Physiology  Topics   

Water Balance

There are two major mechanisms to maintain water balance: the concious thirst sensation and hormonal regulation. An area of the hypothalamus known as the thirsth center governs the urge to drink. Increased nerve impulses from osmoreceptors in the hypothalamus and mouth, baroreceptors in the heart and vasculature, and increased angiotensin II blood levels trigger the thirst sensation.

The hormonal regulation works through the renin/angiotensin system. Decreased renal blood pressure stimulate the juxtaglomerular cells to release renin, leading to the formation of angiotensin II. Angiotensin II and aldosterone promote reabsorption of Na+, Cl- and water in the kidney. The result is water conservation and increased body fluids volume.

As the volume body fluids increases, an increasee in blood volume strretches the atria of the heart and promotes release of atrial natriuretic peptide (ANP), which promotes Na+ excretion. At the same time, renin, angiotensin II and antidiuretic hormone (ADH) levels decrease, slowing renal reabsorption of Na+, Cl- and water. This leads to formation of diluted urine and decreased blood volume.

There is also a concious mechanism to regulate electrolytes, although it is not as well uncerstood as that of thirst. For example, a patient on diuretics that promote loss of K+ may be hungry for bananas, which are high in K+. Cravings for minerals like chalk or dirt may also occur.

Other mechanisms to regulate Na+ levels include:

• Glomerular filtration rate (GFR) regulation. An increased GFR will increase Na+ excretion simply because more ion is being filrtered while reabsorption mechanisms remain unchanged. When baroreceptors sence increased blood pressure, the hypothalamus sends a sympathetic signal to the afferent arteriole to dilate, thus increasing GFR and leading to icreased Na+ excretion and decreased blood volume.
  
  
• Aldosterone/renin system. (discussed previously)
  
  
• Starlin forces. The peritubular capillary bed is responsible for reabsobing Na+ into the blood (passive). If there are cganges in the blood colloidal presure, blood pressure or blood electrolyte composition. This factors, known as Sterling forces, would influence how much fluid is be reabsorbed.
  
  
• Sympathetic stimuli. In a fight-or-fligth situatuion, sympathetic signals will shunt blood away from the kidneys and towards muscle and heart. Afferent arterioles vasoconstric, decreasing GFR. This also conserves water and Na+.
  
  
• ANP. Suppresses the release of ADH and opposes its effect on water channels, and causes vasodilation in order to lower renal blood pressure.
  
  
• Other miscellaneous factors

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