Homeostasis + the kidneys

• Homeostasis


  The maintenance of a constant internal environment within a living organism, irrespective of the external conditions.
• Why is homeostasis important?

  So body cells can function efficiently (independent of fluctuations in external environmental conditions); to provide cells with constant conditions during differing levels of activity.
• Examples of homeostatic control:


  Regulation of: blood glucose levels; solute potential (e.g. when water is lost via excretion, sweating and expiration); core temperature and pH
• Effect of changing temperature and pH on enzymes and importance to life:


  Enzymes denature at extreme temp/pH, they catalyse a lot of metabolic reactions so it is important to maintain a predictable constant rate of these reactions.
• Set point/norm


  The desired level at which the system operates
• Negative feedback:


  A receptor detects any deviation from a set point. Information is sent to a control centre which coordinates a response to return levels back to the set point.
• Detector/receptor (homeostasis):


  Monitors to condition and provides input to the control centre.
• Control centre/ coordinator (homeostasis):


  Evaluates information and provides output to an effector.
• Effector (homeostasis):


  Makes a response designed to take away the deviation (i.e. restore the set point)
• What causes fluctuation around the set point?


  Changes in pH, temperature and solute potential
• Role of positive feedback in contractions at end of pregnancy:


  Oxytocin stimulates contraction of uterus, which stimulates the production of more oxytocin increasing the stimulus
• Role of positive feedback in the first stage of clot formation:

  Platelets adhere to the cut surface and secrete signaling molecules (attract more platelets to the site)
• Role of positive feedback in CO2 removal:

  In high amounts of CO2 we breathe more often (can be harmful)
• Thermoregulation:


  Restoration of core body temperature back to set point.
• Thermoregulatory control centre:


  Hypothalamus
• Thermoreceptors:


  Hypothalamus (detect change in temp of blood); cutaneous receptors (in skin)
• Thermoregulatory effectors:


  Arterioles (contraction of smooth muscle in walls causes arteriole constriction restricting blood flow to skin diverting heat to core of the body); muscle tissue (contracts, shivering release heat from respiration)
• Why does a taller thinner person have more risk of hypothermia in cold conditions?


  Higher SA:V, less thermal insulation so more heat lost by conduction
• Functions of the kidney:

  Nitrogenous excretion, osmoregulation
• Nitrogenous excretion step 1:

  Amine group removed from amino acid
• Nitrogenous excretion step 2:

  Removed amine group converted to ammonia (highly toxic)
• Nitrogenous excretion step 3:


  Adding CO2 results in urea (less toxic) production and is transported in the blood plasma to the kidneys
• Nitrogenous excretion step 4:

  Urea removed by kidneys and excreted in urine
• Why is nitrogenous excretion needed?

  Amino acids cannot be stored; excess amino acids (not used for protein synthesis or other nitrogenous compounds) are deaminated in the liver
• Osmoregulation:


  Homeostatic balance between water gain and water loss; the control of water content and solute composition of body fluids (blood, tissue fluid and lymph)
• How do humans gain water?

  Food and drinks, respiration (metabolic water)
• Humans lose water by:

  Urination and egestion of faeces, sweating, exhalation
• Ultrafiltration:


  Filtration under pressure, separates small soluble molecules from blood plasma
• Water and small soluble molecules separated from blood plasma and filtered from glomerulus to Bowman's capsule through:


  Pores in glomerular capillary wall, the basement membrane (molecular sieve), between the feet of the podocytes (epithelial cells of bowman's capsule)
• What is filtered out of blood plasma into bowman's capsule?

  Water, amino acids, urea, fatty acids, salts, small proteins, glucose
• What remains in the blood (not filtered out)?

  Erythrocytes, leucocytes, thrombocytes, blood plasma proteins
• Kidneys:


  Filtering waste products from blood
• Bladder:


  Store urine prior to elimination
• Sphincter: (urinary system)


  A muscle which consciously controls urine release
• Urethra:


  Conveying urine to outside for elimination
• Ureter


  Conveying urine to the bladder
• Where does ultrafiltration take place in the kidneys?


  The glomerulus (knot of capillaries)
• Where does the filtrate form after ultrafiltration?

  The bowman's capsule
• How is high hydrostatic pressure generated in the glomerulus?


  The afferent arteriole diameter is wider than the efferent arteriole diameter
• Selective reabsorption:


  The process by which useful products (glucose and salts) are re absorbed back into the blood as the filtrate flows along the nephron (filtrate at end of PCT isotonic to blood plasma)