1. HOMEOSTASIS Excretion, osmoregulation & salt balance in animals
Biology Stage 3
Text: Chapters 13 & 15
Pages: ;

2. Keywords Excretion Osmoregulation Kidneys Nitrogenous wastes Ammonia
Urea
Uric acid
Toxicity
Salt balance
Ion concentration
Water balance
Nephron
Bowman’s capsule
Glomerulus
Convoluting tubules
Loop of Henle
Collecting duct
Ureter
Bladder

3. Internal environment
The internal environment in animals consists of cells, interstitial fluid and blood plasma
These intracellular and extracellular fluids need to be regulated to maintain homeostasis
This requires:
the removal of waste products
the maintenance of water balance.
the regulation of ion and salt concentrations
the control of pH levels

4. Homeostasis
Although we will be looking at excretion, osmoregulation and ion concentration (salt balance) separately, they are in reality closely linked homeostatic processes.
In land animals these three processes are mainly regulated by the kidneys
In aquatic animals, these three processes are regulated by the kidneys and gills

5. Excretion
Chapter 13 Pages

6. Excretion of nitrogenous waste
Excretion usually involves loss of water
Excretion of waste and osmoregulation are closely linked homeostatic processes
The most toxic waste comes from the breakdown of proteins (amino acids) because they contain nitrogen
These toxic nitrogenous wastes must be excreted quickly
Animals excrete nitrogen as ammonia, urea or uric acid

7. Excretion of nitrogenous waste
Most animals will excrete several nitrogenous compounds.
The proportion of ammonia, urea or uric acid excreted in different groups of animals are related to:
Availability of water
Energy cost in producing the waste
Toxicity of the nitrogenous waste
The pattern of development in the species (shelled egg or watery environment)

8. Excretion of nitrogenous waste
Animal
Main nitrogenous waste excreted
Most aquatic animals
Amphibians*
Ammonia
High toxicity
High water solubility
Minimal energy requirement
Mammals
Sharks
Urea
Low toxicity
Moderate energy requirement
Birds
Insects
Many reptiles
Uric acid
Insoluble in water
High energy requirement

9. Excretion of nitrogenous waste
Ammonia can be safely excreted by aquatic animals because it is diluted and diffuses away rapidly in the water
Land animals need to conserve water so they excrete either urea or uric acid.
Excreting uric acid will conserve more water but has a higher energy cost

10. Excretory organs in animals
Kidneys excrete:
Nitrogenous wastes
Toxins
Liver breaks down:
Amino acids
Red blood cells
Lungs excrete:
CO2

11. osmoregulation
Chapter 13
Pages
Chapter 15
Pages

12. Osmoregulation
Maintaining the correct internal water balance is called osmoregulation
Many insects, reptiles, birds and mammals have numerous special adaptations that reduce water loss
The excretory system in land animals is particularly adept at conserving water

13. Osmoregulation in aquatic animals
Marine invertebrates
Most marine invertebrates have body fluids with an osmotic concentration equal to that of sea water.
They therefore do not normally need to expend energy to maintain osmotic balance

14. Osmoregulation in aquatic animals
Cartilaginous fish
Retain high levels of urea in their blood
This raises osmotic pressure so that it is about the same as the osmotic concentration of sea water.
As a result, there is no net loss of water to the environment

15. Osmoregulation in aquatic animals
Marine bony fish
Body fluids are less concentrated than sea water
Maintain water & salt balance by:
Constantly drinking
Producing small amounts of concentrated urine
Actively excreting salts through their gills
Freshwater bony fish
Body fluids are more concentrated than the surrounding water
Maintain water & salt balance by:
Rarely drinking
Excreting large amounts of dilute urine
Actively absorbing salts through their gills

16. Osmoregulation in mammals
The kidneys are the main organs that regulate the level and composition of body fluids in mammals
The functional unit of the kidney is the nephron
It filters waste products out of the blood
Each kidney contains about 1.2 million nephrons

17. The nephron Glomerulus Bowman’s capsule Distal convoluting tubule
Proximal convoluting tubule
Blood vessels
Loop of Henle
Collecting duct

18. Renal cortex (Bowman’s capsules)
The kidney
Renal medulla
(loops of Henle)
Renal artery
(blood in)
Renal vein
(blood out)
Renal cortex (Bowman’s capsules)
Ureter
(urine to the bladder)

19. Blood enters the glomerulus (Bowman’s capsule) under high pressure.
Water and small molecules are forced out of the blood vessels.
This filtrate is collected and passes through the convoluting tubules and the loop of Henle
Water and salts are reabsorbed
The rest passes into the collection tubule and through to the ureters
The urine is stored in the bladder for excretion
The kidneys
Renal cortex
Renal medulla

20. Adaptations that conserve water
Kidney
The length of the loops of Henle may vary
In animals adapted for dry, hot conditions, the loops of Henle are much longer
This increases the amount of water and salts that can be reabsorbed, thus conserving water and regulating salt balance more efficiently

21. YouTube clip Osmoregulation (Bozeman Science 9 min)
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The kidneys (6 min)
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