Excretory System

Every organism generates waste in its body and has a mechanism to expel it, and in human beings the undigested food is thrown out through the anus while the metabolic wastes are removed by the excretory system. Excretion is the removal of the harmful nitrogenous and other metabolic wastes produced by the body's chemical reactions, and it must not be confused with defecation, which is the throwing out of undigested food through the anus — defecation is egestion, not excretion. The most important waste in humans is urea, a nitrogenous compound made in the liver from ammonia, and alongside it the body must dispose of several other substances.

Because these wastes form constantly, the excretory system also keeps the water and salt balance of the body steady, a regulating job called osmoregulation, so the system does far more than merely throw out rubbish.

Although the kidneys do most of the work, the body shares the task of excretion among several organs, each removing a particular kind of waste. The lungs handle the gaseous waste, the skin and liver deal with others, and the kidneys remove the bulk of the soluble nitrogenous wastes.

• The lungs, the primary respiratory organ, take in O₂ and release CO₂, and in the process also lose some water as vapour.
• The liver changes the decomposed haemoglobin of old RBCs into bile pigments, which pass with the bile into the alimentary canal for elimination in the faeces.
• The liver also excretes cholesterol, steroid hormones, some vitamins and drugs through the bile.
• Urea is formed in the liver by a cyclic process called the urea cycle or "ornithine cycle".
• Excess amino acids are deaminated by the enzyme oxidase, producing ammonia (NH₃), which, being toxic, is then converted into urea.
• The kidneys are the main organ of the human excretory system.

The two kidneys are paired organs and the main organ of the human excretory system, lying one on each side of the spine. They are a pair of reddish-brown, bean-shaped organs at about the level of the liver, set against the back wall of the abdomen (retroperitoneal). The right kidney lies slightly lower than the left because the large liver presses down on its side, and each kidney is about the size of a clenched fist.

Blood reaches each kidney through the renal artery and leaves by the renal vein, and the cleaned product, urine, drains from the kidney's central basin, the renal pelvis, into the ureter.

If a kidney is sliced lengthwise, it is seen to be built in clearly marked zones, an arrangement worth knowing for the diagram-label questions examiners favour. The outer layer is the cortex, the inner the medulla, and the urine they make is funnelled into the pelvis at the centre.

• The renal hilum carries the renal vein, renal nerve and renal artery.
• The renal pelvis collects urine from the nephrons and narrows into the upper end of the ureter.

Inside this framework lies the real machinery of the kidney — about 1 million nephrons in each kidney — and it is to this tiny unit that we now turn.

The nephron is the structural and functional unit of the kidney, the microscopic tube that actually filters the blood and makes the urine, and each kidney contains about ~1 million (one million) nephrons. A nephron has two main parts: a glomerulus (a tuft of capillaries) and a long, convoluted renal tubule that processes the filtered fluid. Its job is to filter blood, reabsorb useful substances and secrete waste, leading to urine formation.

The glomerulus together with the Bowman's capsule make up the renal corpuscle, and blood is brought to the glomerulus by the afferent arteriole and carried away by the efferent arteriole — the unusual arrangement of one arteriole leading into another keeps the filtering pressure high.

Urine is formed within the nephron by three steps that follow one after another — filtration, reabsorption and secretion — and understanding their order is the key to the whole topic. Blood arrives, is forced through a sieve, the useful parts are taken back, and a last few wastes are added before the leftover liquid leaves as urine.

• Reabsorbed into blood: glucose, amino acids, water, some salts — this process is selective.
• Secreted into the tubule: excess potassium, hydrogen ions, some drugs — this maintains pH balance.

The first step produces about 180 litres of glomerular filtrate a day, yet because nearly all of it is reabsorbed, only about 1–1.5 litres/day of urine is finally passed. The fact that glucose and proteins are normally absent from urine is itself high-yield, since their appearance signals disease (sugar in urine points to diabetes).

The primary mechanism of excretion is urine formation in the kidneys, which keeps the body's internal environment stable through a coordinated effort of filtration, reabsorption and secretion. Following the fluid through each named part of the nephron makes the diagram-label questions easy.

• Diagram labels: afferent arteriole, efferent arteriole, glomerulus, Bowman's capsule, PCT, DCT, Loop of Henle, collecting duct, leading to urine → ureter, then kidney, ureter, urinary bladder, urethra.

The amount and concentration of urine is not fixed but is adjusted moment to moment by hormones, so that the body holds on to water when it is short and lets it go when there is plenty. Two hormones do most of this fine-tuning, and both act on the later parts of the nephron.

When ADH (antidiuretic hormone) is low — for instance after drinking a lot of water — little water is reabsorbed and a large volume of dilute urine is passed, while its lack in the disease diabetes insipidus causes the body to lose enormous amounts of water; aldosterone, by saving sodium, helps to hold up the blood pressure.

Because the kidney is so finely built, several diseases can upset its work, and a few names recur in examinations. They range from infections and inflammations to the familiar stones, tumours and outright failure of the filtering units.

When the kidneys fail, their work can be done artificially by a machine in a process called dialysis (haemodialysis), which filters the blood through an artificial membrane, and a lasting cure is a kidney transplant.

The way an animal gets rid of its nitrogenous waste depends chiefly on how much water it has to spare, and on this basis animals fall into three groups, a classification examiners like to test. Where water is abundant the cheap but toxic ammonia is dumped; where water must be saved, the body spends energy to make the safer, less soluble urea or uric acid.

While the kidneys are the primary excretory organs, other organs also help remove waste, and even plants — which have no special excretory organs — get rid of their wastes by simple methods of their own.