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Intravascular versus Interstitial Fluid
- Intravascular: blood plasma
- Interstitial: fluid outside of the cells and vascular system
Extracellular Fluid (ECF)
made up of the Intravascular and Interstitial Fluid
Substances that dissociate into charged particles known as ions. e.g. Na⁺
Cations versus Anions
- 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.
Relative increases/decreases in sodium are called:
Hypernatremia / Hyponatremia
- Potassium Cation: most common cation in the ICF.
- Maintains osmolarity in ICF.
- Involved in nerve impulses.
Relative increases/decreases in potassium are called:
Hyperkalemia / Hypokalemia
- Calcium Cation:
- Involved in nerve impulses and
- muscle contraction.
Relative increases/decreases are called:
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 versus Alkalosis
- 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
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 / Osmolality
- 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:
Net filtration = (Forces favouring filtration) - (Forces opposing filtration)
Bicarbonate buffer system
- Fastest of the three mechanisms
- H⁺ + HCO₃⁻ ↔ H₂CO₃
- Hydrogen ion + bicarbonate ion ↔ carbonic acid
Healthy ratio of bicarbonate ions to molecules of carbonic acid.
Acid-Base 20:1 ratios and its relation to Respiratory/Metabolic acidosis
- 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
Respiratory Acidosis versus Metabolic Acidosis
- 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
Acidosis in relation to kidney function
- 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⁺
3 mechanisms of H⁺ removal
- 1. Bicarbonate buffer system
- 2. Respiration
- 3. Kidney function
Acidosis in relation to CO₂ stress
- 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⁺
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