12.4 Excretion

12.4 Excretion

Cells produce more than carbon dioxide. Breaking down amino acids frees nitrogen, and water and salts swing every time an animal sweats, drinks, or eats a salty meal. Vertebrate kidneys remove nitrogen waste and, just as carefully, hold the volume and the osmolarity of the blood inside a narrow range. The human nephron is the working model. The chemical form of the nitrogen waste is the comparison across animals.

Three nitrogen wastes

When cells deaminate amino acids, the stripped amino groups become ammonia. Ammonia is very toxic and very soluble in water. Many aquatic animals, including most bony fish, release it across the gills or dilute it in a large volume of urine. Making ammonia costs little energy. Flushing it safely costs a lot of water.

Mammals, including humans, convert ammonia to urea in the liver by the urea cycle. Urea is far less toxic, so it can travel in the blood and leave in a moderate volume of urine. Building each urea molecule costs ATP. Adult amphibians and sharks also rely mainly on urea. Urea is the main mammalian nitrogen waste. Humans also excrete a little uric acid from purine breakdown, but urea remains the main waste of protein metabolism.

Uric acid and the water budget

Birds, many reptiles, and insects convert nitrogen waste to uric acid. Uric acid is poorly soluble and can be voided as a white paste with very little water. That saving matters for an embryo sealed in a shelled egg, and for an animal that rarely gets a drink. The pathway costs more energy than making urea. Insects are not vertebrates, but they belong in the comparison because they use this same water-saving waste.

The human nephron

Each kidney holds on the order of a million nephrons. A nephron is a filter plus a long tubule. Finished urine is what remains after the tubule has reclaimed what the body still needs and has added a few secreted wastes. Urine then flows into the renal pelvis, down a ureter, into the urinary bladder for storage, and out through the urethra. The composition is decided in the nephron, before the bladder ever stores it.

Filtration is not urine yet

Blood pressure drives filtration at the glomerulus, a tuft of capillaries tucked into Bowman's capsule. Water, ions, glucose, amino acids, and urea are pushed into the capsule. Cells and large plasma proteins stay in the blood. The selection is by size, not by a decision that a molecule is a waste. The fluid in the capsule is filtrate. It still contains useful glucose and amino acids. The glomerulus does not secrete urine.

The proximal tubule reabsorbs most of the useful solutes and a large share of the water. Transporters normally reclaim all of the filtered glucose here, along with amino acids and most of the bicarbonate. If the filtered load of glucose rises above what those transporters can carry, glucose continues down the tubule and can appear in the urine.

The loop of Henle builds a salty medulla. This is a countercurrent multiplier, a kidney arrangement, and it is a different mechanism from the countercurrent flow of water and blood in a fish gill. The descending limb is permeable to water and does not pump salt. Water leaves into the salty tissue, and the filtrate inside the limb becomes more concentrated. The ascending limb pumps salt out and is impermeable to water. Salt leaves, water cannot follow, and the filtrate becomes dilute while the medulla stays concentrated. A longer loop, as in many desert mammals, builds a steeper gradient and allows more concentrated urine. Human kidneys can concentrate urine to roughly four times the osmolarity of blood plasma.

The distal tubule continues the adjustment of sodium, potassium, and pH. Some substances, including extra hydrogen ions, potassium, and certain drugs, are secreted from the nearby capillaries into the tubule. Secretion is a second entrance into the tubule. It is not the job of the glomerulus, and it is not the same word as excretion. Excretion is the final elimination from the body.

Hormones on the collecting duct

The collecting duct passes back through the salty medulla and decides how much water the urine will still contain. Antidiuretic hormone, also called vasopressin, is released from the posterior pituitary. It causes cells of the collecting duct to insert aquaporins, water channels, into their membranes. Water then leaves the duct osmotically and the urine becomes concentrated. Without antidiuretic hormone, those channels are not inserted, the duct stays poorly permeable to water, and a large volume of dilute urine leaves.

Aldosterone, from the adrenal cortex, promotes sodium reabsorption in the distal tubule and the collecting duct. Potassium secretion rises as sodium is reclaimed. Water can follow the sodium when the duct is also permeable to water, so aldosterone supports blood volume. It does not block sodium reabsorption.

A runner sweating on a hot day loses water, and blood osmolarity rises. Osmoreceptors in the hypothalamus respond. The posterior pituitary releases more antidiuretic hormone. Aquaporins open the collecting duct to water, water is reabsorbed into the medulla, and the kidneys produce a smaller volume of more concentrated urine. Thirst rises at the same time. Those responses conserve water and pull osmolarity back down. The glomerulus is still only filtering by size. The saving of water happens downstream, under hormonal control.

Finished urine equals filtrate, minus what the tubule reabsorbed, plus what the tubule secreted.