AQA A-level Biology: Osmoregulation and the Kidney (7402)

This guide covers AQA A-level Biology (7402) section 3.6.4.3, Control of blood water potential, from the AQA AS and A-level Biology specification for first teaching 2015. The section is A-level only. It sits in topic 6, Organisms respond to changes in their internal and external environments. Blood glucose control (3.6.4.2) is not covered here. For the other pages see the revision notes and the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. To find weak spots first, try the free diagnostics.

What the specification asks for

Statement	What you must be able to do
Osmoregulation	Describe it as control of the water potential of the blood
Hypothalamus, posterior pituitary, ADH	Explain the role of each in osmoregulation
Nephron: glomerular filtrate	Relate nephron structure to the formation of filtrate
Nephron: proximal convoluted tubule	Explain reabsorption of glucose and water
Nephron: loop of Henle	Explain how it maintains a gradient of sodium ions in the medulla
Nephron: distal convoluted tubule and collecting ducts	Explain reabsorption of water

The specification lists these as content statements. The mechanisms below (pressures, carrier proteins, aquaporins) are the standard explanations used to teach them.

Osmoregulation

Water potential is the tendency of water to move; pure water has the highest value and solutes lower it. Blood water potential changes with sweating, drinking, eating salty food and the water lost in urine. If it falls too far, cells lose water by osmosis and shrink. If it rises too far, cells take in water and swell. Osmoregulation is control of the water potential of the blood so it stays within narrow limits. It is an example of negative feedback, in which the body’s response undoes the change that set it off.

The hypothalamus, posterior pituitary and ADH

1. Osmoreceptors in the hypothalamus detect the water potential of the blood. When it falls, water leaves these cells by osmosis and they shrink.
2. Specialised neurosecretory cells in the hypothalamus make antidiuretic hormone (ADH). Their axons run down into the posterior pituitary gland, where ADH is stored.
3. Shrinking osmoreceptors stimulate these cells, which send nerve impulses to the posterior pituitary. ADH is released into the blood capillaries there.
4. ADH travels in the blood and binds to receptors on cells of the distal convoluted tubule (DCT) and collecting duct, the target cells.
5. Binding makes these cell surface membranes more permeable to water. Vesicles carrying water channels (aquaporins) fuse with the membrane, so more channels are present.
6. More water leaves the filtrate by osmosis and returns to the blood. Less water is lost, and a small volume of concentrated urine is produced.

When blood water potential rises, osmoreceptors swell, less ADH is released, fewer aquaporins are in the membranes, and a large volume of dilute urine is produced. The hormone is broken down fairly quickly, so the response can be reversed.

Structure of the nephron

Each kidney contains about a million nephrons. The cortex (outer) holds the glomeruli, Bowman’s capsules, proximal and distal convoluted tubules. The medulla (inner) holds the loops of Henle and collecting ducts.

Region	Position	Main job
Glomerulus and Bowman’s capsule	Cortex	Ultrafiltration
Proximal convoluted tubule (PCT)	Cortex	Reabsorbs glucose, water and other useful solutes
Loop of Henle	Medulla	Builds the sodium ion gradient
Distal convoluted tubule (DCT)	Cortex	Water reabsorption, controlled by ADH
Collecting duct	Cortex to medulla	Water reabsorption, controlled by ADH

Formation of glomerular filtrate

Blood reaches the glomerulus, a tangle of capillaries, along the afferent arteriole and departs along the narrower efferent arteriole. The narrower exit causes a build-up of hydrostatic pressure in the glomerulus. This is ultrafiltration.

On its way from blood to capsule space, fluid has to cross three layers:

• the capillary endothelium, whose cells are perforated by fenestrations;
• the basement membrane, a gel-like meshwork of proteins and carbohydrates that is the main filter;
• the podocytes, the cells lining the capsule, whose slit-shaped gaps let fluid through.

Water, glucose, amino acids, urea and ions pass into the capsule. Blood cells and plasma proteins are too large, so they stay in the blood. Plasma proteins also keep the water potential of the blood lower than that of the filtrate. Net movement out depends on the balance between the outward hydrostatic pressure and the inward effects of capsule pressure and plasma proteins.

Worked example 1: net filtration pressure

Data supplied: capillary hydrostatic pressure 9.6 kPa; capsule space pressure 2.1 kPa; inward pressure from the plasma proteins 4.5 kPa.

Net outward pressure = 9.6 − 2.1 − 4.5 = 3.0 kPa.

The result is positive, so fluid is forced out into the capsule. Protein or blood cells in urine would suggest the filter is damaged.

Reabsorption in the proximal convoluted tubule

The PCT wall is a single layer of cells with microvilli on the surface facing the lumen, giving a large surface area. The cells contain many mitochondria to supply ATP, and the surfaces next to the blood capillaries are folded to increase area.

Glucose reabsorption happens in these steps:

1. Sodium ions are moved out of the cell into the tissue fluid by active transport (sodium-potassium pumps), using ATP. This keeps the sodium ion concentration inside the cell low.
2. Sodium ions then move into the cell from the filtrate down their concentration gradient, through co-transport proteins that carry glucose in with them.
3. Glucose builds up in the cell and moves into the tissue fluid by facilitated diffusion, then into the capillary.
4. With solutes removed from the filtrate and added to the blood, the water potential of the filtrate rises relative to the blood. Water follows by osmosis.

