instructions to the .
The brings about a response that reverses the change, so the factor returns towards its .
The keeps monitoring the factor, so the information is fed back and the response is switched off once the factor is normal again.

In , the response is always in the opposite direction to the change. A rise leads to a fall, and a fall leads to a rise. Almost all works this way: if blood glucose rises above its , the hormone insulin brings it down, and if it falls, glucagon raises it. Separate mechanisms for correcting a rise and a fall give more precise .

In , the response increases the change, so the gets bigger rather than smaller. It is not used for , but it is useful when something must happen quickly and completely. During birth, stretching of the causes release of the hormone , which makes the muscle of the uterus wall contract more strongly. Stronger contractions cause more stretching and so more , until the baby is born.

The graph below compares how a factor changes after a disturbance under each type of feedback.

Line graph of change from the normal level in arbitrary units, from minus 2 to 10, against time from 0 to 10 arbitrary units. Both lines are flat at 0 until time 1, labelled disturbance at time 1. A solid blue line for negative feedback rises to a peak of about 4.2 at time 2.5, falls below 0 to about minus 0.5 at time 6.2 and returns to 0 by time 8. A dashed red line for positive feedback rises slowly and then ever more steeply, going off the top of the graph at about time 6.5. — How a factor changes after a disturbance under negative and positive feedback. Original BioWelsh graph.

Both lines start at the normal level, 0, until a disturbance at time 1.
Under (solid line), the factor rises to a peak of about 4 units and is then brought back, overshooting slightly before settling at the normal level.
Under (dashed line), the change grows faster and faster and does not return to normal.

Negative and positive feedback compared
Feature	Negative feedback	Positive feedback
Effect on the change	Reverses it	Increases it
Result	Factor returns to the	Factor moves further from the starting level
Example	of blood by	and contractions of the uterus during birth

Lesson 3 of 17

Why do animals need to excrete nitrogenous waste?

is the removal from the body of waste products of metabolism. Carbon dioxide from is one; the other main one comes from protein. The body cannot store excess amino acids, so they are broken down in the liver.

In , the amino group is removed from an amino acid as . The rest of the molecule can be respired or converted to or fat. is very toxic and very soluble, so mammals convert it straight away in the liver into , which is far less toxic. The blood carries to the kidneys, which excrete it in urine.

The three main nitrogenous excretory products
Feature	Ammonia	Urea	Uric acid
Toxicity	Very high	Low	Very low
Solubility in water	Very soluble	Soluble	Almost insoluble
Water needed to excrete it	A lot, to keep it dilute	A moderate amount	Very little: excreted as a paste
Energy needed to make it	None	Some	More
Typical animals	Freshwater fish and many aquatic invertebrates	Mammals and amphibians	Birds, reptiles and insects

The product an animal uses matches the water available. Freshwater fish have plenty of water, so can diffuse away across the gills and be diluted at once. Land animals must conserve water, so they use or . also suits animals that develop inside a shelled egg, because it is insoluble and can be stored safely inside the egg, and flying animals, because excreting it needs very little water, which keeps them light. Plants make only the amino acids and proteins they need, so they have almost no nitrogenous waste to excrete.

Some lungfish show both strategies. In water they excrete , but when their pond dries out they burrow into mud and switch to making , which needs less water to excrete and is safe enough to store until the rains return.

Lesson 4 of 17

What is the structure of the mammalian kidney?

Humans have two kidneys at the back of the abdomen. Each receives blood from a , a branch of the aorta, and returns it through a to the vena cava. Urine leaves each kidney through a and is stored in the bladder, then leaves the body through the urethra.

The photograph below shows a lamb's kidney, which is the same size and shape as a human kidney. The left half has been cut lengthways; the right is whole.

Two lamb's kidneys on a white background. The right one is whole: smooth, reddish brown and bean-shaped, with a notch on its inner side. The left one is cut in half lengthways, showing a dark red outer layer, a paler pink inner region with fine radial stripes, and a creamy white area of fat in the centre next to the notch. — A lamb's kidney cut lengthways (left) beside a whole kidney (right). Photo: Jurfeld, CC BY 4.0. Resized.

The whole kidney is smooth and bean-shaped, with a notch on its inner side where vessels and the enter.
In the cut half, a dark red outer layer, the , runs all round the edge.
Inside it is a paler region with fine radial stripes, the . In a lamb's kidney the pyramids of the are fused into one; human and pig kidneys have several separate pyramids.
The creamy white area in the centre is fat around the , where urine collects before entering the .

The diagram below labels the regions you should be able to identify in a vertical section.

Labelled diagram of a bean-shaped kidney cut in half. A thin dark outer layer is labelled renal capsule. A dark red outer band is labelled cortex. Seven paler striped triangles point inwards and are labelled medulla, pyramid. Their tips fit into small cream cups labelled calyx, which join a cream central cavity labelled pelvis. At the notch on the left, a red renal artery and a blue renal vein enter, and a cream ureter leaves downwards, labelled ureter, to the bladder. — A vertical section through a mammalian kidney. Original BioWelsh diagram.

