16: Cardiovascular System - Blood
Single-celled organisms do not need blood. They obtain nutrients directly from and excrete wastes directly into their environment. The human organism cannot do that. Our large, complex bodies need blood to deliver nutrients to and remove wastes from our trillions of cells. The heart pumps blood throughout the body in a network of blood vessels. Together, these three components—blood, heart, and vessels—make up the cardiovascular system. (Thumbnail image credit: "Drops of Blood Medium" by unknown author is licensed under CC BY 3.0 )
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- 16.1: Introduction to Blood
- Blood serves as the main transport medium of the body, distributing oxygen and other nutrients, housekeeping and sickness fighting cells, and signalling molecules to control body functions throughout the body. This chapter covers the composition and varied functions of blood, the formation of blood cells in red bone marrow, blood typing and its implications for blood transfusions, and several disorders of blood.
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- 16.2: An Overview of Blood
- Like all connective tissues, blood is made up of cellular elements and an extracellular matrix. The cellular elements—referred to as the formed elements—include red blood cells, white blood cells, and cell fragments called platelets. The extracellular matrix, called plasma, makes blood unique among connective tissues because it is fluid. This fluid, which is mostly water, perpetually suspends the formed elements and enables them to circulate throughout the body within the cardiovascular system.
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- 16.3: Erythrocytes
- The erythrocyte, commonly known as a red blood cell (or RBC), is by far the most common formed element. A single drop of blood contains millions of erythrocytes and just thousands of leukocytes. Specifically, males have about 5.4 million erythrocytes per microliter (µL) of blood, and females have approximately 4.8 million per µL. In fact, erythrocytes are estimated to make up about 25 percent of the total cells in the body.
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- 16.4: Leukocytes and Platelets
- The leukocyte, commonly known as a white blood cell (or WBC), is a major component of the body’s defenses against disease. Leukocytes protect the body against invading microorganisms and body cells with mutated DNA, and they clean up debris. Platelets are essential for the formation of blood clots to prevent blood loss in the event of damage to the blood vessel wall.
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- 16.5: Production of the Formed Elements
- Although one type of leukocyte called memory cells can survive for years, most erythrocytes, leukocytes, and platelets normally live only a few hours to a few weeks. Thus, the body must form new blood cells and platelets quickly and continuously. Your body typically replaces the donated plasma within 24 hours and it takes about 4 to 6 weeks to replace the blood cells. The process by which this replacement occurs is called hemopoiesis, or hematopoiesis.
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- 16.6: Blood Typing
- Blood transfusions in humans were risky procedures until the discovery of the major human blood groups by Karl Landsteiner, an Austrian biologist and physician, in 1900. Until that point, physicians did not understand that death sometimes followed blood transfusions, when the type of donor blood infused into the patient was incompatible with the patient’s own blood. Blood groups are determined by the presence or absence of specific marker molecules on the plasma membranes of erythrocytes.