Control of blood water potentialAQA A-Level Biology: Revision notes
Section 1
Osmoregulation and the hypothalamus
Osmoregulation is the control of the water potential of the blood. It matters because blood water potential sets the water potential of tissue fluid and so the water content of cells. If the blood water potential falls (for example after sweating or a salty meal), osmoreceptors in the hypothalamus detect it. If it rises (after drinking a lot), they detect that too.
The hypothalamus contains neurosecretory cells that make antidiuretic hormone (ADH). These pass ADH along their axons to the posterior pituitary gland, where it is stored and released into the blood capillaries when needed.
This is negative feedback: a change in blood water potential triggers a response that reverses the change.
Section 2
The nephron and ultrafiltration
Each kidney contains about a million nephrons. The cortex holds the Bowman's capsule, glomerulus, proximal and distal convoluted tubules. The medulla holds the loop of Henle and collecting duct.
Blood enters the glomerulus in the afferent arteriole, which is wider than the efferent arteriole that carries it away, so the hydrostatic pressure in the glomerulus is high. This forces water, glucose, urea and ions out through three layers: the capillary endothelium (with gaps), the basement membrane and the podocytes of the Bowman's capsule. Large molecules such as plasma proteins stay in the blood. The fluid that enters the capsule is the glomerular filtrate.
Filtration is by pressure and size only. It is not selective, so glucose is filtered; it is reabsorbed later in the proximal convoluted tubule.
Section 3
Reabsorption in the proximal convoluted tubule
The proximal convoluted tubule (PCT) reabsorbs all the glucose and most of the water and ions. Its epithelial cells have microvilli (large surface area) and many mitochondria.
- Sodium ions are actively transported out of the cells into the blood, lowering their concentration inside the cell.
- Sodium ions enter from the filtrate by facilitated diffusion, and glucose enters with them by co-transport.
- Glucose then passes into the blood by facilitated diffusion.
The reabsorbed solutes lower the water potential of the blood, so water follows by osmosis. If the glucose in the filtrate is more than the carriers can take up, some stays in the urine.
Say that the carrier proteins are saturated when explaining glucose in the urine.
Section 4
The loop of Henle and the sodium ion gradient
The loop of Henle creates a gradient of sodium ions in the medulla.
- The ascending limb actively transports sodium ions out into the tissue fluid of the medulla. It is impermeable to water, so water stays in the tubule.
- This gives the medulla a low water potential, lowest deep in the medulla.
- The descending limb is permeable to water, so water leaves it by osmosis, and the filtrate becomes more concentrated as it descends.
Animals with longer loops of Henle, such as desert mammals, build a steeper gradient and make more concentrated urine.
Section 5
The distal convoluted tubule and collecting duct
Filtrate that reaches the distal convoluted tubule (DCT) and collecting duct passes through the medulla, where the water potential of the tissue fluid is low. Water can leave by osmosis only if the walls are permeable to it, and this is controlled by ADH.
- ADH binds to receptors on the cell surface membranes of the DCT and collecting duct.
- This leads to more aquaporins being inserted into the membranes.
- More water is reabsorbed, giving a small volume of concentrated urine.
- With no ADH, the walls stay impermeable, so a large volume of dilute urine is made.
Section 6
The full negative feedback loop
Blood water potential falls: osmoreceptors detect it, the hypothalamus stimulates the posterior pituitary to release more ADH, the DCT and collecting ducts become more permeable, more water is reabsorbed and the water potential rises.
Blood water potential rises: less ADH is released, the walls stay less permeable, more dilute urine is produced and the water potential falls back to normal.
The hypothalamus also triggers thirst, so the person drinks.
In a data question, name the structure that detects the change, the hormone, the target tissue and the effect.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Control of blood water potential
- A marathon runner sweats heavily during a race in hot weather and drinks nothing for two hours. Her blood water potential falls, and her body responds by changing the volume and concentration of her urine.Explain how the release of ADH changes the volume and concentration of the runner's urine.2 marks
- A physiologist is studying filtration in the kidney of a mammal. In each nephron, blood flows from an afferent arteriole into a knot of capillaries called the glomerulus, and leaves in an efferent arteriole. Fluid is forced out of the capillaries into the lumen of the Bowman's capsule.Describe how glucose in the filtrate is reabsorbed into the blood in the proximal convoluted tubule.2 marks
- The kangaroo rat lives in deserts and produces urine far more concentrated than that of a beaver, which lives by rivers. Anatomical studies show that the kangaroo rat has nephrons with much longer loops of Henle than the beaver, which has nephrons with short loops.Describe how the loop of Henle maintains a gradient of sodium ions in the tissue fluid of the medulla.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).