16.1 Introduction to the Heart, Circulation, and Blood Flow

Anatomy of the Heart

The heart is a fist-sized organ positioned in the central thoracic cavity, with the bulk of the heart to the left of the sternum. The heart is surrounded by a fibrous, layered sac called the pericardium. The pericardium protects the heart and provides a low-friction environment for the heart to pump in. The pericardium has two layers: the outer fibrous layer and the inner serous layer. The serous layer is made up of the parietal pericardium (which faces the fibrous layer of the pericardium) and the visceral pericardium (which lines the external heart wall). Between the parietal and visceral pericardium, there is fluid that decreases friction as the heart pumps.

The heart walls are composed of three layers: endocardium (inner layer), myocardium (middle layer), and epicardium (outer layer). The epicardium is identical to, and another term for, the visceral pericardium. The myocardium is the heart muscle that contracts to pump blood throughout the circulatory system. The cells of the myocardium are called cardiac myocytes, which are cardiac muscle cells. The endocardium is the layer that lines the inner chambers and structures of the heart, including the valves.

Figure 16.2 depicts the structure of the heart. The heart is comprised of four chambers. The two upper chambers are called the right and left atria (singular: atrium), which are positioned above the right and left ventricles, respectively.

Figure 16.2 The human heart is structured to pump oxygenated blood to the body for use by the tissues. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Within the heart, valves direct blood flow and prevent the backflow of blood into the heart. These valves open and close based on pressure gradients at coordinated times within the cardiac cycle. Atrioventricular valves are positioned between the atria and ventricles. the tricuspid valve separates the right atrium and ventricle, and the mitral valve separates the left atrium and ventricle. There are also two valves positioned between the ventricles and the blood vessels they pump into. The aortic valve is positioned between the left ventricle and the aorta, and the pulmonary valve (also known as the pulmonic valve) is situated between the left ventricle and the pulmonary artery. These valves are referred to as semilunar valves because their leaflets resemble crescent moons.

The right side of the heart receives deoxygenated blood via the inferior and superior venae cavae (plural term). These are large veins that bring blood from the body and empty into the right atrium. The inferior vena cava (singular term) returns blood from the lower portions of the body, and the superior vena cava returns blood from the upper parts of the body. The heart delivers blood to and receives blood from the lungs via pulmonary arteries and pulmonary veins. The pulmonary veins carry oxygenated blood from the lungs back to the left atrium of the heart.

Although the heart pumps oxygenated blood to all tissues, the cardiac tissue also needs a supply of oxygenated blood. This is accomplished via coronary arteries that branch off of the aorta, which fill with blood during systole (when the heart pumps). Atherosclerotic plaques can form over time in the coronary arteries. This can eventually block blood flow to the heart, leading to ischemia (oxygen deprivation) and tissue death. This is known as a myocardial infarction, or heart attack (see Lipid-Lowering Drugs).

Function of the Heart

The main function of the heart is to pump oxygenated blood to the body for use by the tissues. To accomplish this, the heart receives deoxygenated blood from the venous vasculature and pumps the deoxygenated blood to the lungs, where gas exchange occurs. Gas exchange consists of carbon dioxide leaving the blood via exhalation from the lungs and oxygen moving from the lungs into the bloodstream. Oxygenated blood then returns to the heart and is pumped to the body for use by all the tissues. More information on the cardiac cycle is provided in the next section.

Blood is pumped via the ventricles. The right ventricle pumps against a relatively low pressure in the pulmonary system, whereas the left ventricle must pump against systemic arterial pressure to deliver oxygen. Because of this, the left ventricle is more muscular and powerful than the right ventricle.

Circulation and Blood Flow

Blood circulates from the heart to every tissue of the body. This occurs through a network of arteries, capillaries, and veins. Generally, arteries carry oxygenated blood away from the heart and to the tissues (Arteries = Away). Arteries are thicker-walled blood vessels and give rise to systemic arterial pressure (described in the following section). Arteries branch into arterioles (small arteries), which subdivide into even smaller branches called capillaries. Capillaries are tiny, thin-walled blood vessels that facilitate the exchange of nutrients and oxygen between blood and tissues. Venules (small veins) then collect deoxygenated blood from capillaries, channeling it into larger vessels known as veins, which return deoxygenated blood to the heart.

As mentioned previously, there is a separate circuit by which deoxygenated blood is delivered to and from the lungs for oxygenation (see Figure 16.3). This demonstrates an exception to the previously described system for arteries and veins. In the pulmonary circuit, deoxygenated blood is pumped from the right ventricle into the pulmonary artery (in systemic circulation, arteries carry only oxygenated blood). Once the deoxygenated blood is delivered to the lungs, gas exchange occurs. Carbon dioxide leaves the blood and is exhaled, and oxygen enters the blood. Once oxygenated, the blood returns to the heart via the pulmonary veins and is then pumped to the rest of the body for use.

