Homeostasis and the kidney

• Mammalian tissue


  • Essentially made up of a collection of cells bathed in a fluid medium or 'extracellular' fluid (tissue fluid)
  • The composition of this fluid (and consequentially the blood due to the permeable nature of the capillary walls) must be kept constant in terms of factors such as water and lon content, temperature, pH and oxygen levels, irrespective of the external conditions outside the body
• Homeostatic responses


  • Have three basic features:
  • A control system with sensors (receptors) which provides information allowing the monitoring of the factor being controlled
  • If the receptors show a departure from normal levels (the set point) for the factor being controlled, a corrective mechanism brings about the changes required to return the factor to its normal level
  • The corrective mechanism involves a negative feedback system
• Negative feedback


  1. The return of the factor being controlled to its normal level (set point) causes the corrective measures to be turned off

  2. This prevents over-correction
• Communication in homeostasis


  • Can be by nervous or hormonal pathways between the sensors/receptors and the monitor, and between the monitor and the effectors that bring about the corrective response
• Homeostasis


  The maintenance of constant or steady state conditions within the body
• Homeostatic responses


  • Have a control system with sensors (receptors) which provides information allowing the monitoring of the factor being controlled
  • If the receptors show a departure from normal levels (the set point) for the factor being controlled, a corrective mechanism brings about the changes required to return the factor to its normal level
  • The corrective mechanism involves a negative feedback system
• Negative feedback


  The return of the factor being controlled to its normal level (set point) causes the corrective measures to be turned off, preventing over-correction
• Communication between the sensors/receptors and the monitor (and between the monitor and the effectors that bring about the corrective response) can be by nervous or hormonal control
• Homeostatic control of mammalian body systems is essential for providing the optimum conditions for enzyme reactions and avoiding osmotic problems in cells and in body fluids
• Many other animals have simpler homeostatic controls that are less able to keep the internal environment constant, so they avoid large swings in body conditions by living in a relatively constant external environment
• Kidney


  A major homeostatic organ in mammals with two very important functions: excretion and osmoregulation
• Excretion


  • The removal of the toxic waste of metabolism, mainly urea and creatinine
• Osmoregulation


  • The control of the water potential of body fluids through controlling both the volume and concentration of urine produced
• The urinary (excretory) system includes the kidneys, ureters, bladder and urethra
• Kidney function


  1. Ultrafiltration - the filtration of plasma and substances below a certain size into the Bowman's capsule

  2. Reabsorption - the selective reabsorption of useful products back to the bloodstream from the nephron
• Ultrafiltration


  • Occurs due to the high hydrostatic pressure in the glomerular capillaries, aided by the structure of the capillary walls and Bowman's capsule lining
  • The basement membrane acts as a molecular sieve, allowing small molecules to pass through but retaining blood cells and plasma proteins
• The glomerular filtrate is similar to blood except for the plasma proteins and blood cells that are too large to penetrate the basement membrane
• For filtration to occur

  The water potential within the glomerular capillaries (blood plasma) must exceed the water potential within the Bowman's capsule (glomerular filtrate)
• The difference in water potential is due to the much greater hydrostatic pressure in the blood compared to the filtrate, and the lower solute potential in the filtrate due to the absence of plasma proteins
• The solute potential is represented by the plasma proteins, as there are plasma proteins in the blood in the glomerular capillaries but not in the filtrate
• The filtrate has a less negative solute potential than the blood in the glomerulus
• Although the difference in solute potential opposes filtration, this effect is insignificant when compared to the differences in hydrostatic pressure across the basement membrane
• The water potential of the blood plasma in the glomerulus is higher (more positive or less negative) (20kPa) compared to the water potential of the glomerular filtrate in the Bowman's capsule (0.7kPa), therefore producing the net filtration force or pressure
• Useful blood products temporarily lost to the glomerular filtrate are reabsorbed back into the blood, mainly as the filtrate passes along the proximal convoluted tubule
• Substances reabsorbed from the glomerular filtrate

  • Glucose
  • Amino acids
  • Some salts
• Glucose, amino acids and some salts are actively reabsorbed into the blood, creating an osmotic effect that causes over 70% of the water in the filtrate to re-enter the blood capillaries passively by osmosis
• Small plasma proteins which may have passed through the basement membrane in the glomerular filtrate are reabsorbed by pinocytosis
• Substances filtered into the nephron and subsequently reabsorbed in the proximal tubule


  • Large plasma proteins (100% filtered, 0% reabsorbed)
  • Glucose (100% filtered, 100% reabsorbed)
  • Amino acids (100% filtered, 100% reabsorbed)
  • Urea (100% filtered, <50% reabsorbed by diffusion)
• The epithelial cells of the proximal convoluted tubule have high levels of metabolic activity and continually carry out energy demanding processes such as active transport
• By the time the filtrate reaches the end of the proximal tubule, it will have no glucose or amino acids present as they all will have been reabsorbed
• Although some urea diffuses back into the blood by diffusion along the length of the proximal convoluted tubule, the concentration of urea in the filtrate increases along its length due to the reabsorption of water
• At the end of the proximal convoluted tubule, the filtrate is isotonic with the blood plasma
• Epithelial cells lining the proximal tubule

  • Cell surface membrane contains protein carrier molecules for selective reabsorption by facilitated diffusion and active transport
  • Microvilli increase surface area for reabsorption
  • Nucleus
  • Mitochondria provide ATP for active transport
  • Infolding of membrane further increases surface area
  • Capillaries lie close to cells lining the proximal tubule
• Further regulation of blood composition takes place in the distal convoluted tubule
• The pH and ionic composition of the blood in the capillaries surrounding the distal tubule are adjusted and some toxic substances, for example, creatinine, are secreted from the blood into the filtrate for disposal
• Osmoregulation


  A homeostatic process that controls water balance in the body by controlling water balance in the blood
• The collecting duct is where the water regulation takes place
• Although most water is reabsorbed in the proximal convoluted tubule (and some from the descending limb of the loop of Henle), the process is passive and the exact amount of water reabsorbed back into the blood cannot be controlled
• Reabsorption in the collecting ducts can be controlled by varying the permeability of the collecting duct walls, which is where the fine control of water balance takes place
• Antidiuretic hormone (ADH)


  Crucial in controlling the degree of permeability of the collecting duct walls