The term haemodynamics refers to the physical principles of blood flow, with ‘flow’ being the amount of blood flow in a given time (mL/min). The cardiovascular system’s main aim is to keep blood flowing throughout the capillaries so that capillary exchange can happen.
Capillary exchange is a two-way traffic type movement of substances that occurs between the blood plasma and the interstitial fluid.
It is important to note that tissue perfusion occurs through capillary exchange, making the blood within the capillaries (which is usually around 250-300ml at a given time) the most important blood within the body.
Chemicals pass through capillary walls through 3 possible routes:
- endothelial cell cytoplasm
- intercellular clefts
- filtration pores of the fenestrated capillaries
Movement through the capillary walls happen by:
- Diffusion: allows exchange through the use of the concentration gradient across a permeable membrane (eg. glucose, oxygen, carbon dioxide and waste);
- Transcytosis: through pinocytosis, fluid droplets are picked up by endothelial cells. Vesicles move across the cell and releases fluid through exocytosis (eg. fatty acids, albumin and hormones);
- Filtration: fluids and solutes from blood capillaries move into the interstitial fluid due to blood hydrostatic pressure (BHP), which is the pressure that water within the blood plasma exerts against blood vessel walls, and interstitial fluid osmotic pressure (IFOP), which is the opposing pressure of the interstitial fluid;
- Reabsorption: fluids and solutes from the interstitial fluid move into the blood capillaries due to blood colloid osmotic pressure (BCOP).
Starling’s Law of the Capillaries refers to the near equilibrium existing between the volume of liquid reabsorbed and the volume filtered. The discrepancy in filtration and re-absorption is normally absorbed back into circulation through the lymphatic system.
If filtration exceeds re-absorption in an excessive way, oedema becomes present due to the abnormal increase in interstitial fluid volume. Excessive filtration can be caused by an increase in capillary blood pressure and capillary permeability, while inadequate re-absorption can be caused by a decrease in the concentration of plasma proteins which in turn lowers the blood colloid osmotic pressure (BCOP).
Below you can find a collection of videos that can help provide a more visual approach to Capillary Exchange.
Capillary Exchange and Oedema animation
capillary exchange
Special thanks to the creators of the featured videos on this post, specifically Youtube Channels Alila Medical Media and Medicina Didactica.
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