(For more detail, refer to Chapter 10, Essentials of Exercise Physiology)
The main roles of the cardiovascular system during exercise are:
- To delivery oxygen and nutrients to the working muscles;
- To dissipate heat from the body’s core to the skin (thermoregulation); and
- Transport hormones that help regulate the exercise response throughout the body.
This summary document provides you with an overview of the basic cardiovascular physiology that you will need to know before we discuss the cardiovascular system’s role in exercise.
Components of the Cardiovascular System:
The cardiovascular system consists of a “pump” – the heart itself and the “the pipes”, the arteries and veins. The right atrium receives deoxygenated blood from the inferior and superior vena cava, which then empties into the right ventricle. The right ventricle pumps deoxygenated blood into the lungs through the pulmonary artery. Blood is oxygenated in the lungs and returns to the left atrium via the pulmonary veins. From the left atrium, blood empties into the left ventricle and then this oxygen-rich blood is pumped to the systemic circulation through the aorta. Oxygenated blood is distributed to the organs, brain, muscles and skin through a network of arteries and capillaries and then deoxygenated blood is returned to the heart through veins and the major vessels, the inferior and superior vena cava.
One-way blood flow is maintained in the heart through a series of valves. The atrioventricular (A-V) valves are situated within the heart and allow unidirectional flow from the atria to the ventricles. During ventricular contraction, these valves shut and prevent backflow from the ventricle to the atria. Between the right atrium and ventricle is a tricuspid valve, meaning it has “3 leaves” or cusps. Between the atrium and ventricle on the left is a bicuspid valve, called the mitral valve. The semi-lunar valves (the aortic and pulmonic) are located in the arterial wall of the great vessels (aorta and pulmonary artery, respectively) and prevent backflow into the heart during ventricular contraction.
• HEART ANATOMY TABLE
• HEART VALVES TABLE
Figure 1. The circulation of the blood through the atria, ventricles and lungs is depicted in this diagram. Oxygenated blood is shown in red and deoxygenated blood is shown in blue.
The cardiac muscle consists of specialized myocardial cells, which resemble skeletal muscle cells but are involuntary muscle and have the inherent ability to initiate electrical signal and contract independently. Individual myocardial cells are interdigitated forming a network which allows for quick transmission of the action potential (leading to depolarization) throughout the network of cells. At each interdigitation between neighboring muscle cells, there is an “intercalated disc” which is a specialized synapse that allows for quick transmission of the action potential. Among the cardiac muscle cells, there is also a network of highly specialized conducting, muscle cells known as the Purkinje fibers (named after the scientist who discovered them). This network of cells is located within the endocardium of the heart muscle. These cells are dedicated to the transmission of the electrical signal from the sinoatrial and atrioventricular nodes to the myocardial cells, and assist with the synchronized contraction of the heart.
Figure 2. A) Light microscope image of myocardial cells (H&E stained, longitudinal view) – multinucleated, interdigitated B) Cross-sectional view showing the specialized conducting cells (Purkinje fibers) Note that the conducting cells appear clear since they have fewer contractile elements and are specialized for conducting electrical signal.
Blood Supply of the Heart
The myocardial muscle is highly aerobic, meaning that it requires a continuous supply oxygen to function. The myocardial cells have a great concentration of mitochondria to produce energy from aerobic metabolism and their main sources of energy are glucose, fatty acids and lactate. Even in resting conditions, the heart muscle extracts 60-70% of the total oxygen in the blood, which is much higher than skeletal muscle which only extracts 20-30% at rest. This means that there is little reserve for the heart to extract more oxygen from the circulation at times of stress, such as exercise. The coronary arteries supply oxygenated blood to the myocardium. The major arteries are shown in the Figure below. The coronary arteries are perfused only during diastole, when the heart muscle is relaxed. This allows blood to flow from the vessels into the relaxed muscle fibers. Therefore, as the heart rate increases and the period of diastole becomes shorter, there is less time for myocardial perfusion. The myocardial cells have a “refractory period” of 0.30 sec during which they cannot be depolarized after their last depolarization. This “rest” period for the cell provides sufficient time fore ventricular filling between beats.
Figure 3. Schematic diagram of the major coronary vessels, which supply the myocardium.
The oxygen demand of the heart is an important function to measure during periods of stress, such as exercise. Myocardial oxygen demand or MVO2 is expressed using the “Rate Pressure Product”:
Without oxygen, the cardiac myocytes can only function anaerobically for a few minutes before experiencing damage. Ischemia is a lack of oxygen to the heart muscle, which is completely reversible (no permanent damage to the heart muscle) with rest and/or medications, such as nitrogylcerine, which restore the balance between myocardial oxygen demand and supply. If the ischemic period is prolonged, then an infarction can occur. An infarction is permanent damage to the heart muscle, such that the infarcted area no longer contracts properly. A myocardial infarction is also known as a heart attack.
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