4.9.4Additional lessons
Osmoregulation and Excretion — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 82 of 101
Section 1 of 4Excretion
Definition
- Excretion is the removal from the body of toxic waste products of metabolism, as opposed to egestion of undigested food.
Urea
- The liver breaks down excess amino acids by deamination and converts the toxic ammonia into urea, which is carried in the blood to the kidneys.
Carbon dioxide
- A waste product of respiration that is carried to the lungs and breathed out.
Bile pigments
- Breakdown products of haemoglobin, such as bilirubin, are excreted from the liver in bile.
| Nitrogenous waste | Animals | Reason |
|---|---|---|
| Ammonia | Many aquatic animals | Very soluble, lost with plenty of water |
| Urea | Mammals | Less toxic, dilute solution |
| Uric acid | Birds and insects | Insoluble, saves water |
Section 2 of 4Osmoregulation
Osmoregulation
Definition
- Osmoregulation is the control of the water and solute concentration of the body fluids, so cells neither swell nor shrink.
Kidney roles
- The kidneys excrete urea and also adjust how much water and salt are lost, so they perform both excretion and osmoregulation.
Osmoreceptors
- Cells in the hypothalamus detect the water potential of the blood and trigger the release of the hormone ADH.
Thirst
- Dehydration also stimulates the thirst centre, so behaviour helps the hormonal control.
Water balance
- Water gained from drinking, food and respiration must on average equal water lost in urine, sweat, breath and faeces.
Section 3 of 4Control by ADH
Antidiuretic hormone controls how much water is reabsorbed.
1
When the blood becomes too concentrated, osmoreceptors in the hypothalamus detect the fall in water potential.
2
The hypothalamus stimulates the posterior pituitary gland to release more ADH into the blood.
3
ADH acts on the cells of the collecting duct and makes them insert more aquaporin channels in their membranes.
4
More water is reabsorbed by osmosis into the blood, producing a small volume of concentrated urine.
5
When the blood is too dilute, ADH release falls, fewer aquaporins are present, and a large volume of dilute urine is produced.
| Blood condition | ADH released | Urine produced |
|---|---|---|
| Too concentrated (dehydrated) | More | Small volume, concentrated |
| Too dilute (overhydrated) | Less | Large volume, dilute |
Section 4 of 4Exam Checklist & Tips
4.9.4 – Key Points for the Exam
- Excretion Removal of toxic waste products of metabolism from the body.
- Osmoregulation Control of the water and solute balance of the body fluids.
- ADH A hormone that increases the permeability of the collecting duct to water.
- Deamination Removal of the amino group from an amino acid, forming ammonia.
- Aquaporin A channel protein that allows water to cross a membrane.
Common Mistakes
- Confusing excretion with egestion; faeces are undigested material that never entered cells.
- Saying ADH makes the kidney produce more urine, when high ADH levels cause less urine.
- Naming the pituitary as the receptor; the osmoreceptors are in the hypothalamus.
- Stating that urea is made in the kidney; it is made in the liver.
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