5.1.2 Excretion

• Metabolic reactions create waste products which become toxic if they are not removed. The removal of these products is known as excretion.
• Carbon dioxide is a waste product from respiration and is excreted by the lungs.
• Nitrogenous waste (eg urea) is created from excess amino acids in the diet. Unlike glucose, this can not be stored and so are broken down by the liver to ammonia, then urea. This is excreted from the kidneys.

  • Mammals = Urea as nitrogenous waste.
  • Fish = Ammonia.
  • Birds = Uric acid.
• The liver is a large organ that is located under the right rib cage. It is a glycogen storage unit that also detoxifies and forms urea.

  • Hepatic artery - Oxygenated blood to liver.
  • Hepatic vein - Blood leaves liver.
  • Hepatic portal vein - Blood from digestive system to the liver.
• Hepatocytes are the liver cells and they contain many mitochondria, large nuclei and prominent Golgi apparatus, all enabling for a high metabolic rate.
• Structure and histology of the liver:

  • Blood is delivered from the hepatic portal vein and hepatic artery. This mixes with the sinusoids (spaces surrounded by hepatocytes). This is oxygenated blood which mixes with deoxygenated blood from hepatic portal vein.
  • Kupffer cells in the sinusoids are like macrophages (engulf foreign particles).
  • Hepatocytes produce bile using products from breakdown of old blood and blood cells. Bile is first secreted into the canaliculi, then passes through bile ducts in the gall bladder where it is stored.
• Function of the liver:

  • Hepatocytes - Response to insulin = Absorb excess glucose from blood and convert it to glycogen. Response to glucagon = Hydrolyse glycogen into glucose and releases it in the blood.
  • Detoxification - Neutralisation and breakdown of unwanted chemicals (eg alcohol, hormones and toxins made in chemical reactions in body). Enzymes break these down into non toxic substances.
• Function of the liver:

  • Ornithine cycle - Urea is produced from ammonia and transported to kidneys for excretion. Excess proteins from diet which can not be stored are delivered to the liver for deamination (removal of amine group, converting it to ammonia). Ammonia is highly toxic so it is converted to urea before being transported in the blood. Urea is also toxic but only in high concentrations.
• Mammalian kidney is responsible for excretion of nitrogenous waste and osmoregulation (controlling water potential of the blood).

  • Renal artery - Unfiltered blood to kidney.
  • Renal vein - Filtered blood away from kidney.
• Structure of kidney:

  A) pelvis

  B) medulla

  C) cortex

  D) renal artery

  E) renal vein

  F) ureter

  G) calyces

  
• Structure of kidney:

  • Cortex - Dark outer layer containing many capillary networks (blood from renal arteries to nephrons).
  • Medulla - Contains nephrons.
  • Pelvis - Storage of urine before leaving the kidney and travelling to ureter.
• Nephrons are structures within the kidney where blood is filtered and useful substances are selectively reabsorbed back into the blood.
• Structure of nephron:

  A) glomerulus

  B) proximal convoluted tubule

  C) descending loop of henle

  D) ascending loop of henle

  E) distal tubule

  F) collecting duct

  
• Filtration and reabsorption:

  • Bowman's (renal) capsule - Ultrafiltration occurs here.
  • Proximal convoluted tubule (PCT) - Glucose reabsorption.
  • Loop of Henle - Na+ actively transported out of ascending limb to medulla, creating low water potential.
  • Distal tubule - Water moves out of descending limb to tubule and down collecting duct by osmosis.
  • Collecting duct - Remaining liquid forms urine (contains water, dissolved salts, urea and other substances like hormones).
• Structure of the liver:

  A) hepatocyte

  B) sinusoid

  C) kupffer cell

  D) bile duct

  E) hepatic arteriole

  F) portal venule

  
• Functions of the kidney:

