Human heart| structure, functions and blood flow path through heart|

Table of content

• Human Heart
• Overall size, shape, and location
• Layers of heart
• Heart walls
• Heart chamber
• Valves of heart
• Blood supply to heart 
• Heart beat
• Control of heart function
• Blood flow path through heart 
• Sources

HUMAN HEART

Human heart is a four-chambered, powerful muscular pumping organ, pumping function is coordinated by the nervous system. It is the main organ of the circulation system. 

Overall size, shape, and location

Human heart about the size of a clenched fist and varies with body size. It is cone-shaped and has a broad apex and a pointed posterior end.  It is located in the median position of the chest cavity, between the lungs and behind the sternum. It lies on the left side of the cheat cavity. 

Layers of heart

It is a very sensitive organ. Thus, it is protected in various ways: 

• Rib cage: Heart is protected by the rib cage.
• Pericardium: Heart is enclosed in a membrane called the pericardial membrane. Pericardium is inelastic, thus it protects the heart and prevents it from overextension in extreme conditions. 
• Pericardial fluid: Pericardium encloses a cavity called the pericardial cavity, which contains a lymph like fluid called pericardial fluid it act