Once a nutrient has entered the bloodstream or the lymphatic system, it may be transported to any part of the body, from the tips of the toes to the roots of the hair, where it becomes available to any of the cells. The circulatory systems are arranged to deliver nutrients wherever they are needed.
The Vascular System
The vascular or blood circulatory system is a closed system of vessels through which blood flows continuously in a figure eight, with the heart serving as a pump at the crossover point. On each loop of the figure eight, blood travels a simple route: heart to arteries to capillaries to veins to heart.
The routing of blood through the digestive system is different, however. Blood is carried to the digestive system (as it is to all organs) by way of an artery, which (as in all organs) branches into capillaries to reach every cell. Blood leaving the digestive system goes by way of a vein. The hepatic portal vein, however, directs blood not back to the heart but to another organ—the liver. This vein again branches into a network of small blood vessels (sinusoids) so that every cell of the liver has access to the newly absorbed nutrients that the blood is carrying. Blood leaving the liver then again collects into a vein, called the hepatic vein, which returns the blood to the heart. The route is thus heart to arteries to capillaries (in intestines) to hepatic portal vein to sinusoids (in liver) to hepatic vein to heart.
An anatomist studying this system knows there must be a reason for this special arrangement. The liver is located in the circulation system at the point where it will have the first chance at the materials absorbed from the GI tract. In fact, the liver is the body’s major metabolic organ and must prepare the absorbed nutrients for use by the rest of the body. Furthermore, the liver stands as a gatekeeper to waylay intruders that might otherwise harm the heart or brain.
The Lymphatic System
The lymphatic system is a one-way route for fluids to travel from tissue spaces into the blood. The lymphatic system has no pump; instead, lymph is squeezed from one portion of the body to another like water in a sponge, as muscles contract and create pressure here and there. Ultimately, the lymph flows into the thoracic duct, a large duct behind the heart. This duct terminates in the subclavian vein, which conducts the lymph into the right upper chamber of the heart. In this way, fat-soluble nutrients absorbed into the lymphatic system from the GI tract finally enter the bloodstream.
Transport of Lipids: Lipoproteins
Within the circulatory system, lipids always travel from place to place bundled with protein, that is, as lipoproteins. When physicians measure a person’s blood lipid profile, they are interested in both the types of fat present (such as triglycerides and cholesterol) and the types of lipoproteins that carry them.
VLDL, LDL, and HDL
As mentioned earlier, chylomicrons transport newly absorbed (diet-derived) lipids from the intestinal cells to the rest of the body. As chylomicrons circulate through the body, cells remove their lipid contents, so the chylomicrons gradually become smaller. The liver picks up these chylomicron remnants. When necessary, the liver can assemble different lipoproteins, which are known as very-low-density lipoproteins (VLDL).
As the body’s cells remove triglycerides from the VLDL, the proportions of their lipid and protein contents shift. As this occurs, VLDL become cholesterol-rich low-density lipoproteins (LDL). Cholesterol returning to the liver from other parts of the body for metabolism or excretion is packaged in lipoproteins known as high-density lipoproteins (HDL). HDL are synthesized primarily in the liver.
The density of lipoproteins varies according to the proportion of lipids and protein they contain. The more lipids in the lipoprotein molecule, the lower the density; the more protein, the higher the density. Both LDL and HDL carry lipids around in the blood, but LDL are larger, lighter, and filled with more lipid; HDL are smaller, denser, and packaged with more protein. LDL deliver cholesterol and triglycerides from the liver to the tissues; HDL scavenge excess cholesterol from the tissues and return it to the liver for metabolism or disposal. These different functions explain why some people refer to LDL as “bad” cholesterol and HDL as “good” cholesterol. Keep in mind, though, that there is only one kind of cholesterol molecule; the differences between LDL and HDL reflect proportions of lipids and proteins within them—not the type of cholesterol.
Health Implications of LDL and HDL
The distinction between LDL and HDL has implications for the health of the heart and blood vessels. High concentrations of LDL in the blood are associated with an increased risk of heart disease, as are low concentrations of HDL. Factors that lower LDL concentrations and raise HDL concentrations include:
- Weight management
- Polyunsaturated or monounsaturated, instead of saturated, fatty acids in the diet
- Soluble fibers
- Physical activity
Nutrients leaving the digestive system via the blood are routed directly to the liver before being transported to the body’s cells. Those leaving via the lymphatic system eventually enter the vascular system, but they bypass the liver at first.
Within the circulatory system, lipids travel bundled with proteins as lipoproteins. Different types of lipoproteins include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL).
Elevated blood concentrations of LDL are associated with a high risk of heart disease, as are low levels of HDL.