The Kidneys
One Hundred Eighty Liters of Daily Filtrate and the Salt Hairpin Loop
Your two fist-sized kidneys weigh 300 grams combined and hold barely 60 milliliters of fluid at any instant. Yet every twenty-four hours, they filter 180 liters of blood plasma—enough to fill a bathtub. Tubular pumps reabsorb 99 percent of that water and salt back into the blood, leaving 1.5 liters of urine.
From an engineering standpoint, the kidney cleans blood backward. Instead of building molecular traps for thousands of different wastes, the glomerulus dumps nearly all fluid, salts, sugars, and small molecules into the drainpipe at once. The tubule then pulls back only what it has explicit pumps to keep, automatically flushing out any novel toxin.
Physical chemist Werner Kuhn realized in 1942 that the Loop of Henle is a hairpin countercurrent multiplier. Pumps in the ascending limb create a horizontal gradient of only 200 milliosmoles per liter between the parallel tubes. Folding the tube into a deep hairpin multiplies that gradient lengthwise to 1,200 milliosmoles—four times more concentrated than human blood plasma.
Each human kidney contains roughly one million microscopic filtering units called nephrons. Just like cardiomyocytes and inner-ear hair cells, human nephron formation stops permanently at thirty-six weeks of gestation. You are born with every nephron you will ever have; if disease destroys half, the survivors must hypertrophy to carry the load.
To drive 180 liters of passive filtration a day without a piston, the kidney wires two resistance arterioles in series—afferent then efferent. Constricting the exit pipe while dilating the entrance holds glomerular capillary pressure at 55 millimeters of mercury, more than double normal capillary pressure.
The filtration slit inside Bowman's capsule is not a simple size sieve. Serum albumin has a radius of 3.6 nanometers, slightly smaller than the 4-nanometer slit pores. Albumin stays out of urine by electrostatics: negatively charged sialoproteins lining the foot processes strongly repel negatively charged albumin molecules.
Shipwrecked sailors die of dehydration faster if they drink seawater than if they drink nothing. Human kidneys can concentrate urine to a ceiling of 1,200 milliosmoles per liter. Because seawater sits at 1,000 milliosmoles of salt—plus daily metabolic urea—excreting the salt from one liter of seawater requires 1.3 liters of urine.
Desert kangaroo rats (Dipodomys) never drink liquid water across their entire lives. Their extra-long Loops of Henle concentrate urine to 6,000 milliosmoles per liter—five times the human ceiling. They live entirely on the metabolic water produced inside cells when oxidizing dry seeds ($\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \to 6\text{CO}_2 + 6\text{H}_2\text{O}$).
Your red blood cell count is regulated by the kidneys rather than the bone marrow. Peritubular cells in the renal cortex degrade HIF-2α whenever oxygen is abundant. When oxygen tension dips at high altitude, HIF-2α survives and triggers erythropoietin (EPO) secretion, ordering the marrow to mint new erythrocytes.
Although the kidneys weigh less than 0.5 percent of body mass, they receive 22 percent of resting cardiac output—1.1 liters of blood per minute. Kidney tissue does not need that much oxygen for metabolism; the high flow rate is required strictly to drive rapid glomerular filtration.
Glomeruli filter 180 grams of glucose into raw urine daily, and SGLT2 cotransporters in the proximal tubule reclaim 100 percent of it. However, those pumps saturate when blood glucose exceeds 180 milligrams per deciliter. Above that threshold, uncollected sugar spills into the urine, dragging water with it in diabetes mellitus.
When arterial blood pressure drops by even 10 millimeters of mercury, granular cells in the kidney's juxtaglomerular apparatus release the enzyme renin. Renin launches the angiotensin-aldosterone cascade, constricting blood vessels body-wide and locking sodium and water inside the circulation within minutes.