The Cardiovascular System

 

Introduction

            The cardiovascular system is a key component of the circulatory system. This organ system encompasses three crucial components: blood, blood vessels, and the heart.  The cardiovascular system blood circulation through the pumping action of the heart, in addition to facilitating in the transport of carbon dioxide, oxygen, and nutrients (for example, electrolytes) to and from body cells. This is essential in providing nourishment to the body, maintenance of homeostasis, and regulation of body temperature (Iaizzo, 2010). The goal of this essay is to explore the key roles of the cardiovascular system, including the structure and functions of the three components of the cardiovascular system: the heart, blood, and main blood vessels (arteries, veins, and capillaries).

Blood vessels

            The three main blood vessels involved in the cardiovascular system are arteries, veins, and capillaries. Arteries such as the pulmonary arteries and aorta are involved in the arterial circulation. These arteries help to transport blood from the heart to other parts of the body.

Coronary arteries are an exception though, seeing as they transport oxygen-laden blood to the heart muscle. Healthy arteries tend to be elastic and strong but usually narrow in between heartbeats. In this way, arteries help to maintain consistent blood pressure. Their flexible walls also enable arteries to adjust to the rate and amount of blood that flows to various parts of the body.  Arteries branch into arterioles. These are small passages that permit blood flow.  Arterioles further branch to form the smallest blood vessels known as capillaries. These play a key role in enabling blood circulation between arteries and veins. Capillaries aid in the transport of oxygenated blood from arteries to body tissues. They also help in transporting blood back to the veins.  Following their passage via body tissues, capillaries network congregate to form venules which further merge into veins. Veins are essential in transporting blood from body organs back to the heart.

Heart

            The heart is a central component of the circulatory system. It helps to pump deoxygenated blood to lungs as well as oxygenated blood to various parts of the body.  The human heart is characterised by a single ventricle and one atrium per circulation. Collectively, both the pulmonary and systemic circulation consists of four chambers: left ventricle, right ventricle, right atrium, and left atrium (Tieck, 2011). The right atrium is located on the right side of the heart, in the upper chamber.  The right atrium received deoxygenated blood and permits its passage into the right ventricle where it is then transported to the lungs via the pulmonary artery to facilitate the elimination of carbon dioxide and aid in re-oxygenation.

The re-oxygenated blood then moves from the lungs into the left atrium. The pulmonary vein is also involved in transporting the re-oxygenated blood into the left atrium.  From there, the aorta transports the blood to various body organs.

Diagram 1: A diagram of the heart showing its four chambers (Source: Levick, 2013)

The heart walls consist of three layers: the external layer (epicardium); the middle layer (myocardium); and the inner layer (endocardium). The epicardium is situated outside the myocardium. It is made up of adipose tissue and elastic fibers that coalesce to form loose connective tissue. The main roles of the epicardium are to facilitate in the production of pericardial fluid and also shield the inner heart layers. The pericardial fluid which is contained in the pericardial cavity is essential in minimising friction between pericardial membranes. 

            The myocardium which is made up of cardiac muscle tissue constitutes a larger portion of the cardiac wall. The myocardium aids in the pumping action of the cardiac muscle. It also facilitates heart contractions. The myocardium is thick in nature, but the thickness differs in various sections of the heart. For example, the myocardium located on the left ventricle tends to be the thickest. This is because it plays a role in producing the energy required to transport oxygenated blood to various parts of the body.

The endocardium is the innermost lining of the heart chamber. It protects both the heart chambers and valves. Cardiomyocytes are a type of cardiac muscle cell that forms the myocardium and which aids in the contraction of the heart, thereby ensuring blood flow from the heart chambers into the circulatory system.  Cardiomyocytes are rich in mitochondria. This, along with the rich supply of blood facilitates circulation. The sarcomeres are a key component of the cardiomyocytes. Sarcomeres consist of long, fibrous proteins capable of contraction. Actin and myosin form the fibrous protein membrane. Both actin and myosin form the elongated myofibril filaments and have contractile capabilities.  The myoglobin contained in the cardiac muscle helps to store oxygen essential for metabolism. 

