Introduction to the Mammalian Heart

Describe the structure of the heart and explain how cardiac muscle is different from other muscles

The heart is a complex muscle that pumps blood through the three divisions of the circulatory system: the coronary (vessels that serve the heart), pulmonary (heart and lungs), and systemic (systems of the body). Coronary circulation intrinsic to the heart takes blood directly from the main artery (aorta) coming from the heart. For pulmonary and systemic circulation, the heart has to pump blood to the lungs or the rest of the body, respectively. In vertebrates, the lungs are relatively close to the heart in the thoracic cavity. The shorter distance to pump means that the muscle wall on the right side of the heart is not as thick as the left side which must have enough pressure to pump blood all the way to your big toe.

Illustration shows blood circulation through the mammalian systemic and pulmonary circuits. Blood enters the left atrium, the upper left chamber of the heart, through veins of the systemic circuit. The major vein that feeds the heart from the upper body is the superior vena cava, and the major vein that feeds the heart from the lower body is the inferior vena cava. From the left atrium blood travels down to the left ventricle, then up to the pulmonary artery. From the pulmonary artery blood enters capillaries of the lung. Blood is then collected by the pulmonary vein, and re-enters the heart through the upper left chamber of the heart, the left atrium. Blood travels down to the left ventricle, then re-enters the systemic circuit through the aorta, which exits through the top of the heart. Blood enters tissues of the body through capillaries of the systemic circuit.

Figure 1. The mammalian circulatory system is divided into three circuits: the systemic circuit, the pulmonary circuit, and the coronary circuit. Blood is pumped from veins of the systemic circuit into the right atrium of the heart, then into the right ventricle. Blood then enters the pulmonary circuit, and is oxygenated by the lungs. From the pulmonary circuit, blood reenters the heart through the left atrium. From the left ventricle, blood reenters the systemic circuit through the aorta and is distributed to the rest of the body. The coronary circuit, which provides blood to the heart, is not shown.

Practice Question

After reviewing Figure 1 above, which of the following statements about the circulatory system is false?

    1. Blood in the pulmonary vein is deoxygenated.
    2. Blood in the inferior vena cava is deoxygenated.
    3. Blood in the pulmonary artery is deoxygenated.
    4. Blood in the aorta is oxygenated.

     

    What You’ll Learn to Do

    • Describe the structure of the heart
    • Describe the cardiac cycle

    Learning Activities

    The learning activities for this section include the following:

    • Structure of the Heart
    • The Cardiac Cycle
    • Self Check: The Mammalian Heart