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The heart with the main arteries and veins.
Figure 20.1 The heart is the primary organ of the cardiovascular system, controlling circulation and blood flow for the entire body. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Blood moves through the cardiovascular system to deliver oxygen and nutrients to the various tissues, including those that make up the body’s organs. It is pumped by the heart into arteries, which carry oxygenated and nutrient-rich blood to each tissue. Arteries branch into smaller arteries called arterioles, then into capillaries. Capillaries are thinned-walled blood vessels that facilitate gas exchange, or the exchange of oxygen for carbon dioxide in the tissues. Blood is collected into small veins called venules, which drain into larger veins that eventually bring blood back to the heart for oxygenation. See Heart Failure Drugs for more information on the circulatory system.

Blood is made up of many components, such as erythrocytes (red blood cells), leukocytes (white blood cells, which are important in the immune system), and platelets. Erythrocytes contain hemoglobin, which is the substance that binds with oxygen and facilitates its transport in the blood. Platelets are the sticky substances in the blood that form a scab when a client has a cut or injury. More information on platelets will be discussed later in this chapter. Blood also contains plasma, which is the fluid surrounding the cellular components of blood and is made up of water, electrolytes, and proteins.

The amount of the various components of blood can be monitored by a laboratory blood test called a complete blood count (CBC). The CBC measures the amount of red blood cells, white blood cells, hemoglobin, hematocrit, and platelets. Hematocrit is another way to assess the amount of red blood cells by measuring the ratio of red blood cells to the overall volume of blood. Low hemoglobin and/or hematocrit can signal blood loss or anemia, among other etiologies.


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