Fluid, Electrolyte and Acid-Base Balance I

Major fluid compartments separated by barriers:
2/3 - Intracellular Fluid
1/3 - Extracellular Fluid
            - Blood plasma
            - Interstitial fluid


The barriers separating these compartments are:
- Plasma membrane (between ICF and interstitial fluid) - selectively permeable
- Blood vessel walls (blood and interstitial fluid) - exchange occurs in the capillaries


Regulation of water balance

1. Water intake= water output
2. Regulation of water gain (e.g. thirst) via negative feedback loop!
• When the water content in the body is decreased, this decreases plasma volume. As a result, there is a decrease in blood pressure, stimulating the juxtaglomerular cells in the kidneys to release and begin the renin-angiotensin-aldosterone system. Via a signalling cascade, the production of angiotensin II (a potent vasoconstrictor) is increased. This change is noted by the hypothalamus.
• When the water content in the body is low, this increases plasma osmolality, which decreases saliva production and activates the osmoreceptors in the hypothalamus. Dryness in the mouth will subsequently be detected by the sensory receptors. This change is noted by the hypothalamus. 
• This creates a conscious perception of thirst and leads to drinking water. 
• As a result, blood volume is increased, blood osmolality is decreased and increase hydration levels of the mouth. All these restores normal water balance.
3. Regulation of water loss controlled by 3 hormones
• Na+ and Cl-: main determinant of body fluid volume!
• Renin-angiotensin-aldosterone hormones (renin)
• Decrease in blood volume/ pressure activates this mechanism. Renin is released and via intermediary steps, leads to increased production of angiotensin II (a potent vasoconstrictor). It causes 
• The vasoconstriction of the glomerular afferent arteries, decreasing the glomerular filtration rate.
• Stimulate the activity of Na+/H+ antiporters (cotransporters) in the renal tubules
• Secretion of aldosterone from adrenal cortex cells
• As a result, there is an increased reabsorption of Na+ and Cl- from the tubular filtrate back to the blood, leading to osmosis and increasing blood volume
• Aldosterone
• Increase water retention by acting on the kidney to increase reabsorption of Na+ and Cl- at the loop of Henle back into the blood. 
• Increase secretion of K+ into the tubular filtrate by altering Na+/K+ ATPase activity in principal cells located on the distal convoluted tubule and collecting ducts.
• Antidiuretic hormone/ vasopressin
• Increases water retention.
• When osmolarity of blood plasma and interstitial fluid increases above set point, osmoreceptors of hypothalamus are stimulated and send nerve impulses to the neurohypophysis (posterior pituitary gland), leading to the ADh secretion 
• ADh increase water permeability of principal cells lining the collecting ducts. Adh stimulates the insertion of aquaporin-2 water channels in the apical membrane of these cells. As a result, water permeability is increased and more water is reabsorbed. Blood volume is retained, producing a lower volume of more concentrated urine
• Atrial Natriuretic peptide
• Increase water loss
• When blood vol increases, the atria of the heart are stretched, and cells release ANP.
• ANP 
• causes relaxation of the glomerular mesangial cells, increasing capillary surface area, leading to an increase in gfr.
• inhibit reabsorption of Na+ and water in the proximal convoluted tubule and collecting duct
• inhibit secretion of aldosterone and ADh.