The kidneys

the main organ of osmoregulation

• the role of liver in producing urea

  • urea is produced in the liver and secreted by the kidney
  • the liver is responsible for processing the absorbed products of digestion
  • some amino acids remain in the blood for immediate use by the body
  • amino acids cannot be stored for later use
  • excess amino acids are de-aminated in the liver
  • the amine group is removed and converted to ammonia and then urea.the urea travels via blood to the kidney where it is excreted in the urine

• main process taking place in the kidney

  • ultrafiltration
  • selective reabsorption
  • active secretion
  • concentration

• selective reabsorption

  • useful substances are reabsorbed from the filtrate of the first convoluted tubule
  • the main methods are as follows
  • diffusion
  • facilitated diffusion
  • osmosis
  • active transport

• diffusion and kinetic energy

  • diffusion is the movement of substances from an area of high concentration to an area of low concentration of that substance
  • diffusion occurs because molecules are in constant motion
  • diffusion can occur across the cell membrane

• facilitated diffusion

  • facilitated diffusion requires a concentration gradient
  • facilitated diffusion uses a protein carrier to move substances across the membrane
  • this requires no use of energy in the form of ATP eg:: glucose and amino acids

• active transport

  • moves substances against a concentration gradient (from low to high)
  • uses a protein carrier molecule to move substances across the membrane
  • requires the use of energy from ATP

• reabsorption of water

  • as glucose and amino acids are absorbed from the filtrate then the water potential of the filtrate becomes less negative
  • the water potential of the blood falls
  • the water is reabsorbed from the filtrate into the blood by osmosis

• loop of Henle

  • function is to create a contraction gradient of sodium and chlorine ions down the medulla
  • the concentration i highest closest to the pelvis and lowest close to the cortex
  • 2 limbs of the loop of Henle, the descending and the ascending
  • descending impermeable to sodium
  • ascending permeable to sodium
  • ascending actively transports sodium out of the limb
  • result is that there is an increasing concentration of sodium as you get closer to the bottom of the loop
  • this has no overall effect on the composition of the urine

• 2nd convoluted tubule

  • site of micro adjustments of urine composition
  • level of electrolysis is adjusted depending on body’s needs

• collecting duct

  • aurone passes to collecting duct
  • the site of control of water content of urine
  • as the urine passes down the duct it encounters surrounding tissues that has an increasing concentration of sodium
  • water can leave the duct by osmosis
  • the permeability of the duct to water is affected by the hormone ADH(anti diuretic hormone)

• ADH and water balance

  • the water potential of the blood is monitored by the hypothalamus
  • the water potential falls when a person is dehydrated and rises when a person is overhydrated

• pituitary gland

  • the pituitary gland releases many hormones
  • one of which is ADH

• dehydration

  • hypothalamus detects a fall in water potential
  • signal is sent to the pituitary gland
  • pituitary releases more ADH
  • more water is reabsorbed from the urine
  • volume of urine decreases
  • concentration increases

• over hydration

  • hypothalamus detects a rise in water potential
  • signal is sent to the pituitary
  • pituitary releases less ADH
  • less water is reabsorbed from the urine
  • volume of urine increases
  • concentration decreases

• some hormone disorders connected with ADH

  • diabetes insipidus
  • this is a hormone disorder that occurs when there is a lack of the hormone ADH
  • the symptoms that you get are very similar to those to diabetes mellitus but the cause is different
  • continual thirst
  • large volume of urine production(as much as 20l a day)
  • affects about 1 in 25000 of the population
  • in some cases the body does not produce any ADH as a result of damage to the hypothalamus or a brain tumour
  • this can be treated with the drug desmopressin which mimics the action of ADH
  • in other cases ADH is produced but the kidneys do not respond to the hormone
  • this can be treated by the use of thiazide diuretics which reduce the volume of urine

• SIADH (syndrome of inappropriate antidiuretic hormone secretion)

  • this is caused by an overproduction pf the hormone ADH
  • this is caused by an overproduction pf the hormone ADH
  • sufferers have problems getting rid of excess water causing a buildup of fluid in the body
  • sodium levels will fall leading to cramps
  • nausea,vomiting ,confsuion,hallucination,seizers and coma may follow