The renal capsule is a tough that covers the outside.
The contains the and the convoluted tubules of the , which is why happens here.
The is made of pyramids containing the loops of Henle and the , which give it a striped look.
Each pyramid points into a cup-shaped . The calyces join to form the , which drains into the .

Lesson 5 of 17

What is a nephron and how is it supplied with blood?

Each kidney contains about a million . A is a tiny tubule, about 3 cm long, which filters the blood and then adjusts the to make urine. Its parts are in a set order, and each has a different job.

The diagram below shows one and its blood supply.

Diagram of a nephron on a background divided by a dashed line into cortex above and medulla below. A wide red afferent arteriole enters a knot of red capillaries, the glomerulus, inside a cup-shaped yellow Bowman's capsule; a narrower red efferent arteriole leaves. A yellow tube coils as the proximal convoluted tubule, runs down into the medulla as the descending limb, turns and runs up as the ascending limb of the loop of Henle, coils again as the distal convoluted tubule and joins a vertical collecting duct that runs down to the pelvis. The efferent arteriole continues as capillaries that follow the loop, labelled vasa recta, and lead to a vein. — A nephron and its blood supply. Original BioWelsh diagram.

A wide enters the cup-shaped and forms a knot of capillaries, the . A narrower leaves it.
The flows from the into the (), which is coiled and lies in the .
The dips down into the as a descending limb and returns as an ascending limb.
The () is coiled again in the and joins a , which passes down through the to the .
The divides into capillaries around the tubules, including the alongside the , and then drains to a vein.

The parts of a nephron and their main functions
Part	Region	Main function
and		of the blood
		of glucose, amino acids, most ions and water
		Creates a high solute concentration in the
		Adjusts ions and pH; water reabsorbed under the of
	to	Water reabsorbed under the of

Lesson 6 of 17

What does kidney tissue look like under the microscope?

The specification expects you to recognise the and in prepared slides and to interpret electron micrographs. The key is knowing which parts of the are found in each region.

The two light micrographs below show kidney tissue at low power. The first is from the and the second from the . The black shape in each is a pointer in the microscope eyepiece.

Purple-stained tissue packed with small round and oval tubules. Scattered among them are several larger round structures, each with a pale ring around a darker centre. A black pointer enters from the bottom. — Light micrograph of the cortex of a human kidney. Photo: Jpogi, Public domain.

Purple-stained tissue with no large round structures. On the left, many small tubules appear as white circles; on the right, the tubules appear as long white channels running diagonally in parallel. A black pointer enters from the bottom. — Light micrograph of the medulla of a human kidney. Photo: Jpogi, Public domain.

In the , round structures with a pale ring around them are scattered through the tissue. These are , each surrounded by the space inside its .
The rest of the is packed with small circles and ovals: convoluted tubules cut in many directions, because they are coiled.
In the there are no . Tubules appear as neat circles where they were cut across (left) and as long parallel channels where they were cut lengthways (right).
These straight, parallel tubules are loops of Henle and , which is why the looks striped to the naked eye.

At higher power, the details of the become clear, as in the stained section below.

Pink and purple stained section at high power. In the centre a dark knot of capillaries sits in a clear round space. Around it are cross sections of tubules made of a ring of cells with round nuclei. Labels on the left read macula densa, proximal convoluted tubule, brush border, glomerulus, distal convoluted tubule and basement membrane. — Light micrograph of kidney cortex showing a glomerulus, proximal and distal convoluted tubules. Photo: OpenStax Anatomy and Physiology, CC BY 4.0. Resized.

The is the dark knot of capillaries in the middle, sitting inside a clear space that is the inside of the .
Around it are proximal convoluted tubules, lined with cuboidal cells whose inner edge has a fuzzy of .
Distal convoluted tubules have a wider, clearer and no .
A thin surrounds each tubule. (The macula densa is a group of cells you do not need to know.)

The electron micrograph below shows the filter between the blood and the Bowman's space. FE marks the endothelium, GBM the glomerular , FP the foot processes of a and SD the slits between them.

Grey electron micrograph. A large cell body with a nucleus fills the top. A dashed box at the bottom right enlarges the edge of a capillary: small finger-like foot processes labelled FP's rest on a thin dark band labelled GBM, with gaps between them labelled SD, and a thin layer labelled FE on the other side of the band. Labels in boxes also read nucleus, cell body and MP. — Transmission electron micrograph of a podocyte and the glomerular filtration barrier. Photo: Cunanan J, Zhang D, Peired AJ and Barua M, CC BY 4.0. Resized.

The large grey area at the top is the of a , with its .
The inset enlarges the filter: a thin dark with a layer of capillary endothelium on one side, interrupted by pores.
On the other side, the 's foot processes (FP) rest on the like a row of fingers.
The narrow gaps between the foot processes are the (SD). Molecules pass through all three layers to enter the .

Lesson 7 of 17

How does ultrafiltration happen in the glomerulus?

by the kidney has two steps: , then . is filtration under high pressure: small molecules are forced out of the blood in the into the , while cells and large proteins stay behind.

The diagram below shows why the pressure in the is so high.

