Header image used on licence from Shutterstock The lymphatic system subsequently absorbs this leakage. This results in a small net movement of water out of the capillaries into the interstitial space. These proteins exert an osmotic force called the plasma oncotic pressure that almost balances the hydrostatic pressure imposed on the plasma by the action of the heart. Plasma proteins are the only constituents of the plasma that do not cross into the interstitial fluid under normal circumstances. The principle difference between the interstitial fluid and the plasma is the higher protein content of the plasma. The Donnan equilibrium ensures that electrical force driving the chloride ions out of the cell is balanced by the chemical gradient driving them back in. Because of their large size, these proteins are unable to cross the plasma membrane, and along with other large anions, such as phosphate, these account for most of the anion content of the ICF.Ĭhloride ions, however, are small enough to cross the plasma membrane, and these are forced out of the cell by the charge of the fixed proteins and other large anions. Intracellular proteins are negatively charged at physiological pH. With regards to the ICF and ECF, the Donnan equilibrium mainly influences the movement of chloride ions. 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.
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