5.1.2 - Excretion

• What is excretion? The removal of metabolic waste products from the body maintaining homeostasis and preventing the accumulation of waste products.
• List four functions of the liver. Breaking down amino acids into urea, detoxification of blood, storage of glycogen, production of bile.
• What is the ornithine cycle? The conversion of excess amino acids by the liver into urea preventing the damage of tissues.
• Recall the four main stages of the ornithine cycle. 1 - Amino acids are deaminated: the amino group is removed forming ammonia and organic acids 2 - The organic acids are respired or stored as glycogen. 3 - Ammonia is converted to a less harmful substance as it is toxic. Ammonia is combined with carbon dioxide to convert into urea in a cyclical reaction. 4 - Urea is released into the bloodstream. It's filtered by the kidneys and excreted within the urine.
• What vessels does the liver contain? Hepatic artery hepatic vein hepatic portal vein bile duct.
• What is the role of the hepatic artery? Delivering oxygenated blood to the liver.
• What is the role of the hepatic vein? Taking deoxygenated blood away from the liver
• What is the role of the bile duct? It takes bile from the liver to the gall bladder where it is stored.
• What is the liver made up of? Liver lobules
• What are liver lobules? Cylindrical structures made up of hepatocytes that radiate out from the central vein.
• How does the central vein connect to the hepatic artery and hepatic portal vein? Via special capillaries known as sinusoids.
• How does the central vein connect to the bile duct? Through tubes called canaliculi.
• As blood moves through the sinusoids hepatocytes.. convert toxic substances into less harmful molecules.
• Kupffer cells attached to the walls of the sinusoids... break down old red blood cells and remove bacteria from the bloodstream
• What is the role of the kidneys? To remove excess water excess ions and urea from the blood as urine
• What are the inner and outer parts of the kidney called? The medulla and the cortex.
• How is blood carried to the kidney? Via the renal artery
• Filtered blood is taken away from the kidneys by... the renal vein
• Nephrons are.. individual structures which filter the blood.
• Recall the process of ultrafiltration. - The Bowman’s capsule surrounds the glomerulus where blood is placed under high pressure. - The efferent arteriole is smaller in diameter than the afferent arteriole so blood inside the glomerulus is under high pressure. - Small molecules (glucose urea water and salts) are pushed out of the bloodstream and into the Bowman’s capsule while larger molecules such as blood cells and protein stay inside the capillaries. - They form a substance called glomerular filtrate which moves through the nephron. - When small molecules pass between the capillary and the kidney they pass through three layers: the capillary endothelium the basement membrane and the epithelium of the Bowman’s capsule.
• What is the glomerulus? A ball of capillaries
• What is the efferent arteriole? It leads away from the glomerulus
• What is the afferent arteriole? It leads towards the glomerulus
• Recall the process of selective reabsorption. - In the proximal convoluted tubule (PCT) loop of Henle and the distal convoluted tubule (DCT) useful substances are reabsorbed passing out of the nephron and back into the capillaries. - Glucose is reabsorbed in the PCT by active transport and facilitated diffusion. The PCT epithelium has microvilli to provide a large surface area for reabsorption. - Water is reabsorbed in the loop of Henle DCT and collecting duct by osmosis. - The remaining filtrate is urine containing excess water excess salts and urea. This passes from the collecting duct to the bladder via the ureter.
• Where does the regulation of water potential mostly take place in? The distal convoluted tubule the loop of Henle and the collecting duct.
• What is the loop of Henle made up of? Two limbs called the ascending and descending limb.
• What is the ascending limb permeable to? It is permeable to ions but impermeable to water.
• What is the descending limb permeable to? Water but not ions
• How do organisms that live in dry conditions adapt? By having an extra long loop of Henle meaning more ions can be pumped into the medulla so more water moves out the nephron by osmosis.
• What is the role of ADH? It controls the water content of urine by increasing the reabsorption of water from the collecting duct.
• How does ADH work? It increases permeability of the collecting duct wall making it more porous and allowing more water to pass from the kidney into the blood stream.
• Recall the negative feedback mechanism when blood water potential drops. - Osmoreceptors in the hypothalamus detect a drop in blood water potential. - The hypothalamus signals to the posterior pituitary gland to secrete ADH. - ADH causes the walls of the DCT and collecting duct to become more permeable to water (by the incorporation of more aquaporins into the membranes of the cells that line these parts of the nephron). - More water moves by osmosis out of the DCT/collecting duct and reabsorbed into the bloodstream increasing its water potential. - A smaller volume of concentrated urine is produced.
• When does kidney failure occur? When the kidneys stop filtering toxic substances out of the blood.
• How can infection lead to kidney failure? It causes inflammation in the kidneys damaging cells meaning the nephron cannot reabsorb substances properly.
• How can high blood pressure lead to kidney failure? It damages the glomeruli meaning large molecules pass into the nephron and into the urine.
• How can kidney failure be diagnosed? By measuring the glomerular filtration rate the rate at which blood is filtered from the Bowman's capsule. A low GFR indicates kidney failure.
• List five issues arising from kidney failure. Build up of waste products, ion imbalance, accumulation of fluids in tissues, anaemia, and mortality.
• How does dialysis work? Blood is passed through a partially permeable membrane surrounded by dialysis fluid causing urea to diffuse into the fluid but glucose water and salts to move between the fluid and the blood to reach normal concentrations.
• What are the two methods of treatment for kidney failure? Dialysis and kidney transplant.
• How do pregnancy tests work? by detecting the hormone human chorionic gonadotropin (hCG) that is found only in the urine of pregnant people.