An electrical potential develops between the two sides of the membrane, and the two solutions will have different osmotic pressures. It is the state of equilibrium that exists at a semi-permeable membrane when it separates two solutions containing electrolytes, the ions of some of which are able to permeate the membrane, and some are not. The Donnan equilibrium, or Gibbs-Donnan effect, is the name given to the behaviour of charged particles near a semi-permeable membrane. The NA-K ATPase, image sourced from WikipediaĬourtesy of OpenStax College CC BY-SA 4.0 The Na + K +-ATPase pump, therefore, maintains the gradient of a higher concentration of sodium extracellularly and a higher level of potassium intracellularly. For every single ATP consumed, it pumps 3 Na + out of the cell and K + into the cell, against the concentration gradients. The Na-K ATPase is a transporter found in the outer plasma membrane if cells. The approximate concentrations of ions between the three main fluid compartments are summarised in the table below: Fluid compartment The two main factors that contribute to the maintenance of the cationic differences between the ICF and the ECF are the activity of the sodium-potassium ATPase (NA-K ATPase) and the Donnan equilibrium. Calcium ion (Ca 2+) and chloride ion (Cl –) concentrations are also higher in the ECF.Conversely, the sodium ion (Na +) concentration is much higher in the ECF than in the ICF.
The potassium ion (K +) concentration is much higher in the ICF than in the ECF.The most important difference between the ICF and the ECF is the relative concentration of cations (positively charged ions): The composition of ions between the fluid compartments varies, but within any one compartment, electrical neutrality is maintained with the total number of positive charges always being equal to the total number of negative charges. Fluids leaking from the peritoneal cavity, e.g.Third spacing is the unusual accumulation of fluid in the transcellular space. The barrier between the interstitial fluid and the intravascular fluid consists of the walls of capillaries. Transcellular fluid: This is the final 1.5 L of fluid and comprises intraocular fluid, cerebrospinal fluid, urine in the bladder, and fluid within the lumen of the bowel.Intravascular fluid: This is the plasma, which is the liquid component of the blood and accounts for approximately 25% of the ECF (3.5 L of fluid).Interstitial fluid (ISF): This is the tissue fluid found in the spaces between the cells and accounts for approximately 65% of the ECF (10 L of fluid).The extracellular fluid is further divided into three other compartments: These compartments are separated by the plasma membrane of the cells and differ markedly in terms of the concentrations of the ions dissolved in them. Extracellular fluid (ECF): This is the water outside of the cells and accounts for approximately 35% of total body water (15 L of fluid).This is the matrix in which intracellular organelles are suspended, and chemical reactions take place. Intracellular fluid (ICF): This is the water within the cells and accounts for approximately 65% of total body water (30 L of fluid).There are two major fluid compartments in the human body: Total body water (TBW) accounts for around 60% of an adult male’s total body weight a normally hydrated 75 kg man will consist of approximately 45 L of water. The amount of water in the body varies with a patient’s age, weight, and sex.
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