Excretion in Humans

• Excretion


  A process by which metabolic waste products and toxic materials are removed from an organism's body
• Urea


  A waste product from the deamination of excess amino acids in the liver

  
• Kidney


  • Filters waste products from the blood to prevent the accumulation of the waste products which may be toxic
• Mammalian excretory system


  • Supplied with blood by renal arteries and drained by renal veins
  • Urine exits each kidney through the ureter
  • Both ureters drain into a urinary bladder
  • Urine is expelled through a urethra

  
• Kidney structure


  • An outer renal cortex and an inner renal medulla

  
• Excretion


  A process by which metabolic waste products and toxic materials are removed from an organism's body

  
• Urea


  A waste product from the deamination of excess amino acids in the liver

  
• Kidney


  Filters waste products from the blood to prevent the accumulation of the waste products which may be toxic

  
• Mammalian Excretory System


  • Supplied with blood by renal arteries and drained by renal veins
  • Urine exits each kidney through the ureter
  • Both ureters drain into a urinary bladder
  • Urine is expelled through a urethra

  
• Kidney structure


  • Outer renal cortex
  • Inner renal medulla

  
• Nephron


  Functional unit of the kidney, about 1,000,000 in each human kidney

  
• How blood enters the kidney tubule

  1. Renal artery branches into many arterioles

  2. Blood enters the kidney via the renal artery

  3. Arterioles further divide into a mass of blood capillaries (glomerulus)

  4. Blood capillaries unite to form venules

  5. Venules join to form renal vein

  6. Blood exits the kidney via the renal vein

  7. Blood leaves Malpighian corpuscle and enters tubule

  
• Urine formation - Ultrafiltration


  Water and small molecules (plasma) forced out from blood (due to high pressure in glomerulus) into Bowman's capsule (through basement membrane)
• Ultrafiltration


  • High hydrostatic blood pressure forces plasma (along with solutes) out of the blood
  • Basement membrane (partially permeable membrane) wraps around the glomerular blood capillaries and has small pores that allow water and small molecules to pass through
• Urine formation - Selective reabsorption

  1. Useful substances returned to the blood

  2. Over 80% of filtrate reabsorbed at the proximal convoluted tubule

  3. Glucose, amino acids and salts reabsorbed via diffusion & active transport

  4. Small proteins reabsorbed

  5. 80% of water reabsorbed via osmosis

  
• Urine formation - Selective reabsorption

  1. Loop of Henle: some water and salts reabsorbed from filtrate

  2. Distal convoluted tubule: some water and salts reabsorbed

  3. Collecting duct: water reabsorbed

  4. Water, salts and metabolic waste products pass out of collecting duct into renal pelvis as urine

  
• In healthy people, 100% of glucose is reabsorbed by active transport
• When the maximum threshold is exceeded for people with untreated diabetes, active transport cannot 'keep up' to reabsorb back the glucose, excess glucose is excreted in the urine
• Urine


  95% water, 5% solutes including nitrogenous wastes like urea, uric acid, and creatinine, and other ions
• Abnormally high concentrations of any urinary constituents may indicate disease
• Blood pressure at the glomerulus

  • High pressure forces small molecules through the basement membrane of the glomerular capillaries, and into the Bowman's capsule
• Basement membrane


  • Acts as a filter, allowing small molecules like water, glucose, amino acids, mineral salts and nitrogenous waste products to pass through
• Glomerulus


  • Large molecules like plasma proteins and red blood cells remain in the glomerulus
• Selective reabsorption


  The reabsorption of useful substances from the filtrate
• More than 80% of the filtrate is reabsorbed at the proximal convoluted tubule
• Selective reabsorption


  1. Glucose, amino acids and salts are reabsorbed via diffusion and active transport

  2. Water is reabsorbed via osmosis
• The remaining fluid that passes out of the collecting duct into the renal pelvis forms urine

  
• Urine contains the excess water, excess salts and metabolic waste products such as urea, uric acid and creatinine
• Osmoregulation


  The control of water and solute concentrations (levels) in the blood to maintain a constant water potential in the body
• Water potential of blood plasma

  Needs to be kept relatively constant
• If blood plasma is too concentrated

  RBC dehydrate & undergo crenation
• If blood plasma is too dilute

  RBC would absorb water, expand & burst
• Mechanism of osmoregulation


  1. The amount of water reabsorbed in the kidney tubules is controlled by ADH

  2. ADH secretion by the pituitary gland depends on 'instructions' from the hypothalamus

  3. The hypothalamus has receptors that detect changes in blood water potential

  
• Excess water in the blood

  Less ADH is secreted, less water is reabsorbed, urine is more dilute
• Body is dehydrated


  More ADH is secreted, more water reabsorbed, urine is more concentrated
• Water potential of blood

  Affects blood pressure
• Blood pressure can increase


  Even if blood volume remains the same, if diameter of blood vessels decreases
• Large increase in blood pressure can cause blood vessels in brain to burst, causing a stroke
• Diuretics


  Drugs prescribed to reduce production of ADH, leading to large amount of dilute urine production
• Common causes of kidney failure

  • High blood pressure
  • Diabetes
  • Alcohol abuse
  • Severe accidents that physically damage kidneys
  • Complications from major surgery