Diagram of a cup-shaped capsule with a brown outer wall, opening at the bottom into a tube labelled to the proximal convoluted tubule. Two red vessels enter from the top: a wide one labelled afferent arteriole, wider, and a narrow one labelled efferent arteriole, narrower. Inside the cup is a cluster of pink capillary loops labelled glomerulus, capillary knot, surrounded by a dashed green line labelled podocytes, inner wall. Blue arrows point out of the capillaries into the space labelled Bowman's space, filtrate. Red text reads high hydrostatic pressure: the afferent arteriole is wider than the efferent arteriole. — The glomerulus and Bowman's capsule. Original BioWelsh diagram.

The bringing blood in is wider than the taking blood out.
Blood enters faster than it can leave, so a high hydrostatic (blood) pressure builds up in the glomerular capillaries.
This pressure forces water and small solutes out of the capillaries and into the Bowman's space (blue arrows).
The then flows into the .

Anything leaving the blood must cross three layers, shown in the first diagram below. The second is a drawing of one capillary wrapped by , whose foot processes leave narrow between them.

Cross-section diagram in layers. At the top, pink blood plasma contains red blood cells and purple protein dots. Below is a row of pink endothelial cells with gaps, labelled 1, endothelium with pores (fenestrae). Below that a purple band labelled 2, basement membrane: the main filter. Below that a row of green foot processes labelled 3, podocyte pedicels, filtration slits, joined by green stalks to two oval podocyte cell bodies in the blue Bowman's space. Two blue arrows pass from the plasma through a gap, the membrane and a slit into the Bowman's space; a red arrow from a protein stops at the basement membrane with a red cross. — The three layers of the glomerular filter. Original BioWelsh diagram.

Illustration of a red tube labelled capillary, wrapped by orange cells labelled cell bodies. The cells send out many branching, interlocking processes along the capillary, with narrow gaps between them labelled filtration slits. — Podocytes wrapped round a glomerular capillary, showing filtration slits. Photo: OpenStax College, CC BY 3.0. Resized.

The capillary endothelium has pores (), so it lets plasma through but not blood cells.
The is a mesh of protein fibres. It acts as the main filter, stopping molecules with a relative molecular mass above about 69 000, such as most plasma proteins.
The wrap round the capillaries. Their foot processes interlock, and the between them let the through into the Bowman's space.

What is and is not in the glomerular filtrate
In the filtrate	Not in the filtrate
Water, glucose, amino acids, , sodium, chloride and other ions, vitamins	Red and white blood cells, platelets, and large plasma proteins such as albumin

The pressure that actually forms is the result of three pressures. in the pushes fluid out. Two pressures oppose it: the osmotic pressure due to plasma proteins, which draws water back into the blood, and the pressure of the fluid already in the and tubule.

Widening the or narrowing the raises , so more forms.
Less protein in the plasma (for example, a very low protein diet) lowers its osmotic pressure, so the net pressure and filtration rate rise.
A thicker , as in some kidney diseases, makes the distance longer, so less forms.
The capillaries of all the have a very large total surface area, over a square metre per kidney, so filtration is fast: about 125 cm³ of per minute, or about 180 dm³ per day.

Lesson 8 of 17

What is reabsorbed in the proximal convoluted tubule?

About 180 dm³ of forms each day, but only about 1.5 dm³ of urine is produced. Almost everything useful must be taken back into the blood. that the tubule takes back only particular substances, using specific proteins.

All the glucose and amino acids, as long as their concentration is normal.
About two thirds of the sodium ions and chloride ions.
About two thirds of the water, which follows the solutes by .
Vitamins and small amounts of protein that leaked through the filter, taken up by pinocytosis.
Only a little , which diffuses back passively.

Glucose is reabsorbed against its by with sodium ions, as the diagram below shows.

Diagram of an epithelial cell between the lumen containing filtrate on the left and tissue fluid and blood on the right. On the lumen side, a green co-transporter protein carries sodium ions and glucose into the cell together. On the blood side, a purple sodium-potassium pump moves 3 sodium ions out and 2 potassium ions in, using ATP, and a gold glucose carrier lets glucose out. Text inside the cell reads low sodium ion concentration inside the cell. Three numbered steps below explain that the pump uses ATP, sodium diffuses in carrying glucose, and glucose diffuses out into the blood. — Reabsorption of glucose by co-transport with sodium ions in the proximal convoluted tubule. Original BioWelsh diagram.

Sodium-potassium pumps in the membrane facing the blood actively transport sodium ions out of the cell, using . This keeps the sodium ion concentration inside the cell low.
Sodium ions now diffuse into the cell from the , down their , through proteins. Each carries a glucose molecule in with it.
Glucose builds up inside the cell and diffuses out into the tissue fluid and blood through a different protein.
Removing solutes lowers the of the blood, so water leaves the by .

Because water is reabsorbed but most is not, the concentration of in the rises along the even though the mass of stays about the same. The concentration of glucose falls to zero, because all of it is reabsorbed.

