Homeostasis and the kidney: WJEC A level Biology cram notes

Lesson 1 of 17

What is homeostasis and why does it matter?

Cram notes for topic 3.7, and the kidney, in WJEC A level Biology Unit 3.

is keeping the within narrow limits. Factors such as temperature, pH and blood fluctuate around a : this is a . It keeps working and stops cells gaining or losing too much water.

Lesson 2 of 17

How do negative and positive feedback work?

A detects a change, a compares it with the , and an corrects it. In the response reverses the change. In the response increases the change, as when makes the uterus contract, causing more release.

Lesson 3 of 17

Why do animals need to excrete nitrogenous waste?

Excess amino acids are deaminated in the liver. The amino group becomes , which mammals convert to less toxic .

: very toxic, very soluble, needs a lot of water; used by freshwater fish.
: less toxic, needs less water; used by mammals.
: almost insoluble, needs very little water; used by birds, reptiles and insects.

Lesson 4 of 17

What is the structure of the mammalian kidney?

The diagram below shows a vertical section of a kidney.

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 outer contains and convoluted tubules.
The has pyramids containing loops of Henle and .
Urine drains from the pyramids into the calyces and , then the .
Blood enters in the and leaves in the .

Lesson 5 of 17

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

The diagram below shows the parts of a in order.

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.

inside the : .
: .
: makes the concentrated.
and : water reabsorbed under the of .
The is wider than the .

Lesson 6 of 17

What does kidney tissue look like under the microscope?

The has among coiled tubules cut in all directions. The has no , only straight tubules. cells are cuboidal with a of . = image size ÷ actual size, with both in the same units.

Lesson 7 of 17

How does ultrafiltration happen in the glomerulus?

The is wider than the , so in the is high. Water and small solutes are forced through the capillary endothelium, the and the between .

Filtered: water, glucose, amino acids, and ions.
Not filtered: blood cells and large plasma proteins, which are too large for the .
Net filtration pressure = − osmotic pressure of plasma − pressure in the capsule.
A large surface area of capillaries gives a fast rate of filtration.

Lesson 8 of 17

What is reabsorbed in the proximal convoluted tubule?

In the all glucose and amino acids and most ions and water are reabsorbed. Sodium ions are actively pumped out of the cells into the blood, then sodium ions diffuse in from the with glucose by . Water follows by .

Lesson 9 of 17

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

cells have and basal channels for a large surface area, many to release for , many proteins, and . A stops production, so glucose is no longer reabsorbed.

Lesson 10 of 17

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

The ascending limb actively pumps sodium and chloride ions out and is impermeable to water. The descending limb is permeable to water, which leaves by . This makes the very concentrated, so water leaves the by .

Lesson 11 of 17

How is ADH released from the posterior pituitary gland?

in the detect a fall in blood . in the make , which passes down their and is released from the into the blood by , triggered by calcium ions entering the endings.

Lesson 12 of 17

How does ADH control the reabsorption of water?

The diagram below shows how keeps the of the blood constant by .

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.

Low : more , more inserted in the , more water reabsorbed, small volume of concentrated urine.
High : less , fewer , less water reabsorbed, large volume of dilute urine.
Either way, the returns to normal.

Lesson 13 of 17

What happens when the kidneys fail?

lets and ions build up and leaves excess water in the body, causing and high blood pressure. Protein and blood may appear in the urine.

Lesson 14 of 17

How is kidney failure treated by dialysis and transplantation?

In dialysis, and excess ions diffuse from the blood across a into that has normal plasma concentrations of glucose and ions but no . uses a machine; uses the . A transplant needs a matched donor and .

Lesson 15 of 17

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

Longer loops of Henle pump out more ions, giving a steeper gradient in the , so more water is reabsorbed and urine is more concentrated. Desert mammals such as the kangaroo rat have long loops and a thick ; the beaver has short loops and dilute urine.

Lesson 16 of 17

Specified practical: dissecting a kidney

Cut the kidney lengthways, identify the , , pyramids and , and then view a prepared slide of the .

Specified practical

Dissecting a mammalian kidney and examining kidney tissue

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

Aim To dissect a kidney, identify its regions and relate them to the .

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 kidney
Lamb's kidneyThe specimen.
Scalpel
ScalpelCuts the kidney.
Hand lens
Hand lensShows the stripes.
Light microscope
Light microscopeViews a slide of .

Safety

Scalpel and raw tissueRiskCuts or infection while dissecting.ControlCut away from fingers; gloves; wash hands.

Variables

Independent variableRegion of the kidney

, and .

Dependent variableFeatures observed

Colour, texture and structures.

Controlled variablesPlane of the cut

Vertical section through the .

Method, step by step

1)
Remove fat and identify the , and .
Why?
They enter and leave at the .
2)
Cut lengthways round the convex edge towards the , away from your fingers.
Where to cut a kidney to make a vertical section. Original BioWelsh diagram.
Open full size
3)
Identify the , , pyramids and , and draw them.
4)
View a slide of to find , then wash hands.

Results and analysis

The three main regions of a kidney in vertical section

Region	Appearance	Contains
	Dark red outer layer	and convoluted tubules
	Striped pyramids	Loops of Henle and
	White central cavity	Urine draining to the

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 dark contains and convoluted tubules; the striped contains loops of Henle and ; urine drains into the .

Under the microscope, are found only in the .

Worked calculations

Actual size = image size ÷ = 20 mm ÷ 100 = 0.2 mm

Evaluation

Cut may miss the hilum.

Effect on results not seen.

ImprovementCut slowly towards the .

Nephrons too small to see.

Effect on resultsLink to is indirect.

ImprovementUse prepared slides.

Going further

Sections

Longitudinal section: all regions visible.
Transverse section: as a ring round the .

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

Gloves; remove fat; find artery, vein and .
Cut lengthways towards the , away from fingers.
Identify capsule, , , pyramids and .
Draw with ruled label lines and a title.
Slide of : , so filtration happens there.
stripes: loops of Henle and .

What the answer needs to cover

Safety and hygiene
Longitudinal cut
Regions identified
Labelled drawing
in
to loops

Exam practice

Where does ultrafiltration take place?Show answer

In the , where the and Bowman's capsules are.

Lesson 17 of 17

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

reverses a change; increases it.
separates by size; proteins are too large.
Glucose is co-transported with sodium ions in the .
The ascending limb pumps out Na⁺ and is impermeable to water.
comes from the and inserts .