            The heart undergoes contraction in three stages. During the first stage known as cardiac diastole, the heart chambers tend to be relaxed and undergo passive filling. The second stage known as atrial systole involves contraction of the atria, resulting in the ventricular filling. During the third and final stage known as ventricular systole, blood enters the aorta and pulmonary artery. Cardiac contraction happens once the heart muscle has received signals from specialised cells located in the walls of the heart. The cardiac conduction system encompasses five key components: the AV node, bundle branches, SA node, Purkinje fibers, and a bundle of His. The contraction process is initiated by the SA node by triggering the contraction of the atrial muscles. The bundle of HIS then moves this signal to the AV node, before moving to the bundle branches via the Purkinje fibers. This essentially triggers the contraction of the ventricles (Levick, 2013). The signal forms an electrical current which can be captured using an ECG (Electrocardiogram).  As the heart pumps blood to various parts of the body, it undergoes contraction and relaxation in what is known as cardiac cycle.  Blood vessels responsible for removing blood from, and supplying blood to, the heart are known as coronary circulation. Coronary arteries supply oxygen-rich blood to the heart, while coronary veins transport oxygen-rich blood to the right atrium.

Blood Circulation

            There are two main circulatory paths that characterise the cardiovascular system: pulmonary circulation and systemic circulation. In pulmonary circulation, oxygen-deficient blood is pumped to the lungs for oxygenation and back to the heart. Oxygen-deficient blood passes into the right atrium via the inferior and superior venae cavae, effectively leaving the systemic circulation (Iaizzo, 2010). The tricuspid valve then pumps blood into the right ventricle and into the pulmonary artery via the pulmonary valve. The pulmonary artery branches into two: the left and right pulmonary arteries, and moves into each lung. Here, the alveoli which contain capillary beds permit entry of blood where oxygen is added, carbon dioxide is removed, and gas exchange takes place.  The capillaries in the lungs and the alveoli experience gas partial pressure gradients that facilitate gas exchange. Pulmonary veins then return the oxygenated blood to the left atrium, effectively bringing to an end pulmonary circulation. This also acts as the start of systemic circulation.

Diagram 2: Pulmonary and systemic circulation (Source: Pittman, 2011)

            Systemic circulation involves the transport of oxygen-rich and nutrient-laden blood from the heart to various body tissues as well as the movement of oxygen-deficient blood to the heart. Pulmonary veins permit the movement of oxygenated blood into the left atrium and then into the left ventricle via the mitral valve (Person & Thies, 2012). The aortic valve then moves blood from the left ventricle into the aorta. The aorta divides into arteries, arteriole, and eventually capillaries. The capillaries act as the site for nutrient and gas exchange. Here, glucose and oxygen move into cells from the blood via diffusion, while carbon dioxide and metabolic waste move from the cell into the blood (Carroll, 2010).  The oxygen-rich blood then moves via capillaries which coalesce into venules, veins and ultimately venae cavae. The venae cavae transports blood into the heart's right atrium. Pulmonary circulation then helps to transport blood from the right atrium for purposes of oxygenation. Blood enters the system circulation once more, effectively completing the blood circulation cycle.  

Conclusion    

            In sum, the cardiovascular system encompasses three key components namely, blood, blood vessels, and the heart. Blood is rich in plasma hormones, oxygen, and dissolved proteins, among other components.  The heart, through its pumping action, helps to transport blood to the lungs for oxygenation and elimination of waste products. The deoxygenated blood is then transported back to the heart before being pumped to various organs and tissues of the body.

  

 

 

References          

Carroll, R.G. (2010). Problem-based Physiology. London: Elsevier Health Sciences.

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Levick, J. R. (2013). An introduction to cardiovascular physiology. Oxford: Butterworth-Heinemann.

Manolis, K. (2013). Circulatory System, The. Hopkins, MN: Bellwether Media.

Person, R.J., &Thies, R. (2012). Physiology. New York: Springer.

Pittman, R.N.  (2011). Regulation of Tissues Oxygenation. San Rafael, CA: Morgan & Claypool

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Tieck, S. (2011). Circulatory System.  Minneapolis, Minnesota:  ABDO. 

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