Urine Formation and the Control of the Kidney: Quick Revision

A brief note on Urine Formation and the Control of the Kidney. Read it before a test. Try each question at the end before you open its answer.

Urine formation

• Three steps: glomerular filtration, reabsorption, tubular secretion.
• Filter: endothelium of the capillaries, basement membrane, podocytes with filtration slits. Everything except proteins and cells passes.
• GFR: about 125 mL/min, or 180 L a day. About 99 per cent is reabsorbed. Urine: about 1.5 L a day.
• Active: glucose, amino acids, Na$^+$. Passive: nitrogenous wastes in part, and water in the initial segments.
• Secreted: H$^+$, K$^+$ and ammonia.

The parts of the tubule

• PCT: nearly all the nutrients and 70 to 80 per cent of the electrolytes and water. Brush border.
• Descending limb: permeable to water, filtrate gets concentrated.
• Ascending limb: impermeable to water, electrolytes leave, filtrate gets diluted.
• DCT: conditional reabsorption of Na$^+$ and water, HCO$_3^-$, secretion of H$^+$ and K$^+$.
• Collecting duct: large amounts of water, a little urea into the medulla.

Concentration and control

• Counter current: loop of Henle and vasa recta. The gradient runs from 300 mOsmol/L in the cortex to about 1200 in the inner medulla. The urine can be about four times as concentrated as the filtrate.
• ADH: made in the hypothalamus, released by the neurohypophysis, helps the reabsorption of water. A lack of it causes diabetes insipidus.
• Renin-angiotensin: JG cells, renin, angiotensinogen, angiotensin I and II, vasoconstriction, aldosterone, Na$^+$ and water reabsorbed.
• ANF: from the atria, vasodilation, lowers the blood pressure, a check on the renin-angiotensin mechanism.

A worked example

The GFR is 125 mL/min and the urine is 1.5 L a day. What per cent of the filtrate is reabsorbed?

1. The filtrate is $125 \times 1440 = 180{,}000$ mL, which is 180 L.
2. The reabsorbed part is $180 - 1.5 = 178.5$ L, which is $\dfrac{178.5}{180} \times 100 = 99.2$ per cent.

Practice with new numbers

A program to try

The program finds the filtrate in a day and the part that is urine.

Mistakes to avoid

• Saying that the descending limb is impermeable to water. It is the ascending limb that is.
• Saying that ADH is made in the pituitary. It is made in the hypothalamus.
• Saying that ANF raises the blood pressure. It lowers it.
• Saying that renin is a hormone that narrows the vessels. Renin is an enzyme. Angiotensin II narrows the vessels.

Check yourself

1. What is the GFR in a healthy person?

2. Which limb of the loop lets electrolytes out?

3. What does aldosterone cause?

4. Which hormone lowers the blood pressure and checks the renin-angiotensin mechanism?

Sources

• NCERT (2023). Biology, Textbook for Class XI, chapter 16.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier.