Cardiovascular System – Anatomy and Physiology

 

The cardiovascular system relates to the heart, blood vessels, and blood. Blood contains proteins in its red blood cells called as hemoglobin which carries oxygen to cells and tissues in the body. The cardiovascular system can be deemed as the transport system of the body.

Cardiovascular System Anatomy and Physiology

Anatomy of the heart

One of the most essential and functional parts of the human body is the heart. The size of the heart could be identical to that of a closed fist and it lies beneath the sternum and between the second and the sixth intercostal space. However, the exact position of it varies to some extent in each person.

The heart acts as the body’s pumping station, by which it pumps blood to the lungs and to the systemic arteries. It is surrounded by pericardium, a sac which is composed of outer (fibrous) and inner (serous) layers. Pericardial space, on the other hand, delicately protects and cushions the heart by having pericardial fluid to lubricate its inner layers. 

In addition, the wall of the heart has three layers, namely:

• Epicardium –is the outer part of the heart. It is actually the visceral layer of the serous pericardium, which attaches to the myocardium of the heart. The function of an epicardium is to safeguard the innermost layers of the heart and also to help in the formation of the pericardial fluid, which aids in decreasing friction between the pericardial membranes. 

• Myocardium – is the middle layer of the heart, it is made up of cardiac muscle fiber which aids in heart contractions. The myocardium is the thickest part of the heart wall, wherein the myocardium of the left ventricle is the thickest as it manages the power needed to transport the oxygen pumped by the heart to the rest of the organs. Furthermore, cardiac conduction is made possible by specialized myocardial muscle fibers. These fiber bundles, consisting of the atrioventricular bundle and Purkinje fibers, carry electrical impulses down the center of the heart to the ventricles. These impulses trigger the muscle fibers in the ventricles to contract.
• Endocardium – is the innermost layer of the heart that contains endothelial tissue composed of small blood vessels and bundles of smooth muscle. This layer outlines the inner heart chambers, covers heart valves, and is running alongside the endothelium of large blood vessels. 

Furthermore, the heart is made up of four chambers namely:

1. The right atrium: is part of the heart that receives non-oxygenated blood from the body’s largest veins — superior vena cava and inferior vena cava — and pumps it through the tricuspid valve to the right ventricle.
2. The right ventricle: is part of the heart that pumps the blood through the pulmonary valve to the lungs, where it becomes oxygenated.
3. The left atrium: is part of the heart that receives oxygenated blood from the lungs and pumps it through the mitral valve to the left ventricle.
4. The left ventricle: is part of the heart that pumps oxygen-rich blood through the aortic valve to the aorta and the rest of the body.

  As the heartbeats, it pumps blood through a system of blood vessels called the circulatory system. The vessels are elastic fibers that transport blood to all the tissues in the body.

The four great vessels of the heart are as below:

• Aorta – The main trunk of the systemic artery system and carries the blood away from the left ventricle. After the blood exits the heart via the aortic valve it travels through aorta make a cane-shaped curve that links with other major arteries to deliver oxygen-rich blood to the brain, muscles, and to other cells. The aorta is more than an inch wide in some places and has three layers:
  • • • The inner layer (or intima)
      • The middle layer (or media)
      • The outer layer (or adventitia)

If there is a problem with an aorta, the heart and the entire body’s supply can be hindered. 

• Superior and inferior vena cava – They carry the deoxygenated blood from the body towards the right atrium. The superior vena cava delivers blood from the head and chest area to the heart, while the inferior vena cava returns blood from the lower body regions to the heart
• Pulmonary artery – It carries the blood away from the right ventricle to the lungs. The blood passes through capillaries adjacent to alveoli and becomes oxygenated as part of the process of respiration, whereas the bronchial arteries supply nutrition to the lungs itself.
• Pulmonary veins – They are four of them, two on the left and two on the right which all carry oxygenated blood from the left and right lungs to the left atrium.

Physiology of Heart

The heart generally functions as a highly effective pump. Blood moves around the body due to the force provided by the heart (heartbeat). With every heartbeat, nutrients and oxygen are deployed to cells. A normal young adult’s heart beats approximately 60 to 80 times per minute, ejects roughly 70 ml of blood per beat and has a total of 5 liters per minute of blood circulated.

Intrinsic Conduction System of the Heart

Cardiac muscles have an inherent rhythmic action, meaning to say they can contract spontaneously and independently if maintained under proper conditions. However, the rhythmicity of the heart muscles occurs differently in each area of the heart. Atria and ventricle must contract sequentially to provide the most effective flow. The order of contraction takes place due to specialized cells of the conduction system that generate and conduct electrical impulses to myocardia cells, otherwise the heart would pump inefficiently and without coordination.

Intrinsic Conduction system, otherwise known as the nodal system, is responsible for setting the basic heart rhythm. It is built in the heart tissue which is a cross between the nervous and muscular tissue. Its presence in the heart is important as it causes the heart to depolarize in only one direction – from atria to the ventricles.

The parts of the intrinsic conduction system are:

1. Sinoatrial (SA) node – this crescent-shaped node of tissue is located at the junction of the superior vena cava and the right atrium. SA node is the start of the conduction system. It functions as a pacemaker for the entire myocardium and initiates roughly 60 to 100 beats per minute in a resting normal heart, although it might change depending on the body’s condition and needs.
2. Ventricular (AV) node – this node is located at the right atrial wall near the tricuspid valve. It is conducted along with the myocardial cells when initiated by the SA node. Quite similar to SA node, its intrinsic rate is approximately 40 to 60 beats per minute. One important role of AV node is how it coordinates the incoming electrical impulses to the ventricles through the atrioventricular (AV) bundle or the bundle of his. The bundle of his divides into the left and the right bundle branches, which terminates in the fibers called Purkinje fibers.

