How does this work?
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Very few tissues in the mammalian body have direct supply of ‘whole’ blood – only kidneys, liver and heart muscle. A proportion of the fluid in whole blood passes out through the walls of capillaries to allow osmosis to transfer thousands of substances directly to and from all cells. This proportion doesn’t feed directly back into the bloodstream but, as wh0fuckingcares has said, is returned by the lymphatic system to the venous drainage close to the right side of the heart, mostly.
Take some blood and water and squirt it on a paper towel.
Now wait a while and watch the blood move to the edges of the paper towel.
strange but I found this by accident. and it does happen.
Capillaries actually go EVERYWHERE. Diffusion definitely plays a role, but most cells will be in contact with some derivative of the circulatory system. For example, when people get thrombotic illnesses, you see the manifestation in multiple organs and even on the skin where you might not appreciate there being viable capillaries.
Blood doesn’t reach every cell, It’s simply the primary fluid for sending nutrients, but there are others. The cells make use of the nutrients around them via osmosis.
Blood is made of a yellow liquid called plasma. Plasma carries food,hormones,water, protiens and other essential components. When the blood reaches its target cells by travelling through blood vessels,all these essential components travel out of the blood vessel except for blood protiens as they are too big. Think of it as a fat person unable to move through a narrow door. This liquid is now known as tissue fluid. Tissue fluid and red blood cells can travel out of the blood vessel and into the cell by passing through the plasma membrane. Plasma membrane works like security guards who control what goes into and out of the cell.
If you like anime, I recommend Cells at Work. It’s a cute show that follows a plucky red blood cell and white blood cell doing their jobs circulating the body. I honestly remember way more about immune cells from this show than from school
Interstitial fluid. This is the fluid that surrounds your cells. Nutrients diffuse across thin capillary walls into the interstitial fluid. Nutrients then diffuse or are transported by membrane proteins into the cells. Capillaries do branch out nearly every where in the body. Not every cell is directly next to a capillary but one is usually never very far away.
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