Human Heart

The heart is almost the size of a large fist and weighs between about 280 to 340 grams in men and 230 to 280 grams in women.

Location of the Heart

The heart is situated between the lungs in the thoracic cavity. The name of this area is mediastinum. The cone-shaped heart base is at the top, behind the sternum, and the great vessels are entering or leaving here. The heart's apex (tip) points down and is just above the midline diaphragm to the left. That's why we might think of the heart as being on the left side because here you can hear or feel the strongest beat. 

The heart is confined to the pericardial membranes. The fibrous pericardium is the outermost layer. It is a loose-fitting sac of strong fibrous connective tissue extending inferiorly over the diaphragm and superiorly over the bases of the large vessels entering and leaving the heart. Serous pericardium is a folded membrane; parietal and visceral layers are given by the fold. 

The parietal pericardium is the lining of the fibrous pericardium. The visceral pericardium often called the epicardium, is located on the surface of the heart muscle. Serous fluid is present between the parietal and visceral pericardial membranes, preventing friction as the heartbeats.

Types of Circulation

The heart circulates blood mainly in two ways: pulmonary circulation and systemic circulation.

• The Pulmonary Circulation: In the pulmonary circuit, deoxygenated blood leaves the right ventricle of the heart via the pulmonary artery and travels to the lungs, and then the oxygenated blood returns through the pulmonary vein to the left atrium of the heart.

• The Systemic Circulation: In the systemic circuit, oxygenated blood leaves the heart and travels through the left ventricle to the aorta, and from there enters the arteries and capillaries where it supplies the body's tissues with oxygen.

There is also another type of circulation called coronary circulation in which oxygenated blood is supplied to the heart.

Layers of the Heart Wall

The three layers of the heart's wall are the epicardium (external layer), the myocardium (middle layer), and the endocardium (inner layer).

• Epicardium: It is the outermost layer of the heart.

• Myocardium: It is the middle layer of the heart and conducts the pumping action.

• Endocardium: It is the innermost layer of the heart and thus covers heart valves.

Chambers of the Heart

The walls of the heart's four chambers are made of the myocardium called cardiac muscle. The chambers are lined with the endocardium, a simple squamous epithelium that also covers the heart valves and continues as its lining (endothelium) into the vessels. The endocardium's important physical feature is not its thinness, but its smoothness. This very smooth tissue prevents blood clotting as blood contact with a rough surface would initiate clotting. 

The heart's upper chambers are the right and left atria, with relatively thin walls separated by a common myocardial wall called the interatrial septum. The lower chambers are the right and left ventricles with thicker walls and the interventricular septum separates them. As you can see, the atria receive blood from either the body or the lungs and the ventricles pump blood into the lungs or the body.

Right Atrium

The two big caval veins return blood to the right atrium from the body. The upper vena cava carries upper body blood, and the lower vena cava carries lower body blood. Blood flows into the right ventricle from the right atrium through the right atrioventricular (AV) valve or the tricuspid valve. The tricuspid valve consists of three endocardium flaps (or cusps) strengthened by connective tissue. 

All valves in the circulatory system have the general purpose of preventing blood backflow. The purpose of the tricuspid valve is to prevent the flow of blood from the right ventricle to the right atrium when contracting the right ventricle. As the ventricle contracts, blood is forced to close the valve behind three-valve flaps.

Left Atrium

In the left atrium, blood comes from the lungs through four pulmonary veins. This blood flows through the left atrioventricular (AV) valve into the left ventricle, also known as the mitral valve or bicuspid valve. When the left ventricle contracts, the mitral valve prevents blood from the left ventricle to the left atrium.

Another function of the atria is to produce a hormone that is involved in maintaining blood pressure. When increased blood volume or blood pressure stretches atria's walls, the cells produce atrial natriuretic peptide (ANP), also known as the atrial natriuretic hormone (ANH). 

ANP decreases kidney reabsorption of sodium ions to excrete more sodium ions in urine, which in turn increases water removal. Water loss reduces the volume of blood and blood pressure. You might have noticed that ANP is an antagonist to the aldosterone hormone, which increases blood pressure.

