Excretion in human beings and plants
Excretion removes harmful nitrogenous wastes. In humans, many nephrons in the kidneys filter blood and make urine; plants store or shed wastes in leaves, resins and gums.
Parts of the human excretory system
| Part | Job |
|---|---|
| Kidneys (two) | Filter blood and make urine; lie in the abdomen, one on each side of the backbone |
| Ureters (two) | Carry urine from each kidney to the bladder |
| Urinary bladder | Muscular bag that stores urine; under nervous control |
| Urethra | Carries urine out of the body |
How a nephron makes urine
- 1Blood enters the glomerulus, a cluster of thin-walled capillaries inside the cup-shaped Bowman's capsule.
- 2★Filtration: water, glucose, amino acids, salts and urea pass from the blood into the capsule.
- 3★Selective reabsorption: as the filtrate flows along the tubule, glucose, amino acids, salts and much of the water are taken back into the blood.
- 4How much water is taken back depends on how much extra water the body has and how much waste must be dissolved.
- 5The fluid left is urine. It passes from the collecting duct into the ureter.
- 6Urine is stored in the urinary bladder until the pressure gives the urge to pass it out through the urethra.
★ key step
Natural kidney vs artificial kidney
Dialysis is used when infection, injury or poor blood supply stops the kidneys working, so poisonous wastes build up in the body.
Excretion in plants
- Oxygen made in photosynthesis is a waste for the plant and leaves through stomata.
- Extra water is lost by transpiration.
- Many wastes are stored in cell vacuoles.
- Some wastes are stored in leaves that later fall off.
- Wastes such as resins and gums are stored, especially in old xylem.
- Some wastes are given out into the soil around the roots.
How to draw the excretory system and a nephron
- Draw two bean-shaped kidneys, one on each side of the backbone.
- Show the renal artery and renal vein joining each kidney to the aorta and vena cava in the middle.
- From each kidney, draw a thin tube, the ureter, running down to a bag, the urinary bladder; draw the urethra leaving the bladder.
- Nephron: draw a cup, the Bowman's capsule, holding a knot of capillaries, the glomerulus, fed by a branch of the renal artery.
- Draw a long coiled tubule from the capsule to a collecting duct, with capillaries wrapped around the tubule leading to a branch of the renal vein.
Drinking enough water, especially in a hot Indian summer, helps the kidneys flush out urea. Dialysis units in many district hospitals clean the blood of people whose kidneys have failed.
2 marks: “How is the amount of urine produced regulated?” By how much water the tubules reabsorb, which depends on how much extra water is in the body and how much dissolved waste must be removed.
3 marks: “How do plants get rid of their excretory products? Give three ways.” Extra water by transpiration; wastes stored in vacuoles or in leaves that fall; resins and gums in old xylem; some wastes given into the soil.
5 marks: “Draw a labelled diagram of the human excretory system and explain how urine is formed in a nephron.” Labelled kidneys, ureters, bladder, urethra, renal vessels; filtration in glomerulus and Bowman's capsule; selective reabsorption in the tubule; urine to the ureter and bladder.
A nephron in three words: Filter, Take back, Throw out (filtration, reabsorption, urine).