Body Fluid Compartments: Water moves freely throughout the various body fluid compartments, this movement is in response to two forces: hydrostatic pressure and osmotic pressure. While both forces contribute to fluid movement across a capillary membrane, osmotic pressure is the major force that drives fluid across the plasma membrane of cells.
Total body fluid: Total Body Water (TBW) = 60% x Body Weight.
Extracellular fluid: is made up of Cations (positively charged molecules): Ca2+, Mg2+, Na+; and anions (negatively charged molecules): Cl-, HCO-3 and proteins are in equal concentrations.
Intracellular fluid: Inside cells, cations (positively charged molecules): Ca2+, Mg2+, Na+;and anions (negatively charged molecules): Cl-, HCO-3 and proteins are in equal concentrations.
Fluid Movement Between Compartments
As water and salt are ingested there is a change in the osmolality of the ECF, and subsequently water moves between the ECF and ICF, towards the higher solute concentration. Water moves into the compartment with a higher solute concentration or osmolality, until equilibrium is reached between the two compartments.
Kidney’s Role in Fluid Regulation
The nephron is the functional unit of the kidneys; nephrons are contained within the renal medulla. Nephrons are connected via the collecting duct system; fluid flows collectively from the nephrons into the collecting duct system and ultimately is excreted as urine.
Acid-Base Balance: The acidity and alkalinity of blood is tightly regulated, and is slightly basic within a range of pH= 7.35-7.45.
Henderson-Hasselbalch equation: The Henderson-Hasselbalch equation derives the pH as a measure of the acidity in biological systems.
Acid Formation in the Body: Carbonic Anhydrase is a metalloenzyme that rapidly and reversibly converts carbon dioxide and water into carbonic acid. The main function of this enzyme is to interconvert carbon dioxide and bicarbonate to maintain the acid-base balance in the blood and tissues.
Davenport Diagram: The Davenport diagram is a graphical representation of the relationship between the plasma bicarbonate concentration and the pH of the blood. Disturbance in the normal values of bicarbonate and pH lead to acidosis and alkalosis.
Acidosis and Alkalosis: Acidosis is a state of increased acidity in the body, characterized by a pH < 7.35. Acidosis can be caused by two mechanisms: (A) Respiratory Acidosis results from a build-up of carbon dioxide, due to hypoventilation. Alkalosis is a state of decreased acidity (increased pH) in the body, characterized by a pH > 7.45. Alkalosis can be caused by two mechanisms: (A) Respiratory Alkalosis and (B) Metabolic Alkalosis.
Compensation for a Disruption in the Acid-Base Balance
The human body has 3 main mechanisms to control a change in the acid-base balance of body fluids: (A) Extracellular and intracellular buffering, (B) changing the respiration rate of the lungs and (C) adjustments to renal acid secretion.