Heart rate is determined by the myocardial cells with the fastest intrinsic rate which is the SA node. Consequently, SA node is called the pacemaker. From the SA Node, the impulse travels to atria, to the AV node, then the atria contract. The impulse is delayed to give time for the atria to finish contracting then it travels to the bundle of his, to the Purkinje fibers, then the ventricles contracts starting at the apex of the heart.

Physiology of the Blood Vessels

The circulatory system is the continual system of tubes through which the blood is pumped around the body. It supplies the tissues with their nutritional needs and gets rids of toxins. Resistance, pressure and force are the three main components that influence the blood circulation. Resistance to flow must be overcome to push blood through the circulatory system. If resistance increases, either pressure must increase to maintain flow, or flow rate must reduce to maintain pressure. Numerous factors can alter resistance, but the three most important are vessel length, vessel radius, and blood viscosity. With increasing length, increasing viscosity, and decreasing radius, resistance is increased. The arterioles and capillary networks are the main regions of the circulatory system that generate resistance, arterioles in particular are able to rapidly alter resistance by altering their radius through vasodilation or vasoconstriction.

The resistance offered by peripheral circulation is known as systemic vascular resistance (SVR), while the resistance offered by the vasculature of the lungs is known as pulmonary vascular resistance (PVR).

How the heart works

Cardiac Activity

The cardiac cycle is a regulated filling and emptying the heart if blood by electrical conduction that causes the heart muscles to contract and relax. The cardiac activity has two major phases: 

• The systole referring to pushing out the blood and pumping it via the body followed by
• A relaxation phase of the heart which is known as Diastole, where the heart fills with blood.

In this process atria contracts at the same time, forcing blood via the AV valves into the ventricles. Closing of the AV valves produces a monosyballic“lup” sound. Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery carrying blood to the lungs, closing of the semilunar valves produces a monosyballic “dup” sound.

 Types of blood vessels 

There are three main types of blood vessels:

• Arteries. They carry oxygen-rich blood away from the heart to all of the body’s tissues. 
• Capillaries. These are small, thin blood vessels that connect the arteries and the veins. 
• Veins. These take the blood back to the heart; this blood lacks oxygen (oxygen-poor) and is rich in waste products that are to be excreted or removed from the body.

Pulmonary conduction of the heart

To determine a person’s circulation efficiency, arterial pulse and blood pressure should be measured. Blood pressure is the pressure that blood exerts on the wall of the blood vessels. The pressure originates in the contraction of the heart, which forces blood out of the heart and into the blood vessels. If the flow is impaired through increased resistance then blood pressure must increase, so blood pressure is often used as a test for circulatory health. Blood pressure can be modulated through altering cardiac activity, vasoconstriction, or vasodilation. Whereas, pulse is a pressure wave produced by the alternating expansion and recoil of arteries with each beat of the left ventricle. To measure the blood pressure in the body it is important to get acquainted with two terms: systolic and diastolic pressure. The presence of these two pressures is brought about by the contraction-relaxation cycle of the heart. Blood pressure is measured in millimeters of mercury (mmHg). The systolic pressure is written at the upper portion while diastolic at the bottom. So if the BP is 110/80 mmHg, 110 is the systolic and 80 is the diastolic.

Heart Physiology Video

The below video explains the functions of the heart in more depth. Heart is an iconic part of the body compared to any other parts of the body. Heart transports nutrients, hormones, waste, heat, hormones and immune cells to the rest of the body. Heart is a big muscly organ weighing 250-350gms and its main role is to maintain pressure, by creating hydrostatic pressure to pump blood out of the heart while creating low pressure to bring it back. Heart is made up of three layers namely epicardium, myocardium and endocardium wherein, the general system of chambers, valves, veins and arteries all work together to circulate blood in the body. Heart is divided into two by septum which further branches off and forms two superior atria and two inferior ventricles. Arteries, veins and capillaries form the three blood vessels of the heart. 

 Conclusion 

Heart is a very vital and important part of the human body. In order for it to function properly, it has four layers, four chambers, and very intricate veins and arteries to carry deoxygenated and oxygenated blood to and from the human body. If blood was not constantly being pumped in and out of the human heart, the body would not be functioning or working properly because the cardiovascular system is responsible for the body’s blood flow, nutrients, and oxygen throughout the body. 

Key Points 

•  Cardiovascular is deemed as the main transport system of the body. Heart is one of the most essential and functional parts of the human body is the heart, acts as the body’s pumping station. The size of the heart is identical to that of a closed fist.
• Heart is made up of three layers: epicardium, myocardium and endocardium.
• Further, divided into four chambers: right atrium, right ventricle, left atrium and the left ventricle. 
• Consists of 4 vessels: aorta, superior and inferior vena cava, pulmonary vein and pulmonary artery. 
• Cardiac activity has two major steps which are systolic and diastolic. 

References

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1. Bailey. R(2019). The 3 layers of the heart wall. Retrieved from https://www.thoughtco.com/the-heart-wall-4022792
2. Michigan Medicine (2019). Anatomy of a human heart.Retrieved from https://healthblog.uofmhealth.org/heart-health/anatomy-of-a-human-heart
3. Rice University(2010). The Cardiovascular System: Blood vessels and circulation. Retrieved from https://opentextbc.ca/anatomyandphysiology/chapter/introduction-ch-20/
4. American Heart Association(2020). Your Aorta: The Pulse of Life. Retrieved from  https://www.heart.org/en/health-topics/aortic-aneurysm/your-aorta-the-pulse-of-life
5. Physiology of circulation. Retrieved from https://courses.lumenlearning.com/boundless-ap/chapter/physiology-of-circulation/