B12 - Homeostasis in action

• Your body temperature needs to stay around 37°C. This is the temperature at which your enzymes work best.
• At only a few degrees above or below normal body temperature, your enzymes do not function properly.
• Many things can affect your internal body temperature, including the transfer of energy from your muscles during exercise, fevers caused by disease, and the external temperature rising or falling.
• Thermoregulatory centre


  In the hypothalamus of your brain, contains receptors sensitive to temperature changes in the blood flowing through the brain
• Temperature receptors in the skin


  Send impulses to the thermoregulatory centre, giving information about the skin temperature
• Cooling the body down

  1. Blood vessels supplying surface skin capillaries dilate (open wider)

  2. More sweat produced from sweat glands in skin
• Keeping warm


  1. Blood vessels supplying skin capillaries constrict (close up)

  2. Sweat production reduced or stopped

  3. Skeletal muscles contract and relax rapidly, causing shivering
• Never say that capillaries dilate or constrict. They cannot do this as they have no muscle layer. It is the blood vessels supplying the capillaries that dilate or constrict.
• Blood vessels never move! Either more blood flows in vessels near the skin surface, or more blood flows in the vessels lower down.
• The average person produces up to 900 litres of urine a year.
• The carbon dioxide produced by the body cells during respiration must be removed because dissolved carbon dioxide produces an acidic solution.
• Removing carbon dioxide


  1. Carbon dioxide diffuses out of the cells into the blood

  2. Carbon dioxide diffuses from the blood into the air in the alveoli of the lungs

  3. Carbon dioxide is removed from the body when you exhale
• Urea


  The nitrogenous waste produced by the breakdown of excess amino acids in the liver
• Removing urea


  1. Urea passes from the liver cells into the blood

  2. Urea is filtered out of the blood by the kidneys

  3. Urea is passed out of the body in the urine produced by the kidneys
• Urea is produced when you eat more protein than you need, or when your tissues are worn out.
• Amino acids cannot be used directly as fuel by the cells of the body.
• Deamination


  1. The liver removes the amino group from amino acids, forming ammonia

  2. The ammonia is immediately converted into urea to be excreted from the body
• If the cells of your body lose or gain too much water by osmosis to the fluids surrounding them, they do not function efficiently.
• Ways water is lost from the body

  • Exhaled from the lungs
  • Through the skin in sweat
  • Urea, excess water, and excess mineral ions removed via the kidneys and excreted in urine
• There is no control over water, mineral ion or urea loss by the lungs or skin.
• Urea, along with excess water and mineral ions, is removed via the kidneys and excreted in the urine.
• e methods give no control over the amount of the various substances lost to the environment. Other processes are very important in the control of the water and mineral ion balance in the body.
• Water loss from the body

  1. Water leaves the lungs every time you exhale

  2. Water, mineral ions, and urea are lost through the skin in sweat
• Uncontrolled water loss


  No control over water, mineral ion or urea loss by the lungs or skin
• Removal of urea, excess water, and excess mineral ions from the body

  1. Removed from the body via the kidneys, then excreted in the urine

  2. This process is very tightly regulated to maintain the water and mineral ion content of the blood within narrow limits

  3. Remove urea (the poisonous nitrogenous waste)
• The regular production of urine is an important part of homeostasis - wherever you happen to be
• Urea


  Poisonous nitrogenous waste
• If the body cells lose or gain too much water by osmosis


  They do not function efficiently
• Water, mineral ions, and nitrogenous waste are balanced in the body
• Controlled systems of loss

  • Important for maintaining balance of water and mineral ions in the body
• Uncontrolled losses are also important
• The kidneys remove toxic urea from the body in the urine, along with any excess water and mineral ions not needed by the body
• Urine is produced constantly by the kidneys and stored temporarily in the bladder
• Homeostasis in the water balance of the body

  Kidneys conserve water if short, produce dilute urine to get rid of excess
• Mineral ion balance


  Kidneys remove excess mineral ions and excrete them in the urine
• How the kidneys work

  1. Filter blood, glucose, mineral ions, urea, and water move out of blood into kidney

  2. Blood cells and large proteins too big to leave blood

  3. All blood passes through kidneys about once every 5 minutes

  4. Kidneys filter 180 litres of water per day, 99% returned to blood

  5. Urine trickles into bladder
• Selective reabsorption


  Amount of water and dissolved mineral ions reabsorbed varies depending on what is needed by the body
• Water balance maintained by a sensitive feedback mechanism to keep body fluids and cells within a narrow range
• ADH (antidiuretic hormone)


  Secreted by pituitary gland, affects kidney tubules to reabsorb more or less water to maintain blood solute concentration
• ADH regulation of water balance

  1. If blood too concentrated, pituitary releases more ADH, kidney tubules reabsorb more water, concentrated urine produced

  2. If blood too dilute, less ADH released, kidney tubules reabsorb less water, dilute urine produced