Normally there is almost no protein in the . If the filter is damaged, for example in kidney disease, proteins leak through, but they stay in the urine. They are too large to be reabsorbed and there are no specific proteins for them. Glucose can also appear in the urine, in untreated diabetes mellitus: the blood glucose concentration is so high that there are not enough proteins to reabsorb all the glucose filtered.

Lesson 9 of 17

How are the cells of the proximal convoluted tubule adapted for reabsorption?

The reabsorbs a huge volume every day, so its cells are specialised for moving substances quickly from the to the blood.

The diagram below shows the epithelial cells lining the tubule.

Diagram of three rectangular cells side by side between the lumen of the tubule at the top and a red blood capillary at the bottom. Each cell has a row of finger-like microvilli on its top surface. The middle cell has a nucleus and many orange mitochondria. Black blocks between the cells near the top are tight junctions. Vertical lines at the base of each cell are basal channels. A thin purple basement membrane lies under the cells. Labels read microvilli: large surface area; tight junction: seals cells together; mitochondria: ATP for pumps; nucleus; basal channels: more surface area; basement membrane; close to a capillary: short distance. — Epithelial cells lining the proximal convoluted tubule and their adaptations. Original BioWelsh diagram.

The surface facing the is covered in , forming a that gives a large surface area for absorption.
Folds of the membrane at the base of the cell, the basal channels, increase the surface area on the blood side.
Many sit close to the membranes. They make for the sodium-potassium pumps. (If this link is unfamiliar, read the ATP lesson on why cells need ATP.)
seal neighbouring cells together, so substances pass through the cells rather than leaking between them.
The cells are a single layer close to capillaries, giving a short distance.

Adaptations of proximal convoluted tubule cells
Adaptation	How it helps reabsorption
()	Large surface area for absorption from the
Many	Release for of sodium ions
Many and proteins	Specific uptake of glucose, amino acids and ions
Basal channels	Large surface area for moving substances into the tissue fluid
	Stop leaking back between the cells
Single layer of cuboidal cells near capillaries	Short distance

show how important the are. Oligomycin blocks synthesis in (if this is unfamiliar, read the ATP lesson on chemiosmosis). With no , the sodium ions cannot be pumped out, the sodium gradient disappears, and glucose stops. Glucose then stays in the .

Lesson 10 of 17

How does the loop of Henle help to make concentrated urine?

Mammals can produce urine more concentrated than their blood. To do this, water must be drawn out of the by , which needs a very low (a high solute concentration) in the around them. The creates this.

The diagram below follows the round the . The numbers are solute concentrations in arbitrary units.

Diagram of a U-shaped loop and a collecting duct running down through the medulla, whose background darkens from top to bottom. Tissue fluid concentrations are labelled 300, 600, 900 and 1200 from top to bottom. The descending limb contains filtrate at 300, 600, 900 and 1200, with blue arrows showing water leaving. The ascending limb contains filtrate at 1000, 700, 400 and 150 going upwards, with red arrows showing sodium and chloride ions leaving. The tip of the loop is 1200. Blue arrows show water leaving the collecting duct at every level. Text on the right explains that the descending limb is permeable to water, the ascending limb is impermeable to water and pumps out ions, and water leaves the collecting duct by osmosis, more when ADH is present. — The loop of Henle acting as a counter-current multiplier. Original BioWelsh diagram.

The ascending limb is impermeable to water. Its cells actively pump sodium and chloride ions out into the tissue fluid of the , so the inside becomes more dilute.
This raises the solute concentration of the tissue fluid, lowering its .
The descending limb is permeable to water. Water moves out of it by into the concentrated tissue fluid, so the becomes more concentrated as it goes down; some sodium ions also diffuse in.
The is most concentrated at the tip of the , deep in the .
The passes back down through the same gradient, so water can leave it by all the way down.

The two limbs lie side by side with the flowing in opposite directions. This counter-current arrangement lets a small difference between the limbs at each level add up to a large gradient from the top to the bottom of the . It is called a .

The graph below shows the percentage of the glucose, sodium ions and water filtered that is still in the along the first part of the .

Line graph of percentage remaining in the filtrate from 0 to 100 against distance along the nephron from 0 to 100 arbitrary units, divided by dashed lines into proximal convoluted tubule from 0 to 60, descending limb from 60 to 80 and ascending limb from 80 to 100. A solid green glucose line falls from 100 to 0 by 35. A dashed red sodium ions line falls steadily to 32 at 60, rises to 38 at 80, then falls to 12 at 100. A dotted blue water line falls steadily to 35 at 60, falls to 20 at 80, then stays at 20. — Percentage of filtered glucose, sodium ions and water still in the filtrate along the nephron. Original BioWelsh graph.

In the , all the glucose is reabsorbed by about one third of the way along.
Sodium ions and water both fall steadily in the , to about a third of the amount filtered, because water follows the reabsorbed ions by .
In the descending limb, water falls further but sodium ions rise slightly, because water leaves and some sodium ions diffuse in.
In the ascending limb, sodium ions fall sharply because they are pumped out, while water stays constant because this limb is impermeable to water.

