Concepts: Micturition, other excretory organs, disorders and practice

4.1 From nephron to the outside

Urine leaving the nephrons flows through the collecting ducts, reaches the renal pelvis, and travels down the ureters to the urinary bladder. The bladder holds the urine there until the central nervous system decides to let it out. That decision is triggered when the filling bladder stretches its own wall. Stretch receptors on the wall send signals to the central nervous system, which then sends motor messages that contract the smooth muscles of the bladder and relax the urethral sphincter at the same time, so the urine is released. Passing urine out is called micturition, and the reflex circuit that controls it is the micturition reflex.

A grown adult passes roughly 1 to 1.5 litres of urine in twenty-four hours. The fluid is pale yellow, slightly acidic with a pH close to 6.0, and has its own characteristic smell. The urea leaving the body in that urine comes to about 25 to 30 g a day. The composition of the urine changes with disease, and urine analysis is therefore a routine clinical test: glucose in the urine, called glycosuria, and ketone bodies in the urine, called ketonuria, are signs of diabetes mellitus.

4.2 Other organs that help in excretion

The kidneys are not the only excretory organs. The lungs are a major exit for carbon dioxide: they release roughly 200 mL of it every minute, together with a good deal of water vapour. Bile is the liver's contribution to excretion. This largest gland of the body pours bile into the intestine, and the bile carries bilirubin, biliverdin, cholesterol, broke down steroid hormones, vitamins and drugs. The intestine then carries most of these substances out with the faeces. The sweat glands of the skin produce a watery fluid containing sodium chloride, small amounts of urea and lactic acid; sweat mainly cools the body, but it also carries some wastes. The sebaceous glands remove sterols, hydrocarbons and waxes in sebum, which also forms a protective oily layer on the skin. Small amounts of nitrogenous wastes can also leave through saliva.

4.3 Disorders of the excretory system

When the kidneys stop working properly, the urea level in the blood keeps climbing. This state is called uremia, and if it is not treated it damages the body and can end in kidney failure. The treatment for such a patient is haemodialysis. Blood is taken from a convenient artery, heparin is added so that it will not clot, and the blood is passed through a dialysing unit known as an artificial kidney. Inside the unit the blood flows through a long coiled tube made of cellophane, and the tube is bathed in a fluid that resembles plasma but has no nitrogenous wastes. Urea and the other wastes diffuse across the cellophane into the fluid, while the useful plasma components stay behind because the fluid already contains them. The cleaned blood then returns to the patient. Dialysis performs the filtering task that the failed kidney can no longer do, so it must be repeated regularly.

Other disorders you should be able to name include renal calculi, which are stones formed from crystallised salts inside the kidney; glomerulonephritis, which is inflammation of the glomeruli; and gout, in which uric acid crystals accumulate in the joints. Glycosuria and ketonuria, as noted above, are signs of diabetes rather than kidney disease, but they appear in the same urine test.

4.4 Exercise practice

Exercise 5. The lungs remove carbon dioxide and water; the liver removes bile pigments, cholesterol, degraded steroid hormones, vitamins and drugs; the skin removes sodium chloride, urea and lactic acid through sweat and sterols, hydrocarbons and waxes through sebum.

Exercise 6. Micturition is the release of urine. Stretching of the full bladder stimulates stretch receptors, which send signals to the central nervous system; motor messages then contract the bladder muscle and relax the urethral sphincter, and the urine leaves. The reflex is called the micturition reflex.

Exercise 7. Ammonotelism goes with bony fishes, Bowman's capsule with the renal tubule, micturition with the urinary bladder, uricotelism with birds, and ADH with water reabsorption.

Exercise 8. Osmoregulation is the regulation of the amount of water and the concentration of solutes in the body fluids. The kidneys perform it by adjusting water reabsorption under the control of ADH and by adjusting sodium reabsorption under the control of aldosterone.

Exercise 9. Terrestrial animals cannot afford to lose the large amount of water needed to dilute ammonia, so they convert ammonia into urea or uric acid. Urea needs less water than ammonia and uric acid can be excreted as a semi-solid, so the terrestrial habit favours ureotelism and uricotelism rather than ammonotelism.

Exercise 11. The chordate animal with flame cells is Amphioxus, the cortical portions projecting between the medullary pyramids are the renal columns or columns of Bertini, and the loop of capillary running parallel to the loop of Henle is the vasa recta.