5.1.2 Excretion As An Example Of Homeostatic Control Flashcards
Excretion definition
Removal of metabolic waste from the body
Examples of metabolic waste in mammals
CO2: excreted in lungs
Bile pigments: excreted in bile from liver into small intestine via gall bladder and bile duct
Nitrogenous waste: from breakdown of excess amino acids by the liver, excreted by the kidneys in urine (urea, ammonia, uric acid)
How is the liver supplied with blood
Oxygenated blood is supplied via the hepatic artery
Blood is returned to the heart via the hepatic vein
Majority of blood supplied by the hepatic portal vein which contains blood with products of digestion from the intestines
Structure of hepatocytes
Large nuclei
Prominent Golgi apparatus
Many mitochondria
Can divide and replicate
Structure of the liver
Divided into lobules separated by connective tissues, each lobule supplied with blood from hepatic artery and hepatic portal vein
Blood mixed in spaces called sinusoids, lined with endothelial cells
Ensures the hepatocytes are supplied with enough oxygen to carry out processes
Lobules have a central vein which flows into the hepatic vein
Sinusoids contain Kupffer cells, act as macrophages, ingest foreign particles and protect against disease
Hepatocytes secrete bile from blood into canaliculi, drains into bile ducts and into gall bladder to be stored
Role of the liver in carbohydrate metabolism
When insulin levels rise, hepatocytes stimulated to convert glucose to glycogen
Hepatocytes convert glycogen back to glucose due to glucagon
Role of liver in deamination of excess amino acids
Amine group removed from a molecule
First converted to ammonia
Ammonia toxic so converted to urea in the ornithine cycle
Urea toxic in high concentrations so excreted by kidneys
Remainders of amino acids fed into respiration or converted to lipids for storage
Role of liver in protein metabolism
Hepatocytes synthesise plasma proteins
Transamination
Deamination
Role of liver in detoxification
Toxins are detoxified and made harmless
E.g. hydrogen peroxide
Hepatocytes contain catalase, splits hydrogen peroxide into oxygen and water
E.g. ethanol
Hepatocytes contain alcohol dehydrogenase
Ethanol into ethanal, ethanal converted to ethanoate which can be used to build up fatty acids or used in respiration (acetate)
Structure of the kidney
Cortex: dark outer layer, blood is filtered, dense capillary network
Medulla: lighter, contains tubules of nephrons, form pyramids and collecting ducts
Pelvis: urine collects before it passes down ureter
Supplied with blood from the renal artery, blood removed by renal vein
Structure of the nephron
Bowman’s capsule (glomerulus, afferent and efferent arteriole)
PCT
Loop of Henle (ascending and descending limb)
DCT
Collecting duct
Vasa recta
Describe the process of ultrafiltration
Glomerulus is supplied with blood from the wider afferent arteriole and leaves through a narrower efferent arteriole
Creates high pressure in capillaries of the glomerulus
Blood forced out through capillary walls and through basement membrane
Podocytes in wall of the bowman’s capsule acts as a filter, slits prevent bigger molecules getting through
What is contained in the filtrate in the bowman’s capsule
Glucose, salt, urea, water
Adaptations of the cells lining the PCT
Microvilli, surface area for reabsorption increased
Many mitochondria to provide ATP for active transport
Describe the process of reabsorption in the PCT
Na+/glucose co-transport protein in membrane of cells lining PCT, Na+ and glucose binds and diffuses down concentration gradient
Na+ actively transported out of cells and glucose diffuses down concentration gradient
Glucose reabsorbed
Water diffuses by osmosis down concentration gradient
Can then diffuse into capillaries
Describe what happens in the descending limb of the loop of Henle
Filtrate begins isotonic the blood
Upper section of descending limb is impermeable to water
Lower section is permeable to water so water diffuses by osmosis out of filtrate down concentration gradient due to ascending limb’s actions
Water diffuses by osmosis down concentration gradient into vasa recta
Impermeable to NaCl
Fluid in bend is hypertonic to blood
Describe what happens in the ascending limb of the loop of Henle
Lower section of ascending limb is very permeable to NaCl
So Na+ and Cl- diffuses out of concentrated solution by diffusion
Higher section of ascending limb, Na+ and Cl- is actively transported out into medulla tissue against concentration gradient
Ascending limb of loop is impermeable to water
Fluid in ascending limb is dilute, tissue fluid of medulla very concentrated
So filtrate is hypotonic to the blood at the top of the ascending limb
Describe what happens in the DCT
Cells lining DCT have many mitochondria
If body lacks salt, Na+ actively pumped out and Cl- follows down electrochemical gradient
Water can leave if walls are more permeable to water due to increased ADH levels
Balances pH of the blood
Describe what happens in the collecting duct
Water diffuses out by osmosis down a concentration gradient due to concentrated tissue fluid of renal medulla
Urine becomes more concentrated
What is osmoregulation
Controlling water potential of the blood within narrow boundaries
How is ADH released
ADH is produced in the hypothalamus and secreted into posterior pituitary gland where it is stored
When osmoreceptors in the hypothalamus detect a drop in water potential of the blood, pituitary gland is stimulated to release ADH which is carried in the blood to the cells lining the collecting duct
How does ADH carry out osmoregulation (when water potential of blood is low)
ADH binds to receptors on cell surface membrane of tubule cells
Triggers formation fo cAMP (secondary messenger)
cAMP causes vesicles in cells lining collecting duct to fuse with cell surface membrane in contact with tissue fluid of medulla
Vesicle membranes contain aquaporins which make the cell surface membrane more permeable to water
Provides a route for water to move out into tissue fluid of medulla and then to capillaries by osmosis
What can happen if kidneys are infected or damaged
Protein is present in urine, basement membrane or podocytes damaged so dont act as a filter
Blood in urine, filtering process not working
What can happen after kidney failure
Loss of electrolyte balance, osmotic inbalance in tissues, death
Build up of urea, toxic and can poison cells
High blood pressure
Weakened bones
Pain and stiffness, abnormal protein build up in blood
Anaemia, loss of production of hormone that forms RBCs
