17: Cardiovascular System - Heart
In this chapter, you will explore the remarkable pump that propels the blood into the vessels. The heart functions as a “double pump,” since its contraction develops the pressure that ejects blood into the major vessels for two separate circuits of blood flow: blood from the left side of the heart flows into the aorta for the systemic circuit and blood from the right side of the heart flows into the pulmonary trunk for the pulmonary circuit. From these vessels, blood is distributed to the remainder of the body. Although the connotation of the term “pump” suggests a mechanical device made of steel and plastic, the anatomical structure is a living, sophisticated muscle. As you read this chapter, try to keep these twin concepts in mind: pump and muscle. (Thumbnail image credit: "Human Heart Outside" by Tomáš Kebert & umimeto.org is licensed under CC BY-SA 4.0 )
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- 17.2: Heart Anatomy
- The vital importance of the heart is obvious. If one assumes an average rate of contraction of 75 contractions per minute, a human heart would contract approximately 108,000 times in one day, more than 39 million times in one year, and nearly 3 billion times during a 75-year lifespan. Each of the major pumping chambers of the heart ejects approximately 70 mL blood per contraction in a resting adult. This would be equal to 5.25 liters of fluid per minute and approximately 14,000 liters per day.
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- 17.3: Cardiac Muscle and Electrical Activity
- Recall that cardiac muscle shares a few characteristics with both skeletal muscle and smooth muscle, but it has some unique properties of its own. Not the least of these exceptional properties is its ability to initiate an electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger the contractile mechanism. This property is known as autorhythmicity. Skeletal muscle can not do this. Heart rate is modulated by the endocrine and nervous systems.
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- 17.4: Coronary Circulation
- The period of time that begins with contraction of the atria and ends with ventricular relaxation is known as the cardiac cycle. The period of contraction that the heart undergoes while it pumps blood into circulation is called systole. The period of relaxation that occurs as the chambers fill with blood is called diastole. Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated.
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- 17.5: Development of the Heart
- The human heart is the first functional organ to develop. It begins beating and pumping blood around day 21 or 22, a mere three weeks after fertilization. This emphasizes the critical nature of the heart in distributing blood through the vessels and the vital exchange of nutrients, oxygen, and wastes both to and from the developing baby. The critical early development of the heart is reflected by the prominent heart bulge that appears on the anterior surface of the embryo.