Lesson 11 of 17

How is ADH released from the posterior pituitary gland?

secrete hormones directly into the blood. Hormones travel to target cells that have specific . , the of the of the blood, uses the hormone (), a short peptide of nine amino acids.

is made in the but released from the , as the diagram below shows.

Diagram with a purple box at the top labelled hypothalamus, containing a blue circle labelled osmoreceptors with an arrow to two cells labelled neurosecretory cells make ADH. Their axons run down a stalk into an oval purple lobe labelled posterior pituitary, where gold dots sit at the axon endings. A grey oval beside it is labelled anterior pituitary. A red capillary beside the posterior lobe has an arrow labelled ADH carried in the blood to the kidneys. Numbered notes read 1, osmoreceptors detect a fall in the water potential of the blood; 2, ADH travels down axons in the stalk to the posterior pituitary; 3, vesicles of ADH are released into the blood by exocytosis. — ADH is made in the hypothalamus and released from the posterior pituitary gland. Original BioWelsh diagram.

in the detect the of the blood. When it falls, water leaves them by and they shrink, which stimulates nearby .
are cells that make in their in the .
is packaged into , which pass down the to the , where they are stored.
impulses arriving at the endings trigger the release of into capillaries by . The blood carries it to the kidneys.

An impulse arrives at the end of the and opens sodium ion channels.
Sodium ions flood in, depolarising the membrane.
opens calcium ion channels, and calcium ions diffuse into the cell.
Calcium ions cause the of to move to the membrane and fuse with it, releasing by .

Lesson 12 of 17

How does ADH control the reabsorption of water?

The target cells of are the cells lining the and, especially, the . changes how permeable their walls are to water. Diuresis producing a large volume of dilute urine; the hormone is called antidiuretic because it reduces this.

The diagram below shows how makes a cell more permeable to water.

Diagram of a collecting duct cell between the lumen of the collecting duct, with filtrate of higher water potential, on the left and the tissue fluid of the medulla and blood, with lower water potential, on the right. A gold ADH molecule binds a purple receptor on the right-hand membrane, labelled ADH binds to a receptor, and an arrow labelled cyclic AMP points into the cell. Round vesicles containing blue channel proteins, labelled vesicles with aquaporins, move to the left-hand membrane, where blue aquaporins are inserted. A long blue arrow labelled water moves by osmosis crosses from the lumen through the cell into the tissue fluid. A note reads less ADH: aquaporins are taken back into vesicles. — How ADH makes a collecting duct cell more permeable to water. Original BioWelsh diagram.

binds to specific on the membrane of the cell that faces the blood.
This activates an inside the cell, producing cyclic AMP, which acts as a second messenger.
containing , water channel proteins, move to the membrane facing the and fuse with it, inserting more .
Water moves by from the , which has a higher , through the cells into the concentrated tissue fluid of the and then into the blood.
When falls, are taken back into the cell in , so the wall becomes less permeable.

The whole system is a , shown below.

Flow chart starting from a grey box, normal water potential of the blood, splitting into two columns. Left, in red: water potential falls; osmoreceptors in the hypothalamus detect it; more ADH released from the posterior pituitary; DCT and collecting duct walls more permeable to water; more water reabsorbed: small volume of concentrated urine. Right, in blue: water potential rises; osmoreceptors detect it; less ADH released; walls less permeable to water; less water reabsorbed: large volume of dilute urine. Both columns lead to a green box, water potential returns to normal. — Negative feedback control of the water potential of the blood by ADH. Original BioWelsh diagram.

If the of the blood falls, for example after sweating, more is released. More water is reabsorbed, and a small volume of concentrated urine is produced.
If the rises after drinking a lot, less is released. Less water is reabsorbed, and a large volume of dilute urine is produced.
In each case the change is reversed, so the returns to normal.

In , either too little is released or the cells do not respond to it. The walls stay impermeable, less water is reabsorbed, and the person passes large volumes of dilute urine and is very thirsty. Alcohol reduces the release of , which is why drinking alcohol increases urine output.

Lesson 13 of 17

What happens when the kidneys fail?

In the kidneys can no longer filter the blood or balance water and ions properly. Common causes are diabetes, long-term high blood pressure, infections and diseases that damage the , such as glomerulonephritis.

and other wastes build up in the blood, causing tiredness, nausea and, eventually, death.
Ion concentrations become unbalanced. A high potassium ion concentration can disturb the heart rhythm.
Water is not removed, so body fluids become too dilute. Fluid collects in the tissues (), and water entering cells by can make them swell.
Blood pressure rises because the volume of body fluid increases.
Damaged let protein and blood cells into the urine.

Damage to the filter can also cause indirectly. If the thickens, less forms and more water stays in the blood, making it more dilute. If proteins leak into the urine, the blood loses plasma proteins, its rises, and less water is drawn back from the tissue fluid into the capillaries.

Lesson 14 of 17

How is kidney failure treated by dialysis and transplantation?

