What Is the Cardiovascular System?

The cardiovascular system is one of the eleven organ systems of the human body. Its main function is transporting nutrients to cells and wastes from cells (Figure 2.11 “Cardiovascular Transportation of Nutrients”). This system consists of the heart, blood, and blood vessels. The heart pumps blood, and the blood is the transportation fluid. The transportation route to all tissues, a highly intricate blood-vessel network, comprises arteries, veins, and capillaries. Nutrients absorbed in the small intestine travel mainly to the liver through the hepatic portal vein. From the liver, nutrients travel upward through the inferior vena cava blood vessel to the heart. The heart forcefully pumps the nutrient-rich blood first to the lungs to pick up some oxygen and then to all other cells in the body. Arteries become smaller and smaller on their way to cells, so that by the time blood reaches a cell, the artery’s diameter is extremely small and the vessel is now called a capillary. The blood vessel’s reduced diameter substantially slows the blood flow speed. This dramatic reduction in blood flow gives cells time to harvest the nutrients in the blood and exchange metabolic wastes.

Blood’s Function in the Body and Metabolism Support

You know you cannot live without blood, and that your heart pumps your blood over a vast network of veins and arteries within your body, carrying oxygen to your cells. However, what do you know about your blood beyond these basic facts?

Blood transports absorbed nutrients to cells and waste products from cells. It supports cellular metabolism by transporting synthesized macromolecules from one cell type to another and carrying waste products away from cells. Additionally, it transports molecules, such as hormones, allowing for communication between organs. The volume of blood coursing throughout an adult human body is about 5 liters (1.3 US gallons) and accounts for approximately 8 percent of human body weight.

What Makes Up Blood and How Do These Substances Support Blood Function?

Blood is about 78 percent water and 22 percent solids by volume. The liquid part of blood is called plasma and it is mostly water (95 percent), but also contains proteins, ions, glucose, lipids, vitamins, minerals, waste products, gases, enzymes, and hormones. We have learned that the protein albumin is found in high concentrations in the blood. Albumin helps maintain fluid balance between blood and tissues, as well as helping to maintain a constant blood pH. We have also learned that the water component of blood is essential for its actions as a transport vehicle and that the electrolytes carried in blood help to maintain fluid balance and a constant pH. Furthermore, the high water content of blood helps maintain body temperature, and the constant flow of blood distributes heat throughout the body. Blood is exceptionally good at temperature control, so much so that the many small blood vessels in your nose can warm frigid air to body temperature before it reaches the lungs.

The cellular components of blood include red blood cells, white blood cells, and platelets. Red blood cells are the most numerous of the components. Each drop of blood contains millions of them. Red blood cells are red because they each contain approximately 270 million hemoglobin proteins, which contain the mineral iron, which turns red when bound to oxygen. The most vital duty of red blood cells is to transport oxygen from the lungs to all cells in the body so that cells can utilize oxygen to produce energy via aerobic metabolism. The white blood cells circulating in the blood are part of the immune system, and they survey the entire body for foreign invaders to destroy. They make up about 1 percent of blood volume. Platelets are fragments of cells that are always circulating in the blood in case of an emergency. When blood vessels are injured, platelets rush to the injury site to plug the wound. Blood is in a constant state of renewal and is synthesized from stem cells residing in bone marrow. Red blood cells live for about 120 days, white blood cells live anywhere from 18 hours to one year, or even longer, and platelets have a lifespan of about 10 days.

Nutrients In

Once absorbed from the small intestine, all nutrients are transported to the needed cells. Additionally, molecules manufactured in other cells sometimes require delivery to other organ systems. Blood is the conduit, and blood vessels are the highway that support nutrient and molecule transport to all cells. Water-soluble molecules, such as some vitamins, minerals, sugars, and many proteins, move independently in blood. Fat-soluble vitamins, triglycerides, cholesterol, and other lipids are packaged into lipoproteins that allow transport in the blood’s watery milieu. Many proteins, drugs, and hormones are dependent on transport carriers, primarily by the plasma protein albumin. In addition to transporting all of these molecules, blood transfers oxygen from the lungs to all cells in the body. As discussed, the iron-containing hemoglobin molecule in red blood cells is the oxygen carrier.

Wastes Out

In metabolizing macronutrients to energy, cells produce carbon dioxide and water as waste products. As blood travels through smaller and smaller vessels, the rate of blood flow is dramatically reduced, allowing for efficient exchange of nutrients and oxygen for cellular waste products through tiny capillaries. The kidneys remove any excess water from the blood, and the blood delivers the carbon dioxide to the lungs, where it is exhaled. Also, the liver produces the waste product urea from the breakdown of amino acids. It detoxifies many harmful substances, all of which require transport in the blood to the kidneys for excretion.

All for One, One for All

The eleven organ systems in the body completely depend on each other for continued survival as a complex organism. Blood allows for transporting nutrients, wastes, water, and heat, and is also a conduit of communication between organ systems. Blood’s importance to the rest of the body is aptly presented in its role in glucose delivery, especially to the brain. The brain metabolizes, on average, 6 grams of glucose per hour. To avert confusion, coma, and death, glucose must be readily available to the brain at all times. To accomplish this task, cells in the pancreas sense glucose levels in the blood. If glucose levels are low, the hormone glucagon is released into the blood. It is transported to the liver, where it communicates the signal to ramp up glycogen breakdown and glucose synthesis. The liver does just that, and glucose is released into the blood, transporting it to the brain. Concurrently, blood transports oxygen to support glucose metabolism and provide energy to the brain. Healthy blood conducts its duties rapidly, avoiding hypoglycemic coma and death. This is just one example of the body’s survival mechanisms exemplifying life’s mantra, “All for one, one for all.”

What Makes Blood Healthy?

Maintaining healthy blood, including its continuous renewal, is essential to support its vast array of vital functions. Blood is healthy when it contains the appropriate amount of water and cellular components and proper concentrations of dissolved substances, such as albumin and electrolytes. Like all other tissues, blood needs macro- and micronutrients to function optimally. In the bone marrow, where blood cells are made, amino acids are required to build the massive amount of hemoglobin packed within every red blood cell, along with all other enzymes and cellular organelles contained in each blood cell. Like the brain, red blood cells use only glucose as fuel, and it must be in constant supply to support red-blood-cell metabolism. As with all other cells, the cells in the blood are surrounded by a plasma membrane, which is composed mainly of lipids. Blood health is also more acutely sensitive to vitamin and mineral deficiencies than others.

What Can Blood Tests Tell You About Your Health?

Since blood is the conduit of metabolic products and wastes, measuring the components of blood, and particular substances in blood, can reveal the health of blood and other organ systems. Standard blood tests are performed during an annual physical, and the typical blood tests can tell your physician about the functioning of a particular organ or disease risk.

A biomarker is a measurable molecule or trait connected with a specific disease or health condition. The concentrations of biomarkers in blood are indicative of disease risk. Some biomarkers are cholesterol, triglycerides, glucose, and prostate-specific antigen. The blood test results give the concentrations of substances in a person’s blood and display the normal ranges for a certain population group. Many factors, such as physical activity level, diet, alcohol intake, and medicine intake, can influence a person’s blood test levels and cause them to fall outside the normal range, so results of blood tests outside the “normal” range are not always indicative of health problems. The assessment of multiple blood parameters aids in diagnosing disease risk and indicates overall health status. See Table 2.2 “Blood Tests” for a partial list of substances measured in a typical blood test.^[1] This table notes only a few things their levels tell us about health.