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Exchange Across Capillaries
The primary function of our systemic capillaries is to exchange nutrients and waste products between the tissue and blood plasma portion of our body. Things like water, proteins, hormones, nutrients (glucose, etc), waste products (ammonia), gases (oxygen, carbon dioxide), electrolytes, etc must all be readily exchanged across our capillaries. Most of these things are dissolved in our blood plasma. How exactly do they move across wall of the capillaries? Our capillaries are a single endothelial layer thick and between these endothelial cells, there are tiny slits and junctions. These pores allow the movement of not only the blood plasma but also the things dissolved in that blood plasma. Due to the limited size of the pores, large particles such as red blood cells do not make their way across the capillary wall. The next question is, what exactly creates this movement of fluid across the capillary wall in the first place? It turns out that on the arteriole end of the capillary, the hydrostatic pressure (41.3 mmHg) is greater than the osmotic pressure (28 mmHg) and so there is a net movement of fluid out of the capillary and into the tissue space. This is when the cells of the tissue receive the nutrients such as glucose and gases such as oxygen. On the other hand, the hydrostatic pressure (21.3 mmHg) is less than the osmotic pressure (28 mmHg) on the venule side and that means that the net fluid flow will be in the opposite direction (into the capillary). This is when waste products such as ammonia and carbon dioxide will move out of the tissue and into the blood plasma.
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