Removing wastes- Cell metabolism
- Metabolism: chemical reactions taking place in cells.
- Metabolic reactions break down compounds to form new ones.
- Some compounds produced are wate substances and are harmful if not removed from the body.
- These waste substances build up in the blood.
- Excretion: the removal of these waste substances from the body.
- Carbon dioxide and water are waste products of aerobic respiration.
- Urea is a waste product produced in the liver when excess amino acids are broken down.
- Carbon dioxide is excreted by gas exchange in the lungs.
- Water and urea are excreted in urine.
- In humans and other mammals, the kidney is the main excretory organ.
- The kidney also helps homeostasis by regulating the amount of water and the levels of salts and other substances in the blood.
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Removing wastes- The urinary system
- Urea produced by liver cells- removed in urine produced by the kidneys.
- It is temporarily stored in the bladder before being released from the body.
- Water and salts- needed for cells to work.
- Surplus removed in urine.
- Salts can also be lost in sweat.
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Removing wastes- Nephrons
- Each kidney consists of about a million nephrons.
- Their functions are filtration of soluble wastes from the blood and reabsorption of useful substances back into the blood.
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- Presence of proteins in urine- a symptom of kidney failure.
- Other symptoms: drowsiness, nausea, swollen feet.
- Short-term kidney failure can be caused by infection or blockage of the ureter.
- Can normally be treated by antibiotics or surgery.
- Long-term has many causes including: effects of unregulated diabetes, high blood pressure, tumours in abdomen.
- Amount of urea produced in liver reduced by controlling protein consumption.
- Treated by dialysis (below) or organ donation.
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Regulating water content
- Pituitary gland- produces hormones including ADH which controls the reabsorption of water back into the blood.
- Receptor cells in the hypothalamus of the brain detect the amount of water in the blood. Nerve impulses from receptor cells pass to pituitary gland which releases more or less ADH. It is negative feedback.
- Too much water- less ADH released, walls of collecting ducts become less permeable, less water reabsorbed, more water removed in urine.
- Not enough water- more ADH released, walls of collecting ducts become more permeable, more water reabsorbed, less water removed in urine.
- Water content fluctuates and self-adjusts around a normal value.
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The Menstrual Cycle
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The Menstrual Cycle 
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