  • Ultrafiltration - Blood enters through afferent arterioles and splits into smaller capillaries (make up glomerulus). This causes high hydrostatic pressure of blood. Water and small molecules are forced out of capillaries and form glomerulus filtrate.
  • Filtrate passes through basement membrane (collagen network and proteins). This and capillary act as a sieve.
  • Bowman's capsule wall has podocytes (specialised filter cells). Large proteins and blood cells can't fit through gaps in capillary endothelium so it stays in the blood and leave by efferent arteriole.
• Selective reabsorption:

  • Occurs in PCT. Most of glomerulus filtrate is reabsorbed back into blood, leaving urea and excess minerals behind.
  • Concentration of Na+ in PCT cell is decreased as they are actively transported out and into the blood in capillaries.
  • Due to the concentration gradient, Na+ diffuse down the gradient from the lumen of PCT to the cells lining it. This is cotransport (proteins that carry Na+ also carry glucose). Glucose diffuse from PCT epithelial cell back into blood stream.
• Maintaining a Na+ gradient by Loop of Henle:

  • Filtrate passes into loop. Na+ gradient enables reabsorption (this is maintained in the medulla),
  • Mitochondria in walls of the cell provide ATP to actively transport Na+ out of ascending limb.
  • Accumulation of Na+ outside nephron lowers water potential.
  • Water diffuses out by osmosis into interstitial space then back into blood capillaries.
  • At base of ascending limb, Na+ is transported out by diffusion as there is a dilute solution due to all the water that has been moved out.
• Reabsorption of water at the DCT and collecting duct:

  • Because of all the Na+ being transported out of the loop, when filtrate reaches the top it is very dilute.
  • Filtrate moves to DCT and collecting duct (this section of medulla is very concentrated).
  • Therefore, more water diffuses out of DCT and duct. Whatever remains is transported to form urine.
• Hypothalamus is where changes in water potential of the blood is detected, with the help of osmoreceptors.

  • Too low - Water leaves osmoreceptors by osmosis and they shrivel. This stimulates hypothalamus to produce more ADH.
  • Too high - Water enters osmoreceptors by osmosis. This stimulates hypothalamus to produce less ADH.
• ADH is produced in the hypothalamus which then moves to the posterior pituitary and is released into capillaries in the blood.
• ADH travels through the blood to its target organ, the kidney.
• ADH - Antidiuretic hormone

  • ADH binds to complementary receptors located on target cells in the DCT and collecting duct.
  • After binding, it activates adenyl cyclase to make cAMP.
  • This activates an enzyme which causes vesicles containing aquaporins to fuse with membrane.
  • Therefore, membrane becomes more permeable to water and more will leave to be reabsorbed back into blood.
  • Permeability increases in collecting duct and DCT so more water leaves the nephron and is reabsorbed back into blood, so urine is more concentrated.
• Decreased water potential of blood -> Detected by osmoreceptors in hypothalamus -> Release of ADH which is transported to posterior pituitary and released in blood -> DCT and collecting duct walls more permeable to water -> More water reabsorbed into the blood and less is lost in urine.
• Urine in diagnosis:

  • Can be used to test for diabetes, pregnancy, anabolic steroids and drugs.
• Urine in diagnosis:

  Pregnancy tests use monoclonal antibodies to detect presence of the human growth hormone. (Monoclonal antibody is a single type of antibody that can be isolated and cloned).

  1. Absorbent end of test is submerged in urine.
  2. First mobile antibody (complementary to human growth hormone) has a coloured dye attached.
  3. Second antibody has complementary shape to antigen is immobilised.
  4. Third antibody is immobilised and complimentary in shape to first antibody
• Kidney failure is a condition where the kidneys are unable to function properly.

  • Kidney infections and high bp can damage tubules, podocytes, epithelial cells and basement membrane of Bowman's capsule. This would result in large molecules filtering out the blood.
  • Kidney fails = Blood will not be filtered properly, leading to a build up of urea and an electrolyte imbalance.
  • Glomerular filtration rate (GFR) is affected by failure and can be measured to indicate disease. (Blood tested for creatinine levels - creatinine is the breakdown product of muscles. High = kidneys not filtering properly)