Learning Objectives
By the end of this section, you will be able to:
- Discuss anatomy of the coronaries, valves, arteries, veins, and cardiac muscle
- Review cardiac physiology
- Define cardiac output, stroke volume, preload, and afterload
The cardiovascular system performs the essential role of ensuring that oxygenated blood and essential nutrients are efficiently delivered throughout the body. Understanding the precise anatomy and physiology of the heart and its associated structures, including the heart valves, major arteries and veins, and the cardiac muscle itself, is crucial for grasping their roles in maintaining cardiovascular health. We explore how the heart’s chambers and valves orchestrate the flow of blood, and examine the functional dynamics of cardiac output, stroke volume, preload, and afterload. By integrating this knowledge, you will gain a comprehensive understanding of the heart’s mechanical and physiological processes, which are fundamental to assessing and managing cardiovascular health in clinical practice.
Cardiac Anatomy
The human heart, which is roughly the size of a human fist, resides slightly left to the sternal border (Figure 12.2). The external tissue comprises a thin serous sack, known as the pericardium, that contains roughly 15-50 milliliters of serous fluid (Fender & Zack, 2021). Pericardial fluid permits the contractile action of the heart without adhering to the chest wall.
There are two major venous systems bringing blood to the heart from the systemic circulation, the superior and inferior vena cava, from the right atrium, through the tricuspid valve and into the right ventricle to be sent to the pulmonary unit. The pulmonary vein returns oxygenated blood to the left atrium. The small coronary arteries, which are discussed in 12.5 Myocardial Infarction, bring oxygenated blood to the cardiac muscle for a multitude of reasons. The pulmonary artery delivers deoxygenated blood from the right ventricle to the pulmonary circuit for oxygenation and, once oxygenated, returns the blood to the right atria, through the mitral valve and to the right ventricle to be expelled through the aorta to systemic circulation. Within the four cardiac chambers (left atrium, left ventricle, right atrium, and right ventricle), there is the endocardium that has fibrous structures securing valvular functions inside the heart. The myocardium, which is the middle and thickest part of the cardiac muscle, is encapsulated by the epicardium. This layer is encapsulated by the pericardial sac.
Valves act essentially as “gates” that open and close during different parts of the cardiac cycle. The mitral and tricuspid valves communicate between the atria and ventricles and open during ventricular filling and close during ventricular contraction. In contrast, the aortic and pulmonic valves close during ventricular filling and open during ventricular contraction. The purpose of these structures is to keep a forward flow of blood going to the pulmonary unit for oxygenation and forward flow to the systemic circulation.
Myocardial Cell Physiology
Myocardial cells, also referred to as myocytes, are unique in that they can contract, stretch, and remodel. Also, they can be excited by neurological impulses. An electrical impulse is generated by the native “pacemaker” or sinoatrial node. Deep within the cardiac myocyte is a sodium-potassium ion pump, in which there is an ion exchange that generates an electrical impulse, defined as an action potential (Figure 12.3). However, other influences from the adrenal glands, the brain, and changes in volume status can also influence the nerve impulse. These impulses, ignited by an action potential, occur under the influences of sodium ions going in and potassium ions going out. With these chemical exchanges, an impulse is generated through the membrane and creates a muscular contraction.
Cardiac Physiology Terminology
Each minute, the heart perfuses a total of 2 to 4 liters of blood out through the aorta, known as cardiac output. Cardiac output varies depending on disease state, gender, age, and other factors. The stroke volume is the amount of blood pumped out of the left ventricle during systolic contraction. The initial stretch of the cardiac cells prior to contraction is called preload, while afterload is the amount of pressure the heart needs to exert during ventricular contraction. Medications and certain diseases, such as coronary artery disease and heart failure, can affect these functions. Nurses must have a fundamental understanding of these terms as these measurements will be greatly affected during a cardiac ailment and with pharmacological actions.