Module 5 Excretion

Question 1

Importance of excretion

Answer 1

• Excretion is the process by which toxic waste products of metabolic reactions and substances that are in excess of requirement are removed from the body. Metabolic waste products can have serious negative effects if allowed to build up.
• Metabolic waste examples include CO2, ammonia, urea and bile pigments.
• CO2 is a product of decarboxylation of respiratory substrates and is exhaled from the lungs. An excess of CO2 can cause acidosis as it causes the blood pH to decrease.
• Ammonia is produced by deamination of excess amino acids. It can cause pH to increase and inhibit metabolic processes as well as neurotransmitters in the brain.
• Urea is produced during the ornithine cycle in the liver. Urea will diffuse into cells and cause the water potential to decrease.
• Uric acid can cause crystals to form at joints, causing gout.
• Bile pigments are formed by the breakdown of haem groups in haemoglobin in liver cells. An accumulation of bile pigments causes jaundice.

Question 2

The liver

Answer 2

• The liver requires a constant supply of blood. Oxygenated blood is supplied by the hepatic artery and deoxygenated blood is supplied by the hepatic portal vein. The hepatic portal vein supplies blood from the digestive system which allows the liver to absorb and metabolise nutrients. It is large and branched. Blood leaves the liver by the hepatic vein.
• The liver is directly connected to the gall bladder, which stores bile comprised of bile salts and bile pigment. Bile is released to the duodenum by the bile duct.
• The liver is made up of many hepatocytes. A functional unit of the liver is called a lobule. Lobules are separated by connective tissue and supplied by branches of the hepatic portal vein and hepatic artery. Blood moves in sinusoids, which are wide capillaries with incomplete layers of endothelial cells, that allow the blood to reach the hepatocytes. Hepatocytes have a large surface area to maximise exchange of substances. A central vein is found in each lobule, which allows blood to drain away. Kupffer cells found in sinusoids act as macrophages.
• The functions of the liver include the storage of glycogen, formation of urea, and detoxification.
• Glucose is converted to glycogen through glycogenesis. The regulation of blood glucose concentration is triggered by insulin. Glycogen is stored in hepatocytes as it is compact and insoluble.
• Deamination of amino acids involves the removal of the -NH2 groups. This forms a keto acid and ammonia. Ammonia forms ammonium ions in the cytoplasm and keto acids can be used in respiration in the Krebs cycle or stored as glycogen. Ammonia is very soluble and highly toxic, so it is converted to urea in the ornithine cycle. One molecule of carbon dioxide and two molecules of ammonia form one molecule of urea. Urea is then transported in blood plasma to the kidneys, where it is excreted.
• Detoxification is the breakdown of toxic substances such as lactate, alcohol, hormones and drugs.
• Lactate is the end product of anaerobic respiration. It is energy rich and can be respired. Hepatocytes convert lactate to pyruvate, before it enters mitochondria.
• Alcohol – ethanol is converted to ethanal by alcohol dehydrogenase, which can then be respired or stored. Fats stored in the liver can lead to fatty liver and cirrhosis, which is scarring of the liver. Liver cells also metabolise hormones.

Question 3

The kidneys

Answer 3

• A person has two kidneys. The kidneys are supplied oxygenated blood containing urea by the renal artery and deoxygenated blood without urea is removed from the kidneys by the renal vein. The ureter carrier urine from the kidneys to the bladder, which temporarily stores urine. The urethra releases urine outside the body.
• Kidneys are surrounded by an outer layer called the fibrous capsule.
• The cortex contains the glomerulus, bowman’s capsule, proximal convoluted tubule and distal convoluted tubule. The medulla contains the loop of Henle and collecting ducts. The renal pelvis is where the ureter joins the kidney.
• Nephrons are the functional unit of the kidney. They are comprised of the afferent arteriole, the glomerulus, bowman’s capsule, efferent arteriole, proximal convoluted tubule, loop of Henle, distal convoluted tubule and the collecting duct.
• A network of capillaries surrounds the nephron and carry blood to the renal vein.
• The functions of the kidneys are osmoregulation and excretion.
• Urine is formed by ultrafiltration, where small molecules such as glucose, water, urea, amino acids and inorganic ions move out of the capillaries into the bowman’s capsule to form glomerular filtrate, and selective reabsorption, where useful substances are reabsorbed into the capillaries from the proximal convoluted tubule and filtrate leaves to form urine.
• As the afferent arteriole is wider than the efferent arteriole, a high pressure is created within the capillaries of the glomerulus. This forces out small molecules into the bowman’s capsule. Blood is separated from the capsule by the endothelium of the capillaries, with small caps in-between cells, and the epithelium of the bowman’s capsule that contains podocytes with small caps between them, which are separated by the basement membrane made up of collagen and glycoproteins. This acts as a filter and ensures only small molecules enter the bowman’s capsule.
• The high pressure in the glomerulus creates a high-water potential in the capillaries and a low water potential in the bowman’s capsule. Therefore, water moves down the water potential gradient into the bowman’s capsule. There is a higher concentration of solute in the glomerulus than the bowman’s capsule, as plasma proteins remain in the blood. Therefore, water moves down the water potential gradient into the glomerulus. The pressure gradient is greater than the solute gradient, so there is a net movement of water into the bowman’s capsule.
• Selective reabsorption takes place in the proximal convoluted tubule. Glucose, amino acids, vitamins and inorganic ions are reabsorbed, lowering the water potential of the blood.
• The proximal convoluted tubule has microvilli on the cell surface membrane of lumen cells to increase the surface area for absorption. Co transporter proteins are located on the luminal membrane. Many mitochondria are present for energy for sodium-potassium pumps. Cells are tightly packed to ensure no fluid escapes and blood capillaries are located close to the surface of the PCT.
• Sodium-potassium pumps on the basal membrane of the PCT move sodium ions into the blood, where they are carried away. This lowers the concentration of sodium ions in PCT cells. Sodium ions diffuse from the lumen into PCT epithelial cells, through a co-transporter protein, which allows another solute to be transported at the same time. The solutes then move down their concentration gradient into the capillaries.

