Osmosis

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[box type=”download”] –Definition

–Its importance as the transport mechanism for water based upon gradients [/box]

Osmosis is the passive movement of water across a semi-permeable membrane from regions of low solute concentration to those of higher solute concentration.

Biological membranes are semi-permeable. They usually allow the free movement of water but restrict solutes.


[box type=”download”] Differences between Osmolarity and osmolality.[/box]

The osmotic potential depends on the number of osmotically active particles (molecules) per litre, irrespective of their identity. It is expressed in terms of osmoles, where 1 osmole equals 1 mole of particles, as osmolarity (osmol/L), or osmolality (osmol/kg H2O). The latter is preferred by physiologists as it is independent of temperature, though in physiological fluids the values are very similar.

The osmolality of plasma is 290 mOsmol/kg H2O.

Proteins do not easily pass through capillary walls, and are responsible for the oncotic (or colloidal osmotic) pressure. This is much smaller than crystalloid osmotic pressure, but is critical for fluid transfer across capillary walls. Oncotic pressure in plasma is normally 25 mmHg.


[box type=”download”] Iso- hypo- and hyper-tonicity: –Differences between each and consequences of ingestion of fluids of each type. [/box]

A fluid at the same osmotic potential as plasma is said to be isotonic; one at higher potential (i.e. more concentrated solutes) is hypertonic and one at lower potential is hypotonic.

Drinking fluids of differing osmotic potentials has distinct effects on the distribution of water between cells and extracellular fluid.

Comparison table


[box type=”download”] — Fluid spaces — Relative distribution between intra- and extra-cellular spaces — Key differences between ECF and ICF in terms of cationic concentrations — Role of Na-K ATPase activity in maintaining ionic gradients between ECF and ICF. [/box]

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Body water compartments

Water accounts for some 50–70% of the body mass (i.e. about 40 L in a 70 kg person). There are two major ‘fluid compartments’: the water within cells (intracellular fluid, ICF – about 65%), and the water outside cells (extracellular fluid, ECF), separated by the plasma membranes of the cells.

Approximately 65% of the ECF comprises the tissue fluid found between cells (interstitial fluid, ISF), and the rest is made up of the liquid component of blood (plasma). The barrier between these two fluids consists of the walls of the capillaries.

Within any one compartment, there must be electrical neutrality, i.e. the total number of positive charges must equal the total number of negative charges.

The most important difference between ICF and ECF lies in the relative concentrations of cat-ions. The K+ ion concentration is much higher inside the cell than in ECF, while the opposite is true for the Na+ ion concentration. Ca2+ and Cl ion concentrations are also higher in ECF.

There is always a steady movement of ions across the membrane, with Na+ and K+ following their concentration gradients into and out of the cell, respectively. Uncorrected, the leak would eventually lead to the equalization of the ions, which is prevented by the Na+-K+ ATPase, or Na+ pump.

Of the other ions, most Ca2+ in the cell is transported actively either out of the cell or into the endoplasmic reticulum and mitochondria, leaving very low levels of free Ca2+ in ICF.


[box type=”download”] — The Donnan equilibrium influencing movement of chloride ions.[/box]

Intracellular proteins are negatively charged at physiological pH. These and other large anions that cannot cross the plasma membrane (e.g. phosphate,  PO43-) are trapped within the cell and account for most of the anion content of ICF. Cl ions, which can diffuse across the membrane through channels, are forced out of the cell by the charge on the fixed anions. The electrical force driving Cl ions out of the cell is balanced by the chemical gradient driving them back in, a situation known as the Gibbs–Donnan equilibrium.

Variations in the large anion content of cells mean that the concentration of Cl ions in ICF can vary by a factor of 10 between cell types, being as high as 30 mM in cardiac myocytes, although lower values (around 5 mM) are more common.


[box type=”download”] –Plasma contents. –Basis of oncotic pressure (large proteins).[/box] Plasma contains more protein than does Interstitial fluid.

The presence of impermeant proteins in the plasma exerts an osmotic force relative to ISF  that almost balances the hydrostatic pressure imposed on the plasma by the heart (blood pressure), which tends to force water out of the capillaries, so that there is a small net water movement out of the plasma into the interstitial space which is absorbed by the lymphatic system.

Transcellular fluid is the name given to fluids that do not contribute to any of the main compartments, but which are derived from them. It includes cerebrospinal fluid and exocrine secretions, particularly gastrointestinal secretions, and has a collective volume of approximately 2 L.[/et_pb_text][/et_pb_column]
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Lesson tags: Osmosis
Back to: Physiology > Basic cellular physiology