The body adjusts to:

  • Increased water intake by increasing urine output
  • Decreased water intake or increased exercise by decreasing urine output
  • To do this your body’s nervous system has to communicate with the endocrine system
  • Water balance is regulated by antidiuretic hormone (ADH)
  • ADH regulates osmotic pressure of body fluids by causing the kidneys to increase water reabsorption.

ADH

  • The permeability of the distal tubule and collecting duct is controlled by ADH
  • ADH is produced in the hypothalamus and is stored and released from the pituitary gland.
  • ADH increases the permeability of the distal tubule and collecting duct, so more water is reabsorbed
  • This causes the urine becomes more concentrated

Response to decrease in body water

  • Decrease body water = increase blood solutes = increase blood’s osmotic pressure.
  • Osmoreceptors in the hypothalamus cells detect changes in the osmotic pressure in capillaries
  • Water from the hypothalamic cells moves into the bloodstream (osmosis), causing these cells to shrink.
  • A nerve message is sent to the pituitary to release ADH.
  • ADH travels in the blood to the kidneys.
  • Kidneys reabsorb more water, producing a more concentrated urine.
  • Shrinking of the hypothalamic cells also initiates a sensation of thirst.

As water is consumed…

  • Decrease blood’s osmotic pressure.
  • Blood becomes more dilute, water moves from the blood into the hypothalamus.
  • Hypothalamic cells swell and nerve messages to pituitary gland stops.
  • Less ADH released and less water is reabsorbed.

Diuretics

  • Alcohol and caffeine are examples of diuretics – drugs that increase frequency of urination
  • These drugs block the release of ADH, thereby increasing the volume of urine
  • Excessive use can lead to dehydration

ADH and the Nephron

  • 85% of the water filtered into the nephron is reabsorbed in the proximal tubule.
  • Proximal tubule very permeable to water.

With ADH…

Upper part of distal tubule and collecting duct becomes permeable to water.

Without ADH…

The remaining 15% will not be reabsorbed.

  • Active transport of Na+ from ascending section of loop of Henle concentrates solutes in medulla.
  • High [NaCl] in intercellular spaces creates an osmotic pressure that draws water from the upper part of the distal tubule and collecting duct but only when ADH present.
  • As water passes into the blood, the filtrate in the nephron becomes more concentrated.

Kidneys and Blood Pressure

Increase fluid loss = decrease blood pressure (BP)

  • Kidneys regulate blood pressure by adjusting the volume of blood.
  • BP receptors in the juxtaglomerular apparatus detects low BP.
  • Causes the release of renin which converts angiotensinogen into angiotensin.

Functions of Angiotensin:

  • Constriction of blood vessels.
  • Stimulates release of aldosterone.

Aldosterone and Blood Pressure

  • Produced in the cortex of the adrenal glands which are located above each kidney.
  • Aldosterone acts on distal tubule and collecting duct to increase Na+ ion reabsorption.
  • Cl- is also reabsorbed forming NaCl
  • As NaCl reabsorption increases, the osmotic gradient increases and more water moves out of the nephron, thus increasing blood pressure

pH Balance

  • Body pH around 7.3 – 7.5.
  • If the blood pH fluctuates the secretion of H+ in the nephron either increases or decreases until it returns to normal
  • Acid-base balance is also maintained by buffer systems that absorb excess H+ ions or alkaline ions.

Bicarbonate-Carbon Dioxide Buffer System

  • Excess H+ buffered by bicarbonate ions.
  • Carbonic acid produced and converts to CO2 and water.
  • CO2 expelled from lungs.
  • To remove excess H+ ions, the buffer (HCO3-) must be restored…
  • Kidneys restore the HCO3- by reversing the reaction.
  • CO2 is actively transported from the peritubular capillaries into the cells that line the nephron.
  • CO2 combines with water to produce HCO3- and H+ ions.
  • HCO3- diffuses back into the blood to restore the buffer.
  • The H+ ions combine with phosphate ions or ammonia and are excreted with the filtrate.

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