16Class 11 Biology
Excretory Products and their Elimination short notes
Short notes on nitrogenous waste, the nephron, urine formation and how the body regulates it. Hormone control is a favourite NEET topic.
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Waste types
- Ammonotelic: release ammonia, as in many bony fish and aquatic animals.
- Ureotelic: release urea, as in mammals and many amphibians.
- Uricotelic: release uric acid, as in reptiles, birds and insects.
Nephron
- Each kidney has about one million nephrons.
- Parts: Bowman's capsule and glomerulus (together the Malpighian body), PCT, Henle's loop, DCT and collecting duct.
- Cortical nephrons have short loops, and juxtamedullary ones have long loops.
Urine formation
- Three steps: filtration, reabsorption and secretion.
- GFR is about 125 mL a minute, or 180 L a day. About 1.5 L of urine forms a day.
- The PCT reabsorbs most of the filtrate, including all glucose and amino acids.
- The descending limb lets water pass, and the ascending limb lets out salts.
- The counter-current mechanism in Henle's loop and vasa recta keeps the medulla concentrated.
Regulation
- ADH (vasopressin) makes the kidney save water. Its lack causes diabetes insipidus.
- The JGA releases renin, starting the renin-angiotensin system, and aldosterone saves sodium.
- ANF from the heart works against this system.
Disorders
- Uraemia is the build-up of urea in blood, treated by haemodialysis.
- Renal calculi are kidney stones, and glomerulonephritis is inflammation of the glomeruli.
Where students slip up
- Reversing the permeability of the descending and ascending limbs.
- Thinking 180 L of urine is made daily. That's the filtrate, and most is reabsorbed.
- Mixing up ADH and ANF effects.
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