homeostasis

• Homeostasis


  The maintenance of a constant internal environment
• Importance of homeostasis


  • Internal conditions within the body (such as temperature, blood pressure, water concentration, glucose concentration etc) need to be kept within set limits to ensure reactions in body cells can function and the organism as a whole can live
  • Without medical intervention, if conditions deviate far away from normal, the body will not function properly and the eventual consequence will be death
• Negative feedback


  A process where conditions that change from the ideal or set point are returned to this set point
• Negative feedback mechanism


  1. If the level of something rises, control systems are switched on to reduce it again

  2. If the level of something falls, control systems are switched on to raise it again
• Negative feedback mechanisms


  • Usually a continuous cycle of bringing levels down and then bringing them back up so that overall, they stay within a narrow range of what is considered 'normal'
• Thermoregulation


  The regulation of body temperature
• Thermoregulatory centre


  • Located in the hypothalamus of the brain
  • Contains receptors sensitive to the temperature of the blood
• Skin


  • Contains temperature receptors within the epidermal layer which send nerve impulses to the thermoregulatory centre
• Responses when body temperature is too high

  1. Hair erector muscles relax

  2. Blood vessels dilate (vasodilation)

  3. Sweat is produced from the sweat glands
• Responses when body temperature is too low

  1. Hair erector muscles contract

  2. Blood vessels constrict (vasoconstriction)

  3. Sweating stops

  4. Skeletal muscles contract (shiver)
• Vasodilation and vasoconstriction are mechanisms of heat exchange at the body's surface
• Vasodilation


  Relaxation of muscles in arteriole walls, causing arterioles near the skin to dilate and allowing more blood to flow through capillaries
• Vasoconstriction


  Contraction of muscles in arteriole walls, causing arterioles near the skin to constrict and allowing less blood to flow through capillaries
• Osmoregulation


  The process of maintaining water and salt concentrations (osmotic balance) across membranes within the body
• Importance of osmoregulation


  • Maintaining water levels in the body is vital to prevent harmful changes occurring to cells as a result of osmosis
  • Too much water in the blood results in cells swelling and potential cell lysis
  • Too little water in the blood results in cells losing water by osmosis and potential cell death
• Sources of water in the body

  • Water produced as a result of aerobic respiration
  • Water in the diet
• Ways water is lost from the body

  • Via the lungs during exhalation
  • Lost from the skin as sweat
  • Urine production
• Urinary system


  • Two kidneys joined to the bladder by two tubes called the ureters
  • Urethra carries urine from the bladder to outside the body
  • Kidneys are connected to the renal artery and renal vein
• The kidneys regulate the water content of the blood and excrete toxic waste products and substances in excess of requirements
• There are three regions of the kidney: cortex, medulla, and pelvis
• Note the difference between the 'ureter' and the 'urethra'. These two names are commonly confused by students so take care to learn them and know which tube is which – they are NOT interchangeable!
• Kidneys


  • Located in the back of the abdomen
  • Regulate the water content of the blood (vital for maintaining blood pressure)
  • Excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts)
• Nephrons


  Tiny structures in the kidney, also known as kidney tubules or renal tubules
• Structure of the kidneys

  1. Cortex - the outermost region

  2. Medulla - the inner section of the kidney

  3. Renal pelvis - the tube linking the kidney to the ureter
• Structure of a nephron

  1. Bowman's capsule

  2. Proximal convoluted tubule

  3. Loop of Henle

  4. Distal convoluted tubule

  5. Collecting duct
• Nephron


  • Surrounded by a network of capillaries with a knotted section which sits inside the Bowman's capsule
• Ultrafiltration


  1. Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries (the glomerulus) sitting inside the cup-shaped Bowman's capsule

  2. The capillaries get narrower as they get further into the glomerulus which increases the pressure on the blood moving through them

  3. This eventually causes the smaller molecules being carried in the blood to be forced out of the capillaries and into the Bowman's capsule, where they form what is known as the filtrate
• Ultrafiltration


  The process where smaller molecules are forced out of the capillaries and into the Bowman's capsule
• Reabsorption of glucose


  1. Glucose is the first substance to be reabsorbed at the proximal (first) convoluted tubule

  2. This takes place by active transport

  3. The nephron is adapted for this by having many mitochondria to provide energy for the active transport of glucose molecules

  4. Reabsorption of glucose cannot take place anywhere else in the nephron as the gates that facilitate the active transport of glucose are only found in the proximal convoluted tubule
• People with diabetes


  Cannot control their blood glucose levels and they are often very high, meaning that not all of the glucose filtered out can be reabsorbed into the blood in the proximal convoluted tubule
• As there is nowhere else for the glucose to be reabsorbed, it continues in the filtrate and ends up in the urine
• This is why one of the first tests a doctor may do to check if someone is diabetic is to test their urine for the presence of glucose
• Reabsorption of water & salts

  1. As the filtrate drips through the Loop of Henle necessary salts are reabsorbed back into the blood by diffusion

  2. As salts are reabsorbed back into the blood, water follows by osmosis

  3. Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time
• Urine produced by the kidneys contains a mixture of urea, excess mineral ions, and excess water
• Urine colour and quantity

  • Large quantities of urine are usually pale yellow in colour because it contains a lot of water and so the urea is less concentrated
  • Small quantities of urine are usually darker yellow / orange in colour because it contains little water and so the urea is more concentrated
• Factors affecting urine concentration

  • Water intake - the more fluids drunk, the more water will be removed from the body and so a large quantity of pale yellow, dilute urine will be produced
  • Temperature - the higher the temperature the more water is lost in sweat and so less will appear in the urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced
  • Exercise - the greater the level of exercise, the more water is lost in sweat and so less will appear in the urine, meaning a smaller quantity of dark yellow, concentrated urine will be produced
• ADH


  The hormone that controls water reabsorption in the kidneys
• How ADH regulates water content of the blood

  1. If the water content of the blood is too high, the pituitary gland releases less ADH which leads to less water being reabsorbed in the tubules of the kidney (the tubules become less permeable to water)

  2. If the water content of the blood is too low, the pituitary gland releases more ADH which leads to more water being reabsorbed in the tubules of the kidney (the tubules become more permeable to water)
• The kidney controls water levels using the hormone ADH. It is an example of a negative feedback mechanism.
• Kidney failure


  • Humans can survive with one functioning kidney, but if both are damaged then there will quickly be a build-up of toxic wastes in the body which will be fatal if not removed