Your Kidneys Filter Far More Fluid Every Day Than You Could Ever Drink

Blood entering a kidney carries both wastes and valuable substances such as glucose. The kidney has to throw away the first and keep the second. It does this in three steps inside each nephron: filter almost everything small, take back what is useful, and add a few extra wastes.

This part follows those steps, then covers urine tests, seasonal changes and kidney failure.

Urine forms when high pressure filters blood in the Malpighian capsule, the tubule reabsorbs useful substances, and a few extra wastes are secreted into the tubule.

1. Ultrafiltration — in the Malpighian capsule.

- High pressure in the glomerulus forces water, glucose, amino acids, salts and urea into the capsule
- Blood cells and plasma proteins are too large and stay in the blood
- The fluid formed is the glomerular filtrate

2. Selective reabsorption — mainly in the proximal convoluted tubule.

- All glucose and amino acids return to the blood, by active transport
- Most water and salts are reabsorbed; water follows by osmosis
- Much of the urea stays in the tubule

3. Tubular secretion — mainly in the distal convoluted tubule.

- Extra wastes such as potassium and hydrogen ions, creatinine and some medicines pass from blood into the tubule

Worked example. Filtration at about $125\text{ mL}$ per minute gives, in one day:

$$125\times 60\times 24=180000\text{ mL}=180\text{ L}$$

If $1.5\text{ L}$ of urine is passed, the fraction reabsorbed is:

$$\frac{180-1.5}{180}\times 100\approx 99\%$$

An everyday example. Sorting rice at home, you tip out the grains and pick back the good ones, leaving stones and husk behind.

The misconception. Glucose is absent from normal urine not because it is never filtered, but because all of it is reabsorbed.

Normal urine is mostly water with dissolved urea, salts, uric acid and creatinine; glucose, protein, blood or bile pigments in it point to specific health problems.

Normal constituents:

- Water — by far the largest part
- Urea — the main nitrogen waste
- Uric acid and creatinine — smaller nitrogen wastes
- Salts — mainly sodium chloride
- Urochrome — the pigment that makes urine pale yellow

Abnormal constituents:

- Glucose — glycosuria; blood sugar is so high that reabsorption cannot keep up, as in diabetes mellitus
- Protein — albuminuria; damaged glomeruli let proteins leak through, a sign of kidney disease
- Blood — haematuria; may indicate infection, injury or kidney stones
- Bile pigments — dark yellow urine in jaundice, a sign of liver disease

An everyday example. A routine urine test at a health check-up or during pregnancy screens for exactly these substances.

The link to the previous section. Each abnormal result shows which step has failed — protein means filtration is faulty, glucose means reabsorption is overwhelmed.

The volume of urine depends on how much water the body loses in other ways: heavy sweating or diarrhoea makes the kidneys save water, while little sweating lets them pass more.

- Summer — heavy sweating loses water, so more water is reabsorbed in the tubules; urine is small in volume, concentrated and darker
- Winter — little sweating, so less water is reabsorbed; urine is large in volume, dilute and pale
- Cholera — severe watery diarrhoea and vomiting cause dehydration; the kidneys save almost all water, so urine is very scanty or may stop

How the kidney adjusts. When blood becomes concentrated, the pituitary gland releases more antidiuretic hormone (ADH), which makes the collecting ducts reabsorb more water.

An everyday example. A cholera patient is given oral rehydration solution — water with salt and sugar — because salts are lost along with water.

The boundary case. Drinking lots of water on a cold day gives the most dilute urine of all, because both low sweating and high intake push the same way.

Gout is caused by excess uric acid crystals in joints, kidney stones by crystals of salts forming in the kidney, and dialysis removes wastes from blood by diffusion through a membrane when kidneys fail.

Gout.

- Cause: too much uric acid in the blood, from a diet rich in certain proteins or poor excretion
- Effect: needle-like crystals collect in joints, often the big toe, causing painful swelling

Kidney stones.

- Cause: salts such as calcium oxalate crystallise in the kidney, more likely when too little water is drunk
- Effect: severe pain, blocked urine flow, and sometimes blood in urine

Dialysis — the principle.

- Blood is led from the patient into a dialyser, tubes of a partially permeable membrane
- The tubes are bathed in dialysis fluid containing glucose and salts at normal blood levels but no urea
- Urea and excess salts diffuse out; useful substances stay, since there is no concentration difference
- Heparin is added so blood does not clot; cleaned blood returns to a vein

An everyday example. Patients with kidney failure visit a dialysis centre several times a week, each session doing the job healthy kidneys do all the time.

The boundary case. Dialysis removes wastes but cannot make hormones or fully replace the kidney; a transplant can.