ZIMSEC O Level · Biology · Form 4
Excretion
Removing waste products of metabolism: carbon dioxide (lungs), urea (kidneys), and excess water and salts.
- Urea is made in the liver from excess amino acids (deamination).
- The kidney's nephrons filter blood under high pressure in the glomerulus (ultrafiltration), then reabsorb glucose, some salts and water. Urine (water, urea, salts) passes to the bladder.
- Kidney failure is treated by dialysis or a transplant.
Homeostasis
Keeping internal conditions constant.
- Temperature control: when hot, sweating, vasodilation of skin arterioles and less shivering; when cold, shivering, vasoconstriction and hairs raised.
- Blood glucose: if high, the pancreas releases insulin and the liver stores glucose as glycogen; if low, glucagon causes glycogen to be broken down. Diabetes is when blood glucose cannot be controlled.
- Negative feedback: a change triggers a response that reverses it.
Exam tip: Vasodilation is the widening of arterioles near the skin – the capillaries themselves do not move.
Check yourself
- Where is urea made?
- Name the hormone that lowers blood glucose.
- Give two ways the body loses heat when hot.
Show answers
- In the liver.
- Insulin.
- Sweating and vasodilation (more blood flows near the skin surface).
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