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 the 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 to become 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 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 the pituitary gland stop.
  • Less ADH released and less water is reabsorbed.

Diuretics

  • Alcohol and caffeine are examples of diuretics – drugs that increase the frequency of urination
  • These drugs block the release of ADH, thereby increasing the volume of urine
  • Excessive use can lead to dehydration
READ:
How to Detection Alcohol in Breath, Blood & Urine

ADH and the Nephron

  • 85% of the water filtered into the nephron is reabsorbed in the proximal tubule.
  • The proximal tubule is 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 the loop of Henle concentrates solutes in the 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 the 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
READ:
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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 the 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.
author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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