5.1.2 - excretion

• Excretion is removing metabolic waste products from the body
• excretion enables organisms to maintain pH balance and regulate osmotic pressure
• The liver structure:

  • liver lobules: cylinders of hepatocytes arranged in rows and connected at the centre. These are connected to…
  • Hepatic vein which takes deoxygenated blood away from the liver and is attached to
  • hepatic portal vein which contains products of digestion
  • hepatic artery supplies oxygenated blood via the sinusoid capillaries.
  • Bile duct transports bile to gall bladder for storage
• the functions of the liver are

  • site of gluconeogenesis, glycolysis, glycogenesis
  • stores glycogen
  • deaminates excess amino acids forming ammonia and keto acid, which are then detoxified by adding CO2 in ornithine cycle to make urea
  • detoxifies chemicals, converts alcohol to ethanal then acetic acid.
• The excretory organs are:

  the lungs

  the liver

  the kidneys

  the skin
• substances that are excreted are:

  carbon dioxide from respiration

  nitrogen containing compounds such as urea

  bile pigments found in faeces
• The lungs remove carbon dioxide when you breathe out
• The liver converts excess amino acids to urea.
• the kidneys remove urea from the blood to form the urine
• excretion is important as it products are toxic and can interfere with cell processes and enzyme activity.
• excess amino acids are transported to the liver and the amine group are removed by deamination.

  amino acid + oxygen ——> keto acid + ammonia
• once ammonia is formed from deamination it reacts with CO2 to form urea and to release its energy.

  ammonia + CO2 ——> urea + water
• The gross structure of the mammalian urinary system is kidney

  renal vein

  renal Artery

  ureter

  urethra

  bladder
• The gross structure of the kidney consists of:

  fibrous capsule: protects the kidney

  cortex: outer region consists of bowman’s capsules, convoluted tubules and blood vessels

  medulla: inner region consists of collecting ducts, loops of henle, blood vessels.
• Nephron consists of:

  glomerulus

  afferent arteriole

  efferent arteriole

  Bowmans capsule

  proximal convoluted tubule

  Loop of Henle

  distal convoluted tubule

  collecting duct
• The blood vessels associated with the nephron are

  wide afferent arteriole from renal artery enters renal capsule and forms glomerulus. Branched knot of capillaries which combine to form narrow efferent arteriole. Efferent arteriole branches to form capillary network that surrounds tubules.
• The Bowmans capsule is at the start of the nephron. It is cup shaped and surrounds the glomerulus. It has an inner layer of podocyte cells
• Proximal convoluted tubule (PCT) is a series of loops surrounded by capillaries and the walls are made of epithelial cells with microvilli.
• Loop of Henle is a hairpin loop that extends from the Cortex into the medulla
• Distal convoluted tubule (DCT) is a series of loops surrounded by fewer capillaries than PCT with walls made of epithelial cells.
• Collecting duct is when the DCT from several nephrons empty into collecting duct which leads into the pelvis of the kidney.
• Ultrafiltration occurs in the Bowmans capsule
• Ultrafiltration starts with a high hydrostatic pressure in the glomerulus due to the narrowing efferent arteriole which forces small molecules (urea, water, glucose, mineral ions) out of capillary fenestrations against the osmotic gradient.

  Basement membrane acts as a filter. Blood cells and large molecules will remain in the capillary.
• Cells of the bowmans capsule are adapted for ultrafiltration as there are fenestrations between epithelial cells of capillaries. Fluid can pass between and under folded membrane of podocytes.
• Selective reabsorption occurs in the proximal convoluted tubule
• selective reabsorption is when useful molecules from the glomerular filtrate are reabsorbed into the blood.

  Eg, glucose
• Processes in selective reabsorption:

  glucose from glomerular filtrate ——> cotransport with Na+ ions ——> cells lining proximal convoluted tubule ——> active transport ——> intercellular spaces ——> diffusion ——> blood capillary lining tubule
• The kidney produces urine as after selective reabsorption the filtrate passes through the loop of henle which acts as a countercurrent multiplier. Then through the distal convoluted tubule, where water and mineral ions are reabsorbed. More water is then reabsorbed in the collecting duct and the remaining fluid (urine) contains only waste materials and water.
• In the loop of Henle:

  1. Active transport of Na+ and Cl- out of ascending limb
  2. water potential of interstitial fluid decreases
  3. osmosis of water out of descending limb (ascending limb is impermeable to water)
  4. water potential of filtrate decreases going down descending limb: lowest in medullary region highest at top of ascending limb
• The distal convoluted tubule is involved in reabsorption of:

  water via osmosis

  ions via active transport

  The permeability of walls is determined by action of hormones
• Osmoregulation is the control of plasma water potential via negative feedback homeostatic mechanisms.
• Hypothalamus is involved in osmoregulation as osmosis of water out of Osmoreceptors in hypothalamus causes them to shrink. This triggers hypothalamus to produce more antidiuretic hormone (ADH)
• posterior Pituitary gland is involved in osmoregulation as it stores and secretes the ADH produced by the hypothalamus
• ADH in osmoregulation forms hormone receptor complex on surface membrane of cells in collecting duct. This triggers activation of cAMP as secondary messenger. Triggered cellular processes that increase reabsorption of water. So urine becomes more concentrated.
• More ADH released = more concentrated urine

  less ADH released = more dilute urine
• ADH increases reabsorption of water as it makes cells lining collecting duct more permeable to water.

  Binds to receptor - activates phosphorylase - vesicles with aquaporins on membrane fuse with cell surface membrane. Makes cells lining collecting duct more permeable to urea: water potential in interstitial fluid decreases
• Causes of kidney failure:

  kidney infections cause inflammatory damage = change in GFR

  kidney stones

  uncontrolled diabetes

  high blood pressure damages capillaries = large molecules pass into urine
• The effects of kidney failure are:

  • build up of toxic waste products eg, urea
  • fluid accumulation which then leads to swelling
  • disruption to electrolyte balance - bones are more brittle
• Kidney failure is generally treated by either renal dialysis or kidney transplant
• renal dialysis can either be haemodialysis or peritoneal dialysis