Figure 16.3 The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Systemic Arterial Pressure

Systemic arterial pressure is pressure measured using a sphygmomanometer (blood pressure cuff) and is commonly just referred to as blood pressure. It is the pressure that can be measured in the arteries. It is dependent on both cardiac output (the amount of blood pumped from the left ventricle per unit of time) and systemic vascular resistance (the resistance to blood flow within the artery, determined by the arterial diameter, which changes based on physiologic conditions). A higher cardiac output or higher arterial resistance increases systemic arterial pressure. Systemic arterial pressure is measured in millimeters of mercury (mm Hg) and often is communicated in a clinical setting by systolic blood pressure/diastolic blood pressure. Systolic blood pressure is the pressure in the arteries during systole, or ventricular contraction (heart pumping). Diastolic blood pressure is lower and is the pressure in the arteries during diastole, or ventricular filling/relaxation (between each pump).

Mean arterial pressure (MAP) also provides a measure of systemic arterial pressure throughout the cardiac cycle. A typical MAP is 70–100 mm Hg. It is calculated using the following formula:

$\mathrm{MAP}=\frac{2}{3} \times \mathrm{Diastolic} \mathrm{Blood} \mathrm{Pressure} + \frac{1}{3} \times \mathrm{Systolic} \mathrm{Blood} \mathrm{Pressure}$

Arterial blood pressure facilitates the delivery of nutrients and oxygen to the tissues (perfusion) and, as such, is highly regulated. It must be high enough to maintain perfusion, but chronically high blood pressure has both acute and chronic risks.

Regulation of blood pressure occurs through several interdependent mechanisms. A summary of these mechanisms follows:

• Baroreceptor reflex: Baroreceptors function as sensors and can sense changes in blood pressure and blood volume. When baroreceptors sense low blood pressure or volume, they cause compensatory physiologic changes in the body to raise the blood pressure or volume. One of these changes is net increased activity of the sympathetic nervous system (the fight-or-flight response mediated by epinephrine and norepinephrine, leading to increased vasoconstriction and, thus, increased arterial pressure). This immediate effect on the sympathetic nervous system is responsible for maintaining blood pressure upon abrupt changes, such as when someone moves from a recumbent position to standing quickly. Other changes that occur in response to baroreceptor activation are increased secretion of the hormones renin and aldosterone, which increase circulating volume.
• Antidiuretic hormone: Antidiuretic hormone (also known as vasopressin) is released in response to low blood pressure (among other triggers). It facilitates passive water reabsorption in the collecting duct of the kidneys, which increases circulating blood volume and cardiac output and thus systemic arterial pressure. Vasopressin can be administered therapeutically to maintain blood pressure in clients with vasodilatory or septic shock (Evans et al., 2021).
• Renin-angiotensin-aldosterone system (RAAS): In the RAAS system, renin facilitates the conversion of angiotensinogen to angiotensin I. Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE). Angiotensin II causes vasoconstriction, which 1) increases vascular resistance and thus systemic arterial pressure; 2) increases the amount of sodium and water resorption in the kidneys, which increases circulating blood volume, cardiac output, and thus systemic arterial pressure; 3) increases secretion of antidiuretic hormone, described above; and 4) increases the release of aldosterone. Aldosterone is a hormone that upregulates a pump called the sodium-potassium-ATPase. This pump facilitates the transport of sodium (and water) back into the body, which increases blood volume and cardiac output and thus systemic arterial pressure. Aldosterone also causes increased potassium excretion. Many drugs work on the RAAS system to regulate blood pressure and will be discussed in Heart Failure Drugs. These include angiotensin-converting enzyme inhibitors (ACE inhibitors), angiotensin receptor blockers (ARBs), and aldosterone antagonists such as spironolactone (Unger et al., 2020).

Normal blood pressure is generally less than 120/80 mm Hg (American Heart Association, 2022). Hypertension refers to clients with blood pressure greater than 130–140/80–90 mm Hg (Unger et al., 2020; Whelton et al., 2018), and hypotension refers to clients with low blood pressure (systolic blood pressure less than 90 mm Hg). Any blood pressure higher than 180/120 mm Hg is considered a hypertensive crisis and requires immediate medical attention to prevent or mitigate acute end-organ damage (Whelton et al., 2018). Antihypertensive and Antianginal Drugs includes a detailed description of hypertension.

Venous Pressure

Venous pressure is the pressure within the veins. Venous pressure is much lower than arterial pressure. While arterial blood is driven by pumping of the muscular heart (left ventricle), venous circulation is powered by contraction of the muscles surrounding the veins that squeeze the blood through. Veins have one-way valves that direct blood toward the heart and prevent backflow. Veins deliver deoxygenated blood to the right atrium, which is then pumped to the pulmonary circuit for oxygenation, returned to the heart, and then is pumped throughout the body again. A normal central venous pressure (measured using an invasive technique in the vena cava) is 8–12 mm Hg (Shah & Louis, 2022).