Question 1

Why is urea produced and how is it removed from the body?

Accepted Answer

Urea is produced in the liver from the deamination of excess amino acids. Amino acids cannot be stored, so their nitrogen-containing groups are removed, forming ammonia, which is toxic. The liver converts ammonia into urea via the ornithine cycle, which is less toxic and can be safely transported in the blood. Urea is then filtered out of the blood by the kidneys and excreted in urine.

Question 2

How does ADH control water reabsorption in the kidneys?

Accepted Answer

ADH (antidiuretic hormone) is released from the posterior pituitary gland when blood water potential falls (e.g., due to dehydration). It travels to the kidneys and binds to receptors on the cells of the collecting duct. This triggers a signaling cascade that causes aquaporin-2 water channels to be inserted into the apical membrane, increasing the permeability of the collecting duct to water. Water then moves by osmosis out of the filtrate and into the medullary interstitium, where it is reabsorbed into the blood, concentrating the urine.

Question 3

What is the difference between ultrafiltration and selective reabsorption?

Accepted Answer

Ultrafiltration occurs in the glomerulus, where blood pressure forces small molecules (water, ions, glucose, urea) through the filtration barrier into the Bowman's capsule, forming glomerular filtrate. Large proteins and blood cells are retained. Selective reabsorption occurs mainly in the proximal convoluted tubule, where useful substances like glucose, amino acids, and ions are actively transported back into the blood. Water follows by osmosis. Ultrafiltration is non-selective based on size, while selective reabsorption is specific and energy-dependent.

Question 4

How does the loop of Henle create a concentration gradient?

Accepted Answer

The loop of Henle acts as a countercurrent multiplier. The descending limb is permeable to water but not to salts, so water leaves by osmosis into the medulla. The ascending limb is impermeable to water but actively transports sodium and chloride ions out into the medulla. This creates a high solute concentration in the medullary interstitium, which draws water out of the collecting duct. The longer the loop, the steeper the gradient, allowing for more concentrated urine.

Question 5

What happens to glucose in the nephron?

Accepted Answer

Glucose is filtered out of the blood during ultrafiltration and enters the glomerular filtrate. In the proximal convoluted tubule, glucose is reabsorbed via co-transport with sodium ions. Sodium is actively pumped out of the tubule cell into the blood by the Na+/K+ pump, creating a low sodium concentration inside the cell. Sodium then diffuses into the cell from the filtrate, bringing glucose with it via a co-transporter protein. Glucose then diffuses into the blood. Normally, all glucose is reabsorbed, but if blood glucose exceeds the renal threshold, glucose appears in urine (glycosuria).

Question 6

Why is the kidney considered an example of homeostatic control?

Accepted Answer

The kidney maintains homeostasis by regulating blood water potential, ion balance, and pH. It adjusts the volume and concentration of urine in response to changes in fluid intake, sweating, and diet. For example, when dehydrated, the kidney produces concentrated urine by increasing water reabsorption under ADH control. When overhydrated, it produces dilute urine. It also regulates blood pH by excreting hydrogen ions and reabsorbing bicarbonate. These processes are controlled by negative feedback loops, making the kidney a key homeostatic organ.

Excretion as an example of homeostatic control

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