Chapter 16: Excretory Products and their Elimination (Human Physiology)

GFR is the amount of filtrate formed by the kidneys (both, all nephrons) per minute. In a healthy person it is about 125 mL/min, i.e. about 180 litres per day.

GFR is regulated by the juxta glomerular apparatus (JGA), a sensitive region formed by modified cells of the DCT and the afferent arteriole where they touch. When GFR (or glomerular blood flow/pressure) falls, the JG cells release renin. Renin converts angiotensinogen to angiotensin I and then angiotensin II, a powerful vasoconstrictor that raises glomerular blood pressure and so GFR. Angiotensin II also makes the adrenal cortex release aldosterone, which increases Na⁺ and water reabsorption, raising blood volume, BP and GFR. When GFR is back to normal, renin release falls (feedback).

(a) True (micturition reflex). (b) False: ADH increases water reabsorption, making urine concentrated (hypertonic) and preventing diuresis. (c) True (ultrafiltration). (d) True (counter current). (e) True.

The filtrate flows in opposite directions in the descending and ascending limbs of Henle's loop, and blood flows counter current in the two limbs of the vasa recta, which lies close to the loop. The descending limb loses water; the ascending limb, impermeable to water, transports NaCl out into the interstitium. NaCl is taken up by the descending vasa recta and returned to the interstitium by the ascending vasa recta. Small amounts of urea enter the thin ascending limb and are returned to the interstitium by the collecting tubule. Thus a gradient of osmolarity is kept, rising from 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla. Water therefore passes easily out of the collecting duct, making urine about four times as concentrated as the filtrate: a way of conserving water.

Liver: converts ammonia to urea; secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, which pass out with the faeces. Lungs: remove large amounts of CO₂ (about 200 mL/min) and water vapour. Skin: sweat glands remove water, NaCl, small amounts of urea and lactic acid in sweat (its main role is cooling); sebaceous glands remove sterols, hydrocarbons and waxes in sebum, which also gives a protective oily covering.

Urine from the nephrons is stored in the urinary bladder. As it fills, the bladder wall stretches; stretch receptors send signals to the CNS. When a voluntary signal is given, the CNS sends motor messages causing contraction of the smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter, so urine is released through the urethra. The release is micturition; the neural mechanism is the micturition reflex. An adult passes 1–1.5 L of urine per day.

(a)–(iii); (b)–(v); (c)–(iv); (d)–(i); (e)–(ii).

Osmoregulation is the regulation of the water and ionic (salt) content, and so the osmotic concentration, of the body fluids within normal limits, whatever the intake or the surroundings. Examples: protonephridia (flame cells) of Planaria mainly osmoregulate; in humans the kidneys do it by varying water and Na⁺ reabsorption under ADH and aldosterone.

Ammonia is highly toxic and very soluble, so it must be removed quickly in a large amount of water. Land animals cannot spare so much water; they must conserve water. So they convert ammonia into urea (less toxic, can be stored briefly and excreted with less water) or uric acid (least toxic, almost insoluble, excreted as a pellet or paste with minimum water loss).

The JGA (modified cells of the DCT and afferent arteriole at their contact) monitors and regulates GFR and blood pressure. A fall in GFR/glomerular blood pressure makes the JG cells release renin → angiotensin II (vasoconstriction, higher glomerular pressure) and aldosterone (Na⁺ and water reabsorption) → GFR and BP restored. So it keeps filtration steady and helps regulate blood volume and BP.

(a) Amphioxus (a cephalochordate). (b) Columns of Bertini. (c) Vasa recta.

(a) impermeable; permeable. (b) ADH (vasopressin). (c) nitrogenous wastes. (d) 25–30.