A-LEVEL BIOLOGY OCR NOTES
excretion as an example of homeostatic control
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The Role of the Kidneys in Osmoregulation
- Osmoregulation is maintaining a constant water potential of the blood, despite changes in the level of water and salt intake.
- The kidneys are made if nephrons which help filter the blood. The blood undergoes ultrafiltration at the glomerulus due to the smaller diameter of the efferent arteriole than the afferent arteriole, creating high hydrostatic pressure.
- The filtrate passes into the Bowman’s capsule and travels around the entire nephron, where certain ions and water are reabsorbed into the blood whilst the remaining filtrate is excreted as urine.
- Sodium is actively transported out of the proximal convoluted tubule and into the blood
- Glucose & amino acids are co-transported out of the proximal convoluted tubule via sodium ions diffusing into the epithelial cells.
The Role of the Hypothalamus in Osmoregulation
- The hypothalamus contains osmoreceptors which signal to specialised neurosecretory cells. A fall in water potential causes the release of antidiuretic hormone (ADH) from the posterior pituitary gland.
- ADH travels in the blood to the kidneys, attaching to ADH receptors, activating the intracellular enzyme phosphorylase. This causes vesicles containing aquaporins to fuse with the plasma membrane, reducing water loss by increasing the permeability of the collecting duct and distal convoluted tubule.
Excretion & The Liver
- Excretion is the removal of metabolic waste products from the body
- The main waste products are carbon dioxide, bile pigments and nitrogen containing compounds
- Amino acids are deaminated, so the keto acid can be used in respiration, while the toxic amino group can be excreted as urea
- The liver has three main blood vessels
- Hepatic artery- carries oxygenated blood from the heart to the liver
- Hepatic portal vein carries deoxygenated blood from the small intestine to the liver
- Hepatic vein carries deoxygenated blood away from the liver to the heart
- Hepatocytes remove excess substances and wastes, and secrete substances back into the blood to maintain their concentration within it, while it travels down the sinusoids
- Bile travels down the bile canaliculi to the gall bladder.
- Kupffer cells are present within the sinusoids to protect the liver from disease and break down red blood cells.
- Other liver functions include:
- Glycogen storage
- Detoxification e.g. catalase breaks down hydrogen peroxide to water and oxygen. Cytochrome P450 breaks down drugs
- Detoxification of alcohol
- Excessive alcohol consumption can lead to cirrhosis as NAD is used to detoxify alcohol instead of breaking down fatty acids, which are stored as lipids in hepatocytes.
- Kidney failure hypertension, diabetes, infection or heart disease
- Kidney failure can be assessed by measuring the glomerular filtration rate, which is the volume (cm3) of fluid passing into the nephrons every minute.
- One treatment is renal dialysis, which is a mechanism utilised to artificially regulate the concentrations of solutes in the blood
- Haemodialysis- blood is removed and passed through a machine containing an artificial dialysis membrane, which allows countercurrent substance exchange. Heparin stops blood clots.,
- Peritoneal dialysis- dialysis fluid is inserted into the abdomen; exchange occurs across the peritoneum.
- Kidney transplant can treat kidney failure but requires the use of immunosuppressants because of the risk of rejection.
- Pregnancy tests work by detecting the hormone human chorionic gonadotrophin (hCG) in urine.
- Coloured monoclonal antibodies complementary to hCG form a complex. Which moves up the strip and binds to immobilised antibodies complementary to hCG resulting in the presence of a blue line, indicating pregnancy.