Excretion: MDCAT Biology notes
Excretion for MDCAT: ammonia, urea and uric acid compared, ammonotelic, ureotelic and uricotelic animals, the liver's urea cycle and the urinary pathway.
What is excretion?
Excretion is the removal of metabolic waste from the blood and body. The main wastes are carbon dioxide (from respiration) and nitrogenous wastes formed when excess amino acids are deaminated. Excretion is distinct from osmoregulation (water and salt balance) and egestion (removal of undigested food).
Nitrogenous wastes compared
| Waste | Toxicity | Solubility | Water needed | Animals |
|---|---|---|---|---|
| Ammonia | Highest (most toxic) | Very high | Large amounts | Ammonotelic: most aquatic animals, bony fishes, aquatic invertebrates |
| Urea | Moderate | High | Moderate | Ureotelic: mammals including humans, adult amphibians, cartilaginous fishes |
| Uric acid | Lowest | Lowest (almost insoluble) | Very little; passed as a paste | Uricotelic: birds, reptiles (e.g. snakes), insects |
Ammonia must be diluted with lots of water, which only aquatic animals can afford. Ureotelism is an adaptation to less availability of water: urea is less toxic and can be concentrated. Uric acid saves the most water, which suits egg-laying land animals whose embryos cannot get rid of water.
Role of the liver
The liver is an important excretory organ. It:
- Deaminates excess amino acids, removing the amino group as ammonia.
- Converts toxic ammonia to urea through the urea (ornithine) cycle, which is essentially a detoxification of ammonia: $2NH_3+CO_2\rightarrow CO(NH_2)_2+H_2O$ (overall).
- Forms uric acid from nucleic acid breakdown, and detoxifies many poisons and drugs.
- Breaks down old red cells, excreting bile pigments in bile.
The urea is carried in the blood to the kidneys. Urea is the main nitrogenous waste in human urine; urine also contains small amounts of creatinine, uric acid and ammonia.
Other excretory organs
- Kidneys: filter blood and form urine.
- Lungs: remove carbon dioxide and some water.
- Skin: sweat removes water, salts and a little urea.
Path of urine
Kidney → ureter → urinary bladder (storage) → urethra → outside.
Fish and salt balance
Marine bony fishes live in water saltier than their blood, so they lose water by osmosis. They drink large amounts of seawater, excrete salt actively through their gills and produce small volumes of concentrated urine. Freshwater fishes face the opposite problem: they do not drink and produce large volumes of dilute urine.
Common MDCAT traps
- Ammonia is the most toxic; uric acid the least soluble.
- Humans are ureotelic, not uricotelic, although they excrete some uric acid.
- Snakes and birds are uricotelic; frogs (adults) are ureotelic; freshwater fish are ammonotelic.
- The ureter comes before the bladder; the urethra is the last tube.
- Urea is made in the liver, not in the kidney.
Quick revision
- Excretion = removal of metabolic waste.
- Urea cycle detoxifies ammonia.
- Ureotelism conserves water compared with ammonotelism.
- Uricotelic: birds, reptiles, insects.
- Kidney → ureter → bladder → urethra.