In the early stages of kidney disease, patients may it with a diet low in protein, salt and potassium, and with drugs to lower blood pressure. Patients with need either dialysis, which does the job of the kidneys artificially, or a kidney transplant. In dialysis, blood is separated from by a , and substances diffuse across it.

contains glucose and ions at the same concentrations as normal blood plasma, but no . and excess ions diffuse from the blood into the fluid down their concentration gradients. Glucose does not leave, because there is no for it.

In , the patient's blood is pumped through a dialysis machine, usually for about four hours three times a week. The diagram below shows the circuit.

Diagram of blood flowing in a loop. A red tube from an artery in an arm passes through a pump, has heparin added and enters the top of a tall dialyser containing many red vertical fibres. Blood flows down the fibres and leaves at the bottom in a blue tube that passes a bubble trap and returns to a vein in the arm. Blue arrows show fresh dialysis fluid entering the bottom of the dialyser, flowing up around the fibres in the opposite direction to the blood, and leaving at the top as used fluid containing urea, excess ions and water. — A haemodialysis circuit with counter-current flow in the dialyser. Original BioWelsh diagram.

Blood is taken from an artery in the arm and pumped into the dialyser. is added to stop it clotting.
Inside the dialyser, the blood flows through thousands of thin fibres made of , giving a very large surface area.
flows around the fibres in the opposite direction to the blood. This counter-current flow keeps a all along the dialyser.
Used fluid is removed and fresh fluid is pumped in continuously, which also maintains the gradient.
Blood passes through a bubble trap and returns to a vein. The fluid is kept at body temperature so the patient is not chilled.

The photograph below shows in progress.

A patient's forearm resting on a bed with two blood-filled tubes taped to it. Behind is a dialysis machine with a screen showing numbers, a rotating pump in the middle and a tall red cylinder, the dialyser, on the right. — A patient receiving haemodialysis. Photo: Anna Frodesiak, CC0.

Two tubes, dark red with blood, are taped to the patient's forearm: one takes blood out, the other returns it.
The machine behind has a rotating pump near the centre and a screen showing settings such as flow rate and time.
The upright cylinder on the right, full of blood, is the dialyser.

In (), the patient's own , the membrane lining the abdomen, is used as the dialysis membrane, as shown below.

Diagram of the abdomen as a rounded red outline labelled abdominal wall, lined inside by a dashed green line labelled peritoneum lines the cavity and acts as the dialysis membrane. The cavity is filled with blue dialysis fluid. Wavy red lines at the top represent capillaries, with text urea and excess ions diffuse from the blood into the fluid. A blue tube labelled catheter runs from a bag of fresh fluid into the cavity, and another runs out to a drain bag. A note reads fluid is drained under gravity after a few hours and replaced, several times a day. — Continuous ambulatory peritoneal dialysis. Original BioWelsh diagram.

About 2 dm³ of runs into the abdominal cavity through a permanent catheter.
and excess ions diffuse from the blood in the capillaries of the into the fluid.
After a few hours the used fluid is drained into a bag under gravity and replaced. This is done several times a day, and the patient can move about in between.

Treatments for kidney failure compared
Feature	Haemodialysis	CAPD	Kidney transplant
Where	Hospital or at home with a machine	At home, while going about daily life	Surgery in hospital
Advantages	Very effective at removing ; staff check the patient	No machine; more freedom; blood stays in the body	Most normal life; no dialysis; cheaper in the long term
Disadvantages	Many hours a week; diet and fluid strictly limited; risk of infection or clots	Risk of infection of the (peritonitis); must be done every day	Shortage of donors; risk of rejection; for life increase infection risk

In a transplant, a healthy kidney from a donor is joined to the patient's blood vessels and bladder. The donor's tissue type must match the patient's as closely as possible, because the immune system attacks cells with foreign . The patient takes to reduce the risk of rejection.

Lesson 15 of 17

How is the loop of Henle adapted in animals from different environments?

The longer the , the more ions it can pump out, and the steeper the gradient it builds in the . A steeper gradient lets more water be drawn out of the , so the urine can be more concentrated.

Kidneys contain two types of , compared in the diagram below.

Diagram with cortex above and medulla below a dashed line. A cortical nephron has its capsule high in the cortex and a short loop that only just dips into the medulla. A juxtamedullary nephron has its capsule near the boundary and a long loop that reaches deep into the medulla. Both join a vertical collecting duct on the right. Notes explain that more juxtamedullary nephrons give a steeper gradient and more concentrated urine. — Cortical and juxtamedullary nephrons compared. Original BioWelsh diagram.

A cortical has its in the outer and a short that barely enters the .
A has its near the boundary with the and a long that reaches deep into it.
Animals with a higher proportion of , and a thicker , can produce more concentrated urine.

Desert mammals such as the kangaroo rat, shown below, are the extreme example.

A small sandy-brown and white rodent with large dark eyes, long whiskers, long hind feet and a very long tail with a tufted tip, standing on pale gravel. — Merriam's kangaroo rat, *Dipodomys merriami*, a desert mammal that can survive without drinking. Photo: Federal Government of the United States, Public domain.

The kangaroo rat is a small rodent of North American deserts, with large hind feet for hopping and a long tail.
It may never drink. It gains most of its water as , released by the of the food it eats.
For its body size it has long loops of Henle and a very thick , and produces a tiny volume of extremely concentrated urine.

