Cardiovascular system: Anatomy of Human Heart

Human Heart

How does the Human heart work? What does it do? What is it composed of? How do you examine it?

Introduction

The heart is a muscular organ containing four chambers. Its main function is to pump blood around the circulatory system of the lungs and the systemic circulation of the rest of the body. In the average day the heart beats about 100,000 times and never rests. It must continue its cycle of contraction and relaxation in order to provide a continuous blood supply to the tissues and ensure the delivery of nutrients and oxygen and the removal of waste products.

Factors affecting cardiac output.

• The size and locati on of the heart
• The overall structure of the heart
• The heart muscle and the cells of the heart
• The blood supply to the heart muscle
• The flow of blood through the heart
• The electrical pathways of the heart
• The cardiac cycle

Heart & its Surroundings

The heart weighs around 350g and is roughly the size of an adult’s clenched fist. It is enclosed in the mediastinal cavity of the thorax between the lungs, and extends downwards on the left between the second and fifth intercostal space.

If one draws an imaginary line from the middle of the left clavicle down to below the nipple, this is where the most forceful part of the heart, the apex beat, can be felt.

The heart has a middle muscular layer, the myocardium, made up of cardiac muscle cells, and an inner lining called the endocardium. The inside of the heart (heart cavity) is divided into four chambers – two atria and two ventricles – separated by cardiac valves that regulate the passage of blood.

The heart is enclosed in a sac, the pericardium, which protects it and prevents it from over-expanding, anchoring it inside the thorax. The pericardium is attached to the diaphragm and inner surface of the sternum, and is made up of:

• The fibrous pericardium, composed of a loosely fitting but dense layer of connective tissue;
• The serous pericardium or epicardium, composed of the parietal and visceral layers;
• A film of serous fluid between the fibrous and serous pericardium that allows them to glide smoothly against each other.

The structures of the heart

Atria and ventricles

The atria receive blood returning to the heart, while the ventricles receive blood from the atria – via the atrioventricular valves – and pumps it into the lungs and the rest of the body. The left atrium (LA) and left ventricle (LV) are separated from the right atria (RA) and right ventricle (RV) by a band of tissue called the septum.

The RA receives deoxygenated blood from the head and neck and from the rest of the body via the superior and inferior vena cava, respectively. The RV then pumps blood into the lungs (through the pulmonary trunk, which divides into the right and left pulmonary arteries), where it is oxygenated.

The oxygenated blood is returned to the LA via the pulmonary veins and passes into the LV through the cardiac valves. From the LV, it is delivered to the whole body through the aorta.

The RV does not need a huge amount of force to pump blood into the lungs, compared with the LV, which has to pump blood into the rest of the body. The LV has a thicker wall and its cavity is circular, while the RV cavity is crescent shaped with a thinner wall

The structures of the heart

Heart Walls

Pericardium

The heart is surrounded by a membrane called the pericardium (peri = around). This is often referred to as a single sac surrounding the heart but is in fact made up of two sacs (the fibrous pericardium and the serous pericardium) which are closely connected to each other. These two sacs have very different structures

• The fibrous pericardium, a tough, inelastic layer made up of dense, irregular, connective tissue. The role of this layer is to prevent the overstretching of the heart. It also provides protection to the heart and anchors it in place.
• The serous pericardium, a thinner, more delicate, layer that forms a double layer around the heart.
• The parietal pericardium, the outer layer fused to the fibrous pericardium.
• The visceral pericardium (otherwise known as the epicardium) adheres tightly to the surface of the heart.

Between the parietal and visceral pericardium is a thin film of fluid (pericardial fluid) which reduces the friction between the membranes as the heart moves during its cycle of contraction and relaxation. The space containing the pericardial fluid is known as the pericardial cavity, however it must be noted that this ‘space’ is so small it is normally considered to be a
‘virtual’ space.

Myocardium

Underlying the pericardium is the heart muscle known as the myocardium (myo = muscle). The myocardium makes up the majority of the bulk of the heart. It is a type of muscle only found within the heart and is specialised in its structure and function. The myocardium can be divided into two categories, the majority are specialised to perform mechanical work (contraction); the remainder are specialised to the task of initiating and conducti ng electrical impulses. The cardiac muscle cells (myocytes) are held together in interlacing bundles of fibres that are arranged in a spiral or circular bundles. Compared with skeletal muscle fibres, cardiac muscle fibres are shorter in length and have branches

The ends of the cardiac myocytes are attached to the adjacent cells in an end – to – end fashion. At this point there is a thickening of the sarcolemma (plasma membrane) known as intercalated discs. These discs contain two types of junctions:

• Desmosomes hold the cells together so that the fibres do not pull apart Gap junctions allow the rapid passage of acti on potentials (electrical current) between cells.
  Compared to skeletal muscle cells the cardiac myocyte contains one (occasionally two) nucleus, and the mitochondria are larger, and more numerous, making cardiac muscle cells less prone to fatigue. However, cardiac muscle requires a large supply of oxygen and is less able to cope with reductions in the amount of available oxygen.
• The cardiac muscle cells are divided into two, discrete networks separated by a fibrous layer, the atria and the ventricles, and these two networks contract as separate units. Thus, the atria contract separately from the ventricles (see later).
• Within each myocyte are long contractile bundles of myofibrils. Myofibrils are in turn made up of smaller units known as sarcomeres. Contraction of the cardiac muscle is by the shortening of its sarcomeres.
• The cardiac action potential Unlike the normal skeletal muscle, in response to a single action potential a cardiac muscle fibre develops a prolonged contraction which is approximately 10 – 15 times longer in duration than a skeletal muscle contraction due to a plateau phase.
• Cardiac muscle fibres also have a longer refractory period and thus a new contraction cannot be initiated until muscle relaxation is well advanced. Thus, a maintained contraction (tetany) cannot occur in cardiac muscle

Endocardium

• The endocardium (endo = within) is a layer of smooth simple epithelium lining the inside of the heart muscle and the heart valves. It is connected seamlessly with the lining of the large blood vessels that are connected to the heart.

