The kidney and osmoregulationEdexcel A-Level Biology B: Revision notes
Section 1
Structure of the kidney
Blood enters each kidney through the renal artery and leaves in the renal vein. Urine leaves through the ureter. A section shows an outer cortex, an inner medulla and the pelvis, where urine collects. Each kidney contains about a million nephrons.
In a nephron the Bowman's capsule (with the glomerulus, a knot of capillaries) lies in the cortex and leads to the proximal convoluted tubule (PCT), the loop of Henle (dipping into the medulla), the distal convoluted tubule (DCT) and the collecting duct, which carries urine back through the medulla.
Section 2
Urea and ultrafiltration
Excess amino acids cannot be stored. In the liver they are deaminated (the amino group is removed) and the nitrogen is converted into urea, which is carried in the blood to the kidneys. The details of the ornithine cycle are not required.
In the glomerulus, the afferent arteriole is wider than the efferent arteriole, so the hydrostatic pressure is high. This forces water, glucose, amino acids, urea and ions out through the gaps in the capillary endothelium, the basement membrane (which holds back proteins) and the slits between the podocytes into the Bowman's capsule. Blood cells and large proteins stay in the blood. This is ultrafiltration.
Ultrafiltration is not selective. It is based on size, so glucose and amino acids enter the filtrate and are taken back later.
Section 3
Selective reabsorption in the proximal tubule
Most of the filtrate is reabsorbed in the PCT. Sodium ions are actively transported out of the PCT cells into the blood, lowering the sodium concentration inside. Sodium then diffuses in from the filtrate through co-transport proteins, carrying glucose and amino acids with it. All the glucose is normally reabsorbed. Water follows by osmosis.
PCT cells have microvilli (a large surface area) and many mitochondria (ATP for the sodium pump). Reabsorption has a maximum rate because the carrier proteins can be saturated, which is why glucose appears in the urine in uncontrolled diabetes.
Section 4
The loop of Henle as a counter-current multiplier
The ascending limb actively pumps sodium and chloride ions out into the medulla tissue fluid and is impermeable to water. This lowers the water potential of the medulla. The descending limb is permeable to water, so water leaves by osmosis and the filtrate becomes more concentrated towards the tip of the loop.
Because the filtrate flows in opposite directions in the two limbs, a small difference at each level is multiplied into a large concentration gradient down the medulla. This is a counter-current multiplier. In the collecting duct, water can leave by osmosis along the whole gradient, so a concentrated urine can be made.
Section 5
ADH and negative feedback
Osmoreceptors in the hypothalamus detect a fall in plasma water potential. The posterior pituitary gland releases antidiuretic hormone (ADH) into the blood. ADH makes cells of the DCT and collecting duct more permeable to water by inserting aquaporins into their membranes. More water is reabsorbed, so a small volume of concentrated urine is produced and the plasma water potential rises.
When the water potential is too high the opposite happens: less ADH is released, the walls are less permeable, and a large volume of dilute urine is made. This is negative feedback.
Say what ADH acts on: the DCT and collecting duct, by making them more permeable to water. It does not act on the loop of Henle.
Section 6
Adaptation of the kangaroo rat kidney
The kangaroo rat (Dipodomys) lives in deserts and rarely drinks. Its kidney has very long loops of Henle and a thick medulla. A longer ascending limb pumps out more ions, so the medulla has a very low water potential and a steep gradient along the collecting duct. More water is reabsorbed by osmosis, so it produces very small volumes of highly concentrated urine.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The kidney and osmoregulation
- In a hospital laboratory, samples of blood plasma, glomerular filtrate and urine from a healthy adult are compared. The plasma contains water, glucose, amino acids, urea, mineral ions and large proteins such as albumin. The filtrate is collected from the Bowman's capsule.Explain why red blood cells and albumin are found in the plasma but not in the glomerular filtrate.2 marks
- A scientist follows the glucose concentration along the nephron of a healthy person after a meal. The filtrate entering the proximal convoluted tubule has the same glucose concentration as plasma, but none is detectable by the end of the tubule. The cells lining the tubule have microvilli and many mitochondria.A person with untreated diabetes mellitus has glucose in their urine, although their kidneys are healthy. Explain why.2 marks
- A researcher compares the kidneys of a beaver, which lives in water, with those of a kangaroo rat (Dipodomys sp.), which lives in desert and rarely drinks. The beaver has short loops of Henle. The kangaroo rat has very long loops of Henle for its body size and a thick medulla, and it produces very little urine.Explain how the loop of Henle produces a low water potential 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).