Right Ventricle

The tricuspid valve closes when the right ventricle contracts and the blood is pumped through the pulmonary artery (or trunk) to the lungs. The pulmonary semilunar valve is at the junction of this large artery and the right ventricle. 

When the right ventricle contracts and pumps blood into the pulmonary artery, its three flaps are forced open. Blood tends to come back when the right ventricle relaxes, but this fills the valve flaps and closes the pulmonary valve to prevent blood from flowing back into the right ventricle. 

Columns of the myocardium called papillary muscles are projecting into the lower part of the right ventricle. Fibrous connective tissue strands, the chordae tendineae, range from the papillary muscles to the tricuspid valve flaps. When the right ventricle contracts, the papillary muscles also contract and pull on the tendineae chordae to prevent the tricuspid valve from reversing. 

If you've ever had a strong wind in your umbrella, you can see what would happen if the chordae tendineae and papillary muscles didn't anchor the flaps of the tricuspid valve.

Left Ventricle

The left ventricle walls are thicker than the right ventricle walls, allowing the left ventricle to contract more vigorously. The left ventricle, through the aorta, the body's largest artery, pumps blood to the body. At the junction between the aorta and the left ventricle is the aortic semilunar valve. The left ventricle's contraction force, which also closes the mitral valve, opens this valve. 

When the left ventricle relaxes, the aortic valve closes to prevent blood from the aorta to the left ventricle. When the mitral valve closes, it prevents backflow of blood to the left atrium; the flaps of the mitral valve are also anchored by chordae tendineae and papillary muscles. 

This is a fibrous connective tissue that anchors the valve flaps ' outer edges and prevents stretching of the valve openings. It also separates the atria and ventricles from the myocardium and prevents the contraction of the atria from reaching the ventricles except through the normal conduction path. 

The right side of the heart receives deoxygenated blood from the body and pumps it into the lungs for oxygen collection and carbon dioxide release. The heart's left side receives oxygenated blood from the lungs, pumping it into the body. Both pumps work simultaneously, i.e. both atria and ventricles contract together.

Cardiac Conduction Pathway

The heart cycle is a sequence of mechanical events regulated by the myocardium's electrical activity. Cardiac muscle cells are capable of contracting spontaneously; there is no need for nerve impulses to cause contraction. The heart produces its own beat and the electrical impulses throughout the myocardium following a very specific route. 

The heart's natural pacemaker is the sinoatrial (SA) node, a specialized group of heart muscle cells located in the right atrium wall just below the upper vena cava opening. The SA node is considered to be specialized because it has the fastest contraction rate, it depolarizes faster than any other part of the myocardium (60 to 80 times per minute). 

The rapid entry of Na+ ions and the reversal of charges on either side of the cell membrane is called depolarization. The SA node cells are more permeable to Na+ ions than any other muscle cells in the cardiac. They depolarize faster, then contract and initiate each heartbeat. 

In the lower interatrial septum, impulses for contraction travel from the SA node to the atrioventricular (AV) node. The transmission of impulses from the SA node to the AV node results in atrial systole. Therefore, the only way for impulses from the atria to the ventricles is known as “bundle of his” or AV bundles. 

In the upper interventricular septum, the AV bundle receives impulses from the AV node and communicates them to the right and left branches of the bundle. From the bundle branches, impulses travel along the fibers of Purkinje to the rest of the ventricular myocardium. The electrical activity of the atria and ventricles is easily depicted by an electrocardiogram (ECG).

If the SA node does not work properly, the heartbeat will be initiated by the AV node, but at a slower rate (50 to 60 beats per minute). The ventricle beat can also be generated by the AV bundle, but at a much slower rate (15 to 40 beats per minute). This can happen in certain types of heart disease that block the transmission of impulses from the atria to the ventricles.

Heart Rate

A healthy adult has a 60 to 80 beats per minute resting heart rate (pulse), which is the SA node depolarization rate. A rate of less than 60 (with the exception of athletes) is called bradycardia while tachycardia is the state in which an extended or consistent rate of more than 100 beats per minute is observed.

This is a concise introduction to the anatomy and functioning of the human heart. This organ works non-stop since birth and circulates blood to every nook and corner of the human body.