How Your Kidneys Make Less Urine When You Are Thirsty

After a hot afternoon with little to drink, urine turns dark and scanty; after several glasses of water, it becomes pale and plentiful.

This part covers tubular functions, the counter-current mechanism, hormonal regulation, and micturition, other excretory organs and kidney disorders.

The PCT reabsorbs most useful substances, the descending limb of Henle's loop loses water while the ascending limb moves out salts, the DCT fine-tunes sodium, water and pH, and the collecting duct reabsorbs large amounts of water to concentrate urine.

Proximal convoluted tubule (PCT):

- Reabsorbs nearly all essential nutrients and 70 to 80 per cent of electrolytes and water
- Secretes **H${}^{+}$ and ammonia and absorbs HCO${}_3^{-}$, keeping blood pH balanced

Loop of Henle:

- Descending limb — permeable to water, almost impermeable to electrolytes; filtrate becomes more concentrated as it moves down
- Ascending limb — impermeable to water, but moves electrolytes out actively or passively; filtrate becomes dilute as it moves up

Distal convoluted tubule (DCT):

- Conditional reabsorption** of Na${}^{+}$ and water

Collecting duct:

- Reabsorbs large amounts of water to make concentrated urine
- Lets small amounts of urea into the medulla to keep it hyperosmolar

An everyday example. After drinking several glasses of nimbu pani, urine turns pale, because the DCT and collecting duct reabsorb less water.

The substance. The two limbs of the loop do opposite jobs — water leaves the descending limb, salts leave the ascending limb.

Filtrate flows in opposite directions in the two limbs of Henle's loop, and blood does the same in the vasa recta; this counter-current flow, with salt from the ascending limb and urea from the collecting duct, builds a rising concentration in the medulla that draws water out of the collecting duct and concentrates urine.

How the gradient forms:

- The ascending limb moves NaCl into the medullary interstitium
- NaCl picked up by the ascending limb of the vasa recta is returned to the interstitium by its descending limb
- Small amounts of urea enter the thin ascending limb of Henle's loop and return to the interstitium from the collecting tubule
- The result is an osmolarity gradient from about 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla

Concentrating urine. As filtrate in the collecting duct passes through this increasingly concentrated medulla, water leaves by osmosis. Human kidneys can make urine nearly four times as concentrated as the initial filtrate, since $1200÷300=4$.

An everyday example. Camels in the Thar desert have kidneys well suited to making very concentrated urine, saving scarce water.

The substance. The gradient is kept, not used up — the counter-current flow in the vasa recta stops blood from washing the salt away.

When body fluid volume or blood pressure falls, ADH from the posterior pituitary increases water reabsorption, and renin from the JGA leads to angiotensin II and aldosterone, which raise blood pressure and sodium reabsorption; ANF from the heart acts as a check, and a lack of ADH causes diabetes insipidus.

ADH (vasopressin):

- Excess fluid loss makes the hypothalamus release ADH from the neurohypophysis
- ADH increases water reabsorption from the later parts of the tubule, preventing diuresis

Renin-angiotensin-aldosterone system:

- A fall in glomerular blood flow, pressure or GFR makes JG cells release renin
- Renin converts angiotensinogen to angiotensin I, which is converted to angiotensin II
- Angiotensin II is a powerful vasoconstrictor, raising glomerular pressure and GFR
- It also makes the adrenal cortex release aldosterone, increasing **Na${}^{+}$ and water reabsorption

Atrial natriuretic factor (ANF). Increased blood flow to the atria releases ANF, which dilates blood vessels and lowers blood pressure — a check on the renin-angiotensin mechanism.

Diabetes insipidus. With too little ADH, little water is reabsorbed, so a person passes large volumes of dilute urine and feels very thirsty.

An everyday example. After a long, sweaty cricket match with little to drink, ADH rises and urine becomes dark and scanty.

The substance. Diabetes insipidus has nothing to do with blood sugar** — the name only reflects the large urine output.

Micturition is the reflex release of urine when a filling bladder signals the CNS; lungs, liver and skin also remove wastes; and kidney disorders such as uraemia, kidney failure, stones and glomerulonephritis may need dialysis or a kidney transplant.

Micturition:

- As the bladder fills, stretch receptors in its wall send signals to the CNS
- The CNS sends motor messages that contract the smooth muscles of the bladder and relax the urethral sphincter
- Urine is released — the micturition reflex

Other excretory organs:

- Lungs — remove large amounts of **CO${}_2$ and some water
- Liver — secretes bile carrying bilirubin, biliverdin, cholesterol and degraded steroid hormones
- Skin — sweat removes NaCl, small amounts of urea and lactic acid; sebum removes sterols, hydrocarbons and waxes

Disorders:

- Uraemia — urea accumulates in blood when kidneys malfunction; highly harmful and can lead to kidney failure
- Haemodialysis — blood passes through a dialysing unit where nitrogenous wastes diffuse out into dialysing fluid; heparin prevents clotting
- Kidney transplant — the ultimate treatment for acute kidney failure; a kidney from a close relative lowers the chance of rejection
- Renal calculi — stones of crystallised salts such as oxalates formed in the kidney
- Glomerulonephritis — inflammation of the glomeruli

An everyday example. Doctors often advise people prone to kidney stones to drink plenty of water, keeping the salts dilute.

The substance. Dialysis replaces filtration but not every kidney function** — it cannot make hormones such as erythropoietin.