excretion

• excretion is removal of metabolic waste from the body
• why do we have to excrete ?

  • prevent from becoming toxic and changing pH
  • maintain homeostasis
  • prevent end-product inhibition from stopping pathways
• Our bodies excretes nitrogenous waste, bile pigments and carbon dioxide
• nitrogenous waste is when excess amino acids cannot be stored so have to be broken down as ammonia in fish, ammonia then urea in mammals and uric acid in birds/insects
• bile pigments are formed when haemoglobin from old erythrocytes is broken down by hepatocytes and then excreted in bile
• too much carbon dioxide can cause pH to decrease and cause respiratory acidosis
• too little carbon dioxide causes pH to increase and cause alkalosis
• the liver is divided into 2 lobes and which then divided into 100,00 lobules
• each lobule has 6 sides and has cylindrical in shape. Hepatocytes radiate from the central vein
• hepatocyte

  • microvilli increase surface area which increases the toxins that can enter
  • big nucleus
  • lots of mitochondria so more ATP for reactions
  • dense cytoplasm with increased Golgi apparatus and smooth endoplasmic reticulum
• flow through liver

  1. oxygen rich blood enters liver via hepatic artery
  2. capillary bed brings in nutrients and potential toxins
  3. intestinal vein brings in deoxygenated blood and travels up hepatic portal vein
  4. oxygen and deoxygenated blood mix in sinusoids
  5. flows through central vein and then leaves liver back to the heart
  6. bile canaliculi carry bile out of liver
• kupffer cells line the sinusoids and resident to macrophages that breakdown bacteria and old red blood cells
• why is the liver so important ?

  • breaks down old red blood cells and haemoglobin
  • stores vitamins A,D and B12
  • breaks down lactic acid
  • breaks down toxins
  • controls blood glucose levels
  • breaks down excess amino acids
• when blood glucose levels are high liver cells take up excess glucose and converts to glycogen (glycogenesis)
• when blood glucose levels are low, liver cells breakdown glycogen into glucose and release into the blood stream (glycogenolysis)
• excess amino acids can either be transanimated or broken down in two ways :

  • deamination
  • ornithine cycle
• deamination is combining amino acids with oxygen to produce keto acid and ammonia
• ornithine cycle converts ammonia into urea in 4 steps

  1. ornithine +ammonia +carbon dioxide = citruline
  2. citruline + ammonia = arginine
  3. arginine + water = urea +ornithine
• ultrafiltration the filtration of small molecules on molecular level. Anything small enough to leave the glomerular capillaries will enter the Bowmans capsule
• ultrafiltration goes from afferent arteriole into efferent arteriole
• What gets out of ultrafiltration ?

  • hormones
  • water
  • urea
  • electrolytes
  • glucose
  • amino acids
  • drugs
  • vitamins
• what can't get out of ultrafiltration ?

  • red blood cells
  • white blood cells
  • platelets
  • plasma proteins
• ultrafiltration works in two ways ; bottleneck effect and using a 3 layer system
• Bottleneck effect is when it forces the solutes out of the blood because the diameter of the afferent arteriole is larger than the efferent arteriole
• the 3 layers in ultrafiltration are ; endothelium of the glomerurler capillaries, basement membrane and the epithelium of the Bowmans capsule.
• basement membrane is made of collagen fibres and glycoproteins forming molecular mash. Only allows molecules of a RMM >69,000
• pedocils keep podocytes away from the capillaries to create gap so solutes can get through and holds adjacent podocytes together
• selective reabsorption is where useful substances are reabsorbed from glomerular filtrate back into the blood at the proximal convoluted tube
• 85 % of the filtrate is reabsorbed into the blood including :

  • glucose
  • most amino acids
  • 60 % of water
  • some sodium, chloride, potassium
  • most hormones
  • 50 % of urea
• adaptations to the proximal convoluted tube

  • convoluted for a longer time to reabsorb
  • microvilli to increase surface area
  • may mitochondria for active transport
• selective reabsorption

  1. sodium and potassium pumps pump sodium from cell to blood and potassium from blood to cell
  2. sodium diffused into cell via a co-transport protein that takes glucose and amino acids with it
  3. glucose and amino acids move from cell into the blood via facilitated diffusion
  4. water diffuses into cell via osmosis
  5. as the water leaves the proximal convoluted tube, concentration of chloride and urea will increase so chloride and urea can diffuse into the blood
• loop of henule is a countercurrent multiplier system. Its purpose is to multiply the effect of osmosis by maintaining a water potential gradient allowing water to leave the nephron and re-enter the blood
• filtrate from the PCT includes 40 % of water, sodium, chloride, potassium and urea
• descending limb is permeable to water, urea, sodium and chloride
• ascending limp is permeable to sodium and chloride by active transport and urea. But its not permeable to water
• Flow through loop of Henle

  1. Sodium and chloride are pumped out of the ascending limb

  2. Decreases the water potential of the medulla

  3. Water leaves the descending limb by osmosis

  4. Some sodium and chloride will diffuse into the descending limb

  5. As filtrate moves down descending limb, water is moving out and sodium and chloride entering so the concentration of sodium and chloride will increase

  6. Some sodium and chloride diffuses out of bottom of descending limb making it more salty at the bottom and maintaining the water potential gradient so water can keep moving out

  7. As filtrate moves up the ascending limb, water can not get out but sodium and chloride are being pumped out so at the top of the ascending limb it has a high water potential

  8. As filtrate moves down the collecting duct, water leaves by osmosis

  9. As filtrate moves along the collecting duct, water leaves so concentration of urea increase so some urea diffuses out into blood and further decrease the water potential of medulla so even more water can enter medulla/blood
• collecting duct is permeable to water and urea but not permeable to sodium and chloride
• ADH is an antidiuretic hormone
• osmoregulation is achieved by adjusting the amount of ADH in the blood
• ADH is released when your dehydrated and increases the permeability of the collecting duct so more water is reabsorbed into blood