The blood supply to the heart

Although small the heart receives about 5% of the body’ s blood supply. Ensuring that the heart receives a plentiful supply of blood is essential to ensure the constant supply of oxygen and nutrients and the efficient removal of waste products required by the myocardium.

Only the inner part of the endocardium (about 2 mm in thickness) is supplied with blood directly from the inside of the heart chambers. The rest of the heart is supplied by the coronary arteries. The coronary arteries come directly off the aorta just aft er the aortic valve. They continuously divide into smaller branches, forming a web of blood vessels to supply the heart muscle.

Each artery (and its branches) supplies different areas of the heart muscle, It is important to note that the table gives the anatomy as it pertains to most people but there are normal variations in this pattern of blood supply in as much as 30% of the population. These variations have no significance in the normal, healthy person but can be important in the treatment of
cardiac patients.

Each of the main coronary arteries has branches coming off it and supplies different areas of the heart. Other (normal) variations can occur with each artery supplying a greater or lesser area of the heart.
As the coronary arteries are compressed during each heartbeat blood does not flow through the coronary arteries at this time. Thus, blood flow to the myocardium occurs during the relaxation phase; this is the opposite of every other part of the body.

Names of the coronary arteries, their major branches and the areas of the heart they supply.

Blood flow through the heart

Though the heart is a single organ it is best to think of it as two pumps, the right and the left heart pumps. Each pump is made up of two chambers (atrium and ventricle) and their associated valves.

• The right heart pump receives blood from the systemic circulation (the body) and pumps it through the pulmonary circulation (the lungs)
• The left heart pump receives blood from the pulmonary circulation and pumps it out around the systemic circulation.
  It is important to note that deoxygenated blood’ does not refer to blood that has no oxygen in it but blood that has given up some of its oxygen to the tissues. Typically, deoxygenated blood contains 75% of the oxygen that oxygenated blood carries.
• So as can be seen, deoxygenated blood returns from the body to the right atrium and then into the right ventricle from where it is pumped out to the lungs. In the lungs the waste gases are exchanged for oxygen and the oxygenated blood flows into the left atrium and into
  the left ventricle. From the left ventricle the blood is then pumped into the circulation of the body.
• Blood enters the right atrium via the superior vena cava and inferior vena cava and leaves the right ventricle via the pulmonary arteries. Note that even though it is deoxygenated blood leaving the right ventricle it is the vessels that the blood is carried in that makes it arterial or venous. Thus, Blood entering the atria is carried in veins and is therefore venous blood.
• Blood leaving the ventricles is carried in arteries and is arterial blood.
  Blood is transported through the pulmonary circulation and returned to the left atrium through the pulmonary veins, it is then pumped out by the left ventricle into the aorta.

Cardiac valves

When working correctly, the cardiac valves ensure a one-way system of blood flow. They have projections (cusps) held in place by strong tendons (chordae tendinea) attached to the inner walls of the heart by small papillary muscles.

The RA and RV are separated by the tricuspid valve, which has three leaflets. The tricuspid valve allows deoxygenated blood to move from the RA into the RV. From the RV, blood passes through the pulmonary valve (situated between the RV and the pulmonary artery), allowing deoxygenated blood to enter the lungs.

On the left side of the heart, oxygenated blood from the lungs enters the LA from the pulmonary vein. The LA is separated from the LV by the mitral valve (also called bicuspid valve, as it has two leaflets) and blood flows through this valve into the LV. It then passes through the aortic valve into the aorta, which transports oxygenated blood throughout the body.

Key points

• The heart is a muscle that contracts and relaxes, pumping blood through the body
• The heart cavity is divided into two atria and two ventricles separated by cardiac valves
• Blood supply goes into the heart via the coronary arteries and is drained via the coronary veins
• The heart has its own conduction system, the sinoatrial node being its natural pacemaker
• During the cardiac cycle, the chambers of the heart contract and relax, and blood flows from high-pressure to low-pressure areas

REFERENCES

1. Fundamentals of anatomy and physiology for student nurses / edited by Ian Peate and Muralitharan Nair. Blackwell Publishing, ISBN 978-1-4443-3443-2
2. Douglas G et al (2013) Macleod’s Clinical Examination, 13th edn. Edinburgh: Churchill Livingstone.
3. Jarvis S, Saman S (2017) Heart failure 1: pathogenesis, presentation and diagnosis. Nursing Times; 113: 9, 49-53.
4. Marieb EN, Hoehn KN (2015) Human Anatomy and Physiology (10th edn). London: Pearson.
5. National Heart, Lung, and Blood Institute (U.S.) How the Heart Works https://www.nhlbi.nih.gov/health-topics/how-heart-works. Last updated 3/24/2022. 

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