All the glucose in healthy filtrate is reabsorbed, so none appears in urine. Amino acids and many ions are reabsorbed in a similar way. The number of carrier proteins is limited. If the filtrate holds more glucose than the carriers can handle, some stays in the filtrate and is lost in urine.

Worked example 2: reabsorption of water

An adult forms 165 dm³ of filtrate in a day and produces 1.8 dm³ of urine.

Water-containing fluid reabsorbed = 165 − 1.8 = 163.2 dm³.

Percentage reabsorbed = 163.2 ÷ 165 × 100 = 98.9%.

Worked example 3: glucose and carrier limits

The following values are supplied as data. Filtrate forms at 0.125 dm³ per minute, and the carriers can reabsorb at most 375 mg of glucose per minute.

• Healthy plasma glucose is 0.9 g dm⁻³. Glucose filtered = 0.9 × 0.125 = 0.1125 g min⁻¹ = 112.5 mg min⁻¹. This is below 375, so all is reabsorbed.
• Uncontrolled diabetes gives plasma glucose of 3.2 g dm⁻³. Glucose filtered = 3.2 × 0.125 = 0.400 g min⁻¹ = 400 mg min⁻¹. Excess = 400 − 375 = 25 mg min⁻¹, which stays in the filtrate and is excreted. Extra glucose lowers the water potential of the filtrate, so less water is reabsorbed and urine volume rises.

The loop of Henle and the sodium ion gradient

The loop of Henle is a hairpin dipping into the medulla, with a descending limb and an ascending limb. It makes the tissue fluid of the medulla very concentrated, with a low water potential, so that water can be pulled out of the collecting duct. This is the countercurrent multiplier principle: fluid flows in opposite directions in the two limbs, and this multiplies the effect of the ion movement.

1. In the ascending limb, sodium ions (with chloride ions) are moved out into the medulla tissue fluid by active transport. The wall of this limb is impermeable to water, so water cannot follow.
2. The medulla tissue fluid now has a low water potential, and the gradient is steepest at the base of the loop.
3. The descending limb is permeable to water. Water leaves the filtrate by osmosis into the medulla tissue fluid, down the water potential gradient, and is carried away by nearby capillaries. The filtrate becomes more concentrated as it moves down.
4. The concentrated filtrate reaches the bottom of the loop, then rises in the ascending limb, where more ions leave. This keeps the ion gradient in place along the whole medulla.

A longer loop of Henle sets up a steeper gradient over a greater depth. More water can then be reabsorbed from the collecting duct, producing more concentrated urine. Mammals from dry habitats tend to have long loops.

Worked example 4: loop length and urine concentration

The data below are supplied. Urine concentration is given as a multiple of blood plasma concentration.

Species	Loop length / mm	Urine concentration (× plasma)
P	5	2.0
Q	14	4.5
R	32	10.0

Loop of R compared with P = 32 ÷ 5 = 6.4 times longer. Urine of R compared with P = 10.0 ÷ 2.0 = 5.0 times more concentrated. Urine concentration per mm of loop: P 0.40, Q 0.32, R 0.31. The relationship is positive, but not directly proportional.

Water reabsorption in the DCT and collecting duct

Filtrate entering the DCT has a higher water potential than the medulla tissue fluid. The collecting duct runs down through the medulla, so filtrate in it meets tissue fluid of steadily lower water potential. If the walls are water-permeable, water leaves by osmosis all the way down. ADH controls permeability:

Blood water potential	ADH	Water permeability of DCT and collecting duct	Urine
Low (dehydrated)	More	Higher	Small volume, concentrated
High (well hydrated)	Less	Lower	Large volume, dilute

Worked example 5: effect of ADH

After a long walk without drinking, a person’s urine flow falls from 2.0 to 0.7 cm³ min⁻¹.

Percentage change = (0.7 − 2.0) ÷ 2.0 × 100 = −65%.

The urine is more concentrated because more water was reabsorbed into the blood.

Common errors

• Writing that ADH “makes the kidney reabsorb water”. Say it makes the DCT and collecting duct cell membranes more permeable to water.
• Saying water is actively transported. Water always moves by osmosis. The active step is the movement of sodium ions.
• Saying the ascending limb is where water leaves. It is impermeable to water. Water leaves from the descending limb, DCT and collecting duct.
• Mixing up glucose reabsorption and ultrafiltration. Filtration is passive and driven by pressure; reabsorption of glucose needs carriers and ATP.
• Writing that proteins are filtered out in the PCT. Proteins and cells do not enter the filtrate in the first place.

Next steps

Condense this with the revision notes, then test yourself with the practice questions. Tick off the section on the printable checklist.

Official syllabus

AQA, AS and A-level Biology specification (7401 and 7402), for first teaching 2015, section 3.6.4.3 Control of blood water potential (A-level only). Published by AQA.