Capillary diffusion allows blood to deliver nutrients to cells.
Human blood flows through blood vessels to deliver nutrients to cells. Blood is really liquid connective tissue. Like other tissues of the body it has cells, but cells in blood are suspended in a fluid matrix. In addition to moving essential nutrients to feed cells, the function of blood includes the transportation of dissolved gases, hormones, and metabolic wastes. Oxygen from the lungs is carried by the blood to tissues and carbon dioxide from these tissues is taken away in the blood and carried to the lungs.
Nutrients enter into the blood from three locations in the body. The blood picks up nutrients that were absorbed at the digestive tract and transports this "food" to hungry cells. Nutrients are also transported from stored reserves in adipose tissue and from storage reserves in the liver. Waste products, generated by cells, are carried away in the blood and flow to the kidneys to be excreted.
The blood is approximately one half plasma (plasma proteins, water, and other solutes) and the other half of the blood is made up of formed elements (platelets, white blood cells, and red blood cells).
So what exactly are the nutrients that blood must deliver to the cells of our tissues? And how are they transported in the blood?
Plasma. The substances that cells need include substances that are transported by plasma proteins. Plasma proteins makes up 7% of the plasma. More than half of the plasma proteins are composed of albumins. Albumins contribute to osmotic pressure of the plasma but also transport lipids and steroid hormones.
A smaller percentage of plasma equals globulins that move ions, hormones, and lipids. Globulins also have immune functions. A small amount of fibrinogen porteins are found in the plasma. Fibrinogen helps blood clot and can be converted to insoluble fibrin.
Less than 1% of plasma proteins are regulatory proteins. Regulatory proteins in plasma are enzymes, proenzymes, and hormones.
Water. Other substances that cells need are carried with water in the blood. Water accounts for 92% of blood plasma. Organic and inorganic molecules, formed elements, and heat are all transported by water in the blood.
Other Solutes. The remaining 1% of plasma composition is made up of other solutes. Other solutes include electrolytes. Two major plasma electrolytes are sodium and potassium, but there are also a handful of others. Electrolytes are vital for cellular activities and needed to regulate osmotic pressure of the fluids in our bodies.
Organic nutrients are used by cells for ATP (energy) production, growth, and upkeep of the cells. Organic nutrients are included in the 1% of other solutes that make up the plasma portion of the blood. These nutrient solutes are the lipids (fatty acid, cholesterol, and glycerides), carbohydrates (glucose), and amino acids.
Oraganic wastes are included in this 1% of other solutes that is a portion of the plasma composition of blood. Urea, uric acid, creatinine, bilirubin, and ammonium ions are all discarded by cells and carried away in the blood to sites in the body where they are broken down or excreted.
Delivery by Diffusion. The blood flows with nutrients and oxygen into the capillaries. Capillaries penetrate most every tissue in the body. The capillaries are the only blood vessels with walls thin enough to allow diffusion. Oxygen and nutrients diffuse directly across the capillary walls and into the cells of the tissues. Simultaneously, waste diffuses out of tissue cells, across capillary walls, and into the blood stream. Waste is dumped into venules from the capillaries and then the blood enters veins and continues the circulatory circuit.
The balance of nutrients and waste in the plasma is managed by diffusion and the red blood cells keep equilibrium of diffused substances from being reached. When oxygen or carbon dioxide concentrations are high, then the RBCs remove more molecules. When plasma concentrations drop, RBCs contribute their stored reserves.
Diffusion is made possible by capillaries that slow the flow of blood down, only allowing one red blood cell to pass through the capillary at a time. Each red blood cell participates in the two-way exchange of diffusion that allows nutrients to be absorbed out of the blood and waste to be deposited into the blood.
Diffusion is the method, and capillaries are the instrument, by which blood delivers nutrients to cells.