The bar chart below compares the most concentrated urine that different mammals can produce, as a ratio of urine concentration to plasma concentration.

Bar chart of maximum urine concentration divided by plasma concentration, from 0 to 35, for five mammals. Beaver 1.7, human 4.7, camel 9.3, kangaroo rat 18.3 and hopping mouse 31.3. — Maximum concentration of urine produced by five mammals, as a ratio to plasma concentration (approximate published values). Original BioWelsh graph.

The beaver, which lives in , can only concentrate its urine to about 1.7 times its plasma, because it has short loops of Henle and never needs to save water.
Humans reach about 4.7 times.
Desert mammals reach much higher values: about 9 for the camel, 18 for the kangaroo rat and 31 for the Australian hopping mouse.
The pattern matches the relative length of the loops of Henle and the thickness of the .

Desert mammals often have a high level of , so the are highly permeable to water.
Many are nocturnal and stay in cool, humid burrows during the day, reducing evaporation.
Using fat as a respiratory releases a lot of .

Lesson 16 of 17

Specified practical: dissecting a kidney

Dissecting a lamb's or pig's kidney lets you see the regions of the kidney for yourself and match them to the . The specification also asks you to examine prepared slides of kidney, so the practical ends with a slide of the under the microscope.

Specified practical

Dissecting a mammalian kidney and examining kidney tissue

Unit 3, section 7, specified practical 1 (BioWelsh entry number)

Aim To dissect a mammalian kidney, identify its external features and internal regions, relate them to the structure of the , and examine a prepared slide of kidney .

Five panels in order: a kidney, remove the fat, peel it off by hand; a scalpel, cut lengthways round the outer edge; a hand lens, identify regions, cortex, medulla, pelvis; a seeker, trace the pelvis into the ureter; a microscope, view cortex tissue at ×100 and ×400. A footer says use a sharp scalpel with care, cut away from you and wash your hands afterwards. — Dissecting a kidney at a glance. Original BioWelsh diagram.

Equipment

Lamb's or pig's kidney
Lamb's or pig's kidney1Fresh from a butcher, with the and vessels attached if possible.
Dissecting board
Dissecting boardA firm, washable surface to cut on.
Scalpel
ScalpelSharp bladeMakes a clean cut through the kidney.
Seeker (mounted needle)
Seeker (mounted needle)Traces tubes such as the without cutting them.
Forceps
ForcepsHold and lift tissue such as the capsule.
Hand lens
Hand lens×10Shows the striped pattern in the .
Light microscope
Light microscope×100 and ×400Shows and tubules in prepared slides.
Prepared slide of kidney
Prepared slide of kidneyA stained section of and .

Safety

Sharp scalpelRiskCutting your fingers while cutting the kidney.ControlCut on a board, away from your fingers, and keep the blade covered when not in use.
Raw animal tissueRiskInfection from bacteria when handling the kidney.ControlWear gloves, disinfect surfaces and wash hands afterwards.
Glass slideRiskCuts from a broken slide or coverslip.ControlHandle slides by the edges and report breakages.

Variables

Independent variableRegion of the kidney examined

Capsule, , , and in turn.

Each region contains different parts of the .

Dependent variableFeatures observed

Colour, texture and structures seen by eye, hand lens and microscope.

These identify each region.

Controlled variablesPlane of the cut, Magnification

Plane of the cut

A vertical section through the .

A different plane would not show all the regions together.

State the objective lens used for each observation.

Structures look different at different magnifications.

Method, step by step

1)
Put on gloves and place the kidney on the dissecting board. Peel away any fat by hand.
Why?
Fat hides the vessels and at the , and gloves reduce the risk of infection from raw tissue.
2)
Identify the , and at the notch (), and use a seeker to trace the .
Why?
The has a thick, pale wall; the vein has a thin wall and the artery a thicker, elastic wall.
3)
Hold the kidney flat and cut lengthways round its outer, convex edge towards the , cutting away from your fingers, until it opens into two halves.
Why?
A vertical (longitudinal) section shows the , and all at once.
Where to cut a kidney to make a vertical section. Original BioWelsh diagram.
Open full size
4)
Peel back a corner of the renal capsule with forceps.
Why?
This shows the capsule is a separate tough layer covering the .
5)
Identify the , , pyramids, calyces and , using a hand lens to see the stripes in the .
Why?
The stripes are loops of Henle and running towards the .
6)
Make a labelled drawing of the cut surface, with a title and the plane of section.
Why?
A drawing records what you actually saw and shows the positions of the regions.
7)
View the prepared slide under low power, then high power, and find and tubules in the .
Why?
are only in the , which links the regions to the .
8)
Dispose of the tissue as instructed, disinfect the board and scalpel, and wash your hands.
Why?
Raw tissue may carry bacteria such as Salmonella.

