Your bloodstream is critical to your survival, because it's what carries oxygen and nutrient molecules to your body cells. Nutrient molecules that you absorb from the digestive tract can dissolve in the blood, and cells remove them from the blood as needed. Oxygen doesn't dissolve in blood, but red blood cells in the bloodstream carry it to the cells.
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Your circulatory system is an efficient mechanism for getting oxygen and nutrients to the body cells. The system consists of a heart, which functions as a pump, plus a network of vessels and the liquid -- blood -- that flows through them. Your heart pumps blood to the lungs for oxygen, after which blood returns to the heart. The heart then sends the blood out to the body cells, after which the blood once again returns to the heart.
Because blood from the heart travels to the lungs before being sent out to the body, the blood going to your body cells is rich in oxygen. Oxygen doesn't dissolve well in water, and blood is water-based. Red blood cells can carry oxygen, however, using a protein called hemoglobin. Hemoglobin contains four oxygen-binding units called heme, each of which picks up one oxygen molecule, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Each red blood cell contains many, many hemoglobin proteins.
To get nutrients to the cells, your body relies upon the circulatory system. One of the organs to which the heart sends blood is the small intestine; as you digest your food, the intestine absorbs the nutrient molecules into the blood vessels that pass through it. The nutrient molecules then travel directly to the liver, through a specialized vessel called the hepatic portal vein. This ensures that nutrients go to the liver -- an important metabolic organ -- first.
Like oxygen, fat isn't soluble in blood. As such, it can't travel to the liver through the hepatic portal vein. Instead, fat that you consume ends up in the lymphatic system, explains Dr. Lauralee Sherwood in her book "Human Physiology." The lymphatic system is a network of vessels similar to blood vessels, but without a pumping organ. The fluid in the vessels, called lymph, is similar to blood without the red blood cells. From the lymphatic vessels, fat eventually makes its way into the bloodstream in the form of packaged particles.