All of the living cells of our bodies need nutrients to maintain themselves and carry out their functions. We obtain these nutrients by eating and drinking, with our gastrointestinal (digestive) tract breaking down the food into the components absorbed, mainly through the small intestine. Once in our body, it is the cardiovascular (CV) system that transports them to where they are needed.
The human cardiovascular (CV) system is made up of the heart and three types of blood vessels: arteries, capillaries and veins. Our CV system has two closed circuits, the pulmonary that takes blood from the heart to the lungs and back, and the systemic that takes blood from the heart through the rest of the body and back. The arteries carry blood from the heart to the capillaries, the veins carry it from the capillaries back to the heart. Blood travels these circuits like a double loop, through the pulmonary circuit, then the systemic, the pulmonary again and so on until the day we die.
The purpose of this circulation is to transport substances from one part of the body to another in a speedy manner. The stations of this transport system, where substances enter and leave, are the capillary webs or beds. Capillaries are the smallest of blood vessels, only wide enough to allow the passage of erythrocytes (red blood cells) in single file. They occur in networks, like a web or lace, that spread throughout the tissue that they serve.
Nutrients enter our body as part of the food we eat and drink. The digestive system works to break the food down into the smallest, usable, organic molecules by the time it reaches the small intestine where most absorption occurs. The wall of the small intestine is in folds with capillary beds inside the folds, although on the outside of the intestine. Inside the intestine, covering the folds are villi, frond-like cells. The villi and the folds increase the surface area of the intestine, maximizing the amount of nutrients that can be absorbed. The nutrients follow a gradient path, from high concentration to low. So they pass through the villi into the interstitial fluid coating the small intestine and on into the capillaries, at the end that connects to the venules (smallest veins).
The blood, high in nutrients, travels from the intestines to the liver where it is sifted for any nasties we may have consumed, such as toxins. It then proceeds to the inferior vena cava, the main vein from the lower body, that empties into the right atrium of the heart at the end of the systemic circuit. Then through the pulmonary circuit, feeding nutrients to the lungs through their capillary beds, back to the heart and out through the systemic circuit to feed the rest of the body via the capillary beds laced throughout the body’s tissues.
Arteries coming from the heart divide into smaller and smaller blood vessels; the smallest are called arterioles and they supply relatively high pressured blood to the capillaries. The walls of capillaries are just one cell thick, the blood pressure at the end attached to the arteriole forces fluid out between the cells of the wall. The fluid is thinner than plasma, the non-blood cell component of blood, because the larger blood proteins can’t fit through.
Nutrients are carried through the cell wall with this fluid, to mix with the interstitial fluid that surrounds the capillary and the cells of the tissue around it. The blood pressure within the capillary eases the further you get from the arteriole end, as the fluid leaks out. The flow of fluid from the arteriole end increases the pressure of the interstitial fluid fractionally, so that it leaks into the tail end of the capillary. Several capillaries combine into a venule, the smallest of the veins.
The result of this fluid exchange is that any substance that was in greater concentration within the blood, such as glucose, will leave an increased amount behind in the interstitial fluid, while any substance that was in greater concentration in the interstitial fluid, such as nutrients around the small intestine, will pass more into the blood, reducing the amount in the interstitial fluid. This is a continuous process, occurring with every beat of your heart, every day of your life.
With the nutrients in the neighboring interstitial fluid, cells can uptake them at need. Depending on what and how large the nutrient is, this may occur through diffusion, passive or active transport, or carrier or vesicle mediated transport. But that’s a whole other story!