CH8 Excretion

• Excretion


  The removal of metabolic waste products and toxic materials from the body of an organism
• When humans undergo metabolic activities, metabolic waste products are produced
• Carbon dioxide is produced when cells respire and urea is produced when amino acids are deaminated
• If left to accumulate within the body, these metabolic waste products can be toxic
• It is crucial for our bodies to be able to remove them through excretion
• Kidneys


  • Filter the blood to form urine and in doing so, removes urea, excess water, and excess mineral salts from the blood stream
  • Play important role in osmoregulation and excretion of urea

  
• Ureter


  • Transports urine from a kidney to the urinary bladder
• Urinary bladder


  • Stores urine temporarily
• Urethra


  • Transports urine from the urinary bladder to the outside of the body
• Urine formation


  1. Ultrafiltration

  2. Selective reabsorption

  3. Role of anti-diuretic hormone (ADH) in osmoregulation

  
• Ultrafiltration


  • The efferent arteriole has a narrower lumen than the afferent arteriole, generating a region of high hydrostatic pressure in the glomerulus
  • This forces small molecules out of the glomerulus into the Bowman's capsule
  • Basement membrane surrounding the glomerulus is partially permeable and acts as a filter
  • Only smaller particles like water, mineral salt ions, glucose, amino acids, and urea pass into the Bowman's capsule to form the glomerular filtrate
  • Red blood cells, white blood cells, platelets, and plasma proteins are too large to pass through the basement membrane, hence remain in the blood stream

  
• Selective reabsorption


  1. In the proximal convoluted tubule, ALL glucose and amino acids molecules are reabsorbed into the surrounding blood capillaries by diffusion and active transport, most of the water molecules are reabsorbed by osmosis, and most of the mineral salts are reabsorbed by diffusion and active transport

  2. In the loop of Henle, distal convoluted tubule, and collecting duct, some water and mineral salts are reabsorbed for final adjustments to concentration of urine, and excess water, mineral salts, and most of the urea passes along the collecting duct to form urine for excretion

  
• Anti-diuretic hormone (ADH)


  Plays a role in osmoregulation
• Mechanism of dialysis in the case of kidney failure

  1. Blood is extracted from an arterio-venous fistula/graft/shunt

  2. Blood pump generates force to pump blood through apparatus of the kidney machine

  3. Heparin pump prevents the clotting of blood in the apparatus

  4. Dialyser dialysis membrane is partially permeable, allowing urea to diffuse from blood to dialysis fluid while retaining blood cells and plasma proteins in the blood stream

  5. Dialysis tubing is long and highly coiled to increase surface area to volume ratio for faster diffusion of urea out of the blood into the dialysis fluid

  6. Direction of flow of blood is opposite of that of dialysis fluid (counter current) to maintain steep concentration gradient of urea between blood and dialysis fluid, allowing faster diffusion of urea from blood to dialysis fluid

  7. Dialysis fluid contains correct concentration of glucose, amino acids, and mineral salts as the blood plasma of a healthy individual, and has no urea, to prevent loss of nutrients from the patient's blood and replenish any nutrient / mineral deficiencies, and to set up steep concentration gradient for urea to diffuse from blood stream into dialysis fluid

  8. Bubble trap removes air bubbles from blood stream to prevent introduction of air bubbles into vascular system of patient, which may cause obstruction of blood flow

  9. Warm water surrounding dialysis tubing warms and maintains temperature of dialysis fluid at about 37°C to prevent loss of heat from patient's blood to dialysis fluid