Results and analysis

What each region of the kidney looks like in the dissection, and the parts of the it contains

Region	Appearance to the naked eye	Parts of the found there
Renal capsule	Thin, tough, shiny membrane that peels off the surface	None
	Dark red, grainy outer layer, 5 to 10 mm thick	, Bowman's capsules, proximal and distal convoluted tubules
	Paler, finely striped region; separate pyramids in a pig's kidney, fused into one in a lamb's	Loops of Henle, ,
and calyces	White, funnel-shaped cavity, often surrounded by fat	Receives urine from the
	Pale tube with a thick muscular wall	Carries urine to the bladder

Two halves of a kidney side by side on a white background, cut faces up. Each has a dark red outer band, a paler pink inner region with fine radial stripes and small white vessels at its boundary, and a creamy white area of fat in the centre at the notch. — The two halves of a lamb's kidney after a lengthways cut. Photo: Jurfeld, CC BY 4.0. Resized.

What the results show

The cut surface shows a dark red all round the edge and a paler, striped . Its pyramids (fused into one in a lamb's kidney) point towards the white , which leads into the .

The looks grainy because it is packed with and coiled tubules. Under the microscope it shows round , each in a , among tubules cut in many directions.

The looks striped because the loops of Henle and run in parallel towards the . Under the microscope it has no .

This fits the : filtration happens in the , the loops of Henle build a gradient in the , and urine drains through the into the .

Worked calculations

Actual thickness of the = measured thickness on a photograph ÷ , for example 24 mm ÷ 3 = 8 mm

Actual diameter of a = image diameter ÷ = 20 mm ÷ 100 = 0.2 mm = 200 µm

Evaluation

The cut may not pass exactly through the hilum.

Effect on resultsSome regions, such as the , may be missing or small.

ImprovementCut slowly round the convex edge, aiming at the .

Butchers often remove the ureter and vessels.

Effect on resultsExternal features cannot all be identified.

ImprovementOrder a kidney with the and vessels attached.

Individual nephrons are too small to see by eye.

Effect on resultsThe link between regions and parts must come from the slide.

ImprovementUse prepared slides or micrographs at high power.

Going further

Longitudinal or transverse section?

A longitudinal (vertical) section cuts the kidney into a left and right half through the and shows every region at once. A transverse section cuts across the kidney, like the scans used in hospitals, and shows the as a ring around the , with the vessels at the .

Other ways to carry out this practical

Inject coloured dye or ink into the before cutting, to show the blood vessels in the .
Use photographs or a virtual dissection if fresh kidneys are not available.
Tease apart a tiny piece of in water on a slide with a seeker, add a coverslip and look for tubules under the microscope.
Compare a lamb's kidney, which has a single pyramid, with a pig's kidney, which has several, like a human kidney.

Drawing tips

Use a sharp pencil and single lines.
Draw large, with no shading or colouring.
Add a title, the plane of section and a scale or .

Model answer: describe the method

Describe how you would dissect a kidney to show its internal structure, and how you would relate the regions to the .

Original practice response · 6 indicative marks

Wear gloves, remove the fat and identify the , and at the .
Place the kidney on a board and cut lengthways round the convex edge towards the with a sharp scalpel, cutting away from the fingers.
Identify the capsule, , with its pyramids, and the leading to the .
Make a labelled drawing with ruled label lines and a title stating the plane of section.
Examine a prepared slide of the : and convoluted tubules show that happens in the .
Relate the striped to the loops of Henle and , then disinfect equipment and wash hands.

What the answer needs to cover

Safe use of scalpel and hygiene
Longitudinal cut through the
, and identified
Labelled drawing with ruled lines
in the to
to loops of Henle and

Exam practice

Explain why ultrafiltration happens in the cortex and not in the medulla.Show answer

All the and Bowman's capsules are in the . The contains only the loops of Henle and , so no filtration of blood happens there.

only in the
has loops of Henle and

Suggest how you could tell the ureter from the renal vein in the dissection.Show answer

The has a thick, pale, muscular wall and contains no blood. The has a thin, dark wall and contains blood.

has a thicker, paler wall
Vein contains blood and has a thin wall

Explain why the medulla appears striped.Show answer

The loops of Henle and run in parallel lines from the towards the , so the cut surface shows them as stripes.

Parallel tubules
Loops of Henle and

Lesson 17 of 17

How do the ideas in homeostasis and the kidney fit together?

How the ideas in this topic connect

Zoom 100%

Zoom in to read each branch. Swipe or scroll sideways to explore.

: a constant in
- : , , reverse a change
- increases a change, as with in birth
- The kidney excretes and controls water
  - : high pressure in the , filtered by size
  - in the : , ,
  - : makes the concentrated
  - from the inserts in the
  - : dialysis or transplant
- Adaptations: excretory product and length match water available

keeps conditions within narrow limits, not perfectly constant.
reverses a change; increases it.
is the most toxic excretory product and needs the most water.
forms because the is wider than the .
Proteins are absent from the because they are too large to cross the .
concentration rises in the because water is reabsorbed but little is.
Glucose is reabsorbed by with sodium ions; drives the sodium pump.
The ascending limb pumps out sodium ions and is impermeable to water.
is released from the and inserts .
Longer loops of Henle give a steeper gradient and more concentrated urine.