Excretion
Definition:
removal from the, body (of)
● poisons / toxins / harmful substances
● waste products of metabolism
● substances in excess (of requirements)
Excretory organs / glands
Products excreted
Skin / Sweat glands
urea and excess water and salts
Liver
Urea, bile pigments, toxins from drugs
Kidneys
urea and excess water and salts
Lungs
carbon dioxide
Deamination
Definition: the removal of the nitrogen-containing part of amino acids to form urea
Process:
• Amino groups of excess amino acids removed and converted to ammonia
• Ammonia is toxic / harmful to the body so converted to urea. This is carried to the
kidneys [by blood] for excretion.
• The rest of the amino acid molecule is converted into carbohydrates and stored or
respired.
• some amino acids are used to make proteins e.g. fibrinogen.
Role of the liver in excretion:
• deamination of amino acids to produce urea which passes into blood.
• breakdown of, hormones / toxins / drugs / excess vitamins.
• breakdown of, worn out red blood cells to form bile pigments.
Role of the kidney:
• Filters [ultrafiltration of] blood plasma in the glomerulus.
• followed by reabsorption of all glucose and most water and salts
• removes / excretes nitrogenous waste; e.g. urea, excess salts, excess water and spent
hormones.
Ultrafiltration:
• Products filtered due to high pressure:
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Created because the afferent arteriole has a wider lumen than the efferent.
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Filtered from: Glomerulus / glomerular capillaries
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Filtered into: Renal capsule / Bowman’s capsule
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Plasma constituents not filtered out: Plasma proteins and blood cells as they are too
big to pass through the pores of the glomerular capillaries.
● Glomerular filtrate contains:
o Water
o Salts /Ions
o Urea
o Glucose
o Toxins, spent hormones etc.
Selective Reabsorption:
• What is reabsorbed?
• All glucose
• Most water
• Most salts
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From where? Lumen of kidney tubule [PCT, Loop of Henle and DCT]
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Into where? Blood in capillaries surrounding the kidney tubule. These capillaries join
the renal vein.
The volume and concentration of urine produced is affected by:
• water intake
• temperature
• exercise
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Dialysis
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Blood taken from the arm of the patient flows into the machine.
• Usually radial vein – easier to find / superficial and has low pressure blood.
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Blood flows into a dialysis tubing, made of partially permeable membrane.
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Dialysis fluid flows around the tubing and contains:
• Glucose, water and salts - same concentration as (should be) in blood
• No urea
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Blood and dialysis fluid move in countercurrent direction – makes diffusion faster.
All urea and excess salts diffuse from blood into dialysis fluid.
Excess water moves by osmosis from blood into dialysis fluid.
No net loss of glucose.
Dialysis fluid is continually refreshed to maintain concentration gradient.
Blood is returned to the vein in the hand of patient.
Disadvantages of dialysis:
• Patients need to visit hospitals multiple times a week for dialysis – time consuming and
inconvenient.
• Patients will have to be on a restricted diet.
• Quality of life affected.
Kidney transplants.
advantages
1. patients do not need to return to clinic for dialysis
2. Patients do not need to eat a restricted diet
3. No long term discomfort / pain.
4. periods of feeling unwell reduced / absent . Or quality of life improved.
disadvantages
1. rejection of kidney
2. finding a compatible donor
3. need, immunosuppressant drugs throughout life
4. risk of death / infection, during / after, the operation
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