fluids.txt

• Intravascular: blood plasma
• Interstitial: fluid outside of the cells and vascular system

made up of the Intravascular and Interstitial Fluid

Substances that dissociate into charged particles known as ions. e.g. Na⁺

• Cations: positively charges ions (e.g. Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺)
• Anions: negatively charges ions (Cl⁻, HCO₃⁻, HPO₄⁻)

• Sodium Cation: most common cation in the ECF.
• Maintains osmolarity in ECF.
• Involved in water movement and
• nerve impulses.

Hypernatremia / Hyponatremia

• Potassium Cation: most common cation in the ICF.
• Maintains osmolarity in ICF.
• Involved in nerve impulses.

Hyperkalemia / Hypokalemia

• Calcium Cation:
• Involved in nerve impulses and
• muscle contraction.

Hypercalcemia / Hypocalcemia

• Magnesium Cation: several biochemical processes and closely associated to phosphates.
• Relative increases/decreases are called: Hypermagnesemia / Hypomagnesemia

Chloride Anion: involved in fluid balance and renal function.

Bicarbonate Anion: principle buffer of the body, neutralising highly acidic hydrogen ions (H⁺)

• Phosphate Anion: used in ATP.
• Closely associated with Mg⁺⁺ in renal function.
• Acts as a buffer primarily in the ICF.

• Acidosis: pH below 7.35
• Alkalosis: pH above 7.45
• The body is constantly creating hydrogen ions (H⁺) through metabolism, creating acidity.
• A variation of around 0.4 pH can be fatal

Passage of a solvent (usually water) through a membrane

Passage of molecules through a membrane from an area of greater concentration to an area of lesser concentration.

Equal concentration of solute molecules on either side of a membrane

less concentrated

more concentrated

water moves to saltier enviro

• Movement of a substance against Osmotic gradient
• Is faster than diffusion but requires energy.

• Diffusion of a substance, such as glucose, through a cell membrane with the assistance of a "helper" or carrier protein.
• It may or may not require energy.

• Osmolarity: concentration of solute/kg of water
• Osmolality: concentration of solute/litre of water

Pressure exerted by the concentration of solutes. Is a pull, not a push force.

Colloid osmotic pressure

• Total loss of water from plasma to interstitial space.
• Value is usually zero, because of Starling's Hypothesis

• BP. It tends to force water out of the capillaries into the interstitial space, by a process called:
• Filtration

Net filtration = (Forces favouring filtration) - (Forces opposing filtration)

• Fastest of the three mechanisms
• H⁺ + HCO₃⁻ ↔ H₂CO₃
• Hydrogen ion + bicarbonate ion ↔ carbonic acid

20:1

• 20:1 is normal pH [bicarbonate ions : molecules of carbonic acid]
• 20:4 is an excess of carbonic acid and is Respiratory acidosis
• 15:1 is a deficit of bicarbonate and is Metabolic acidosis

• RA: 20:4
• MA: 15:1

• Erythrocytes contain the enzyme carbonic anhydrase which rapidly converts carbonic acid to CO₂ and water. This process can go in both directions:
• H₂CO₃ ↔ H₂O + CO₂

• pH can be lowered (made more acidic) by excretion of bicarbonate ions, because there is less bicarbonate available to bind with the H⁺ ions:
• ↓ HCO₃⁻ → ↑ H⁺

• 1. Bicarbonate buffer system
• 2. Respiration
• 3. Kidney function

• Hypoventilation leads to a build up of CO₂ which creates a 'stress' that shifts the following equation to the left, thus leading to an excess of H⁺ and Respiratory acidosis:
• H⁺ + HCO₃⁻ ↔ H₂CO₃ ↔ H₂O + CO₂
• So an increase of CO₂ leads to an increase of H⁺ and a drop in pH (acidosis):
• ↑ CO₂ ↔ ↑ H⁺
• So you can see that an increase in respiration will lower CO₂ levels and thus lower the excess of H⁺, resolving the acidosis:
• ↑ Respiration → ↓ CO₂ → ↓ H⁺