Question 4

Osmoregulation

Answer 4

• Osmoreceptors in the hypothalamus monitor the water potential of the blood. If the water potential decreases nerve impulses are sent to the posterior pituitary gland, which releases ADH. ADH is transported in the blood and causes the kidneys to absorb more water, reducing water loss.
• ADH causes more water to be reabsorbed from the filtrate in the collecting duct by osmosis, by making the luminal membrane more permeable to water.
• ADH binds to cell surface receptors on cells of the collecting duct, which causes phosphorylation of aquaporin molecules. Collecting ducts contain vesicles with aquaporins in their membranes. These vesicles fuse with the luminal membrane, increasing permeability.
• The filtrate becomes more concentrated, and a small volume of concentrated urine is produced.

Question 5

Kidney failure

Answer 5

• Kidney failure can be caused by high blood pressure, infections or blood loss.
• If the kidneys fail urea, salts and toxins will be retained in the body and not excreted. Less blood will be filtered so the glomerular filtrate rate will decrease. There will be a build-up of toxins in the blood and the electrolyte balance will be disrupted.
• Excess potassium ions can cause cramps and tiredness. They can also impact the frequency of impulses emitted from the SAN, leading to cardiac arrest. An imbalance of sodium ions can lead to high blood pressure.
• In renal dialysis, toxic metabolic waste products and excess substances are removed from the blood by diffusion, via a dialysis membrane. A kidney transplant will replace a non-functioning kidney.
• Haemodialysis involves regular treatment from a haemodialyser. Partially permeable membranes separate the blood in tubes from the dialysis fluid. The dialysate contains substances in the blood at the correct concentrations but no urea. Urea is removed from the blood, as well as other excess substances. The dialysate is continually refreshed. Heparin is used as an anticoagulant to prevent blood clots from forming.
• Peritoneal dialysis involves the dialysate entering the abdominal cavity through a catheter. Urea diffuses from the blood supply across the peritoneum into the dialysate, which is removed and replaced. Dialysis involves regular trips to the hospital and a restricted diet.
• Kidney transplants only require one kidney to be replaced, from a donor with a compatible blood group. The disadvantages are that the kidney may be rejected due to an immune response, immunosuppressant drugs must be taken that leave the individual vulnerable to infection and that there are not enough donors. Benefits include that patients have more freedom as they don’t need to visit the hospitable multiple times a week, they have a less restrictive diet, it is cheaper and a more long-term solution.

Question 6

Medical Diagnosis

Answer 6

• Urine tests are used to test for glucose, ketones and proteins. Glucose should not be present in the urine as it is completely reabsorbed. Therefore, glucose indicates a problem with the homeostatic control of blood glucose concentration. Ketones are found in the urine of individuals with diabetes. Proteins in urine indicate a kidney infection, or that blood pressure is too high.
• Pregnancy tests contain monoclonal antibodies complimentary to hCG.
• The test should be held under a stream of urine for 5 seconds. hCG binds to mobile monoclonal antibodies which move up the test strip. If hCG is bound to the antibodies, it will attach to the first layer of fixed monoclonal antibodies, that have a coloured bead attached. All monoclonal antibodies will attach to the second layer. One top line = negative. Two lines = positive. One bottom line = false.
• Anabolic steroids can be detected in urine by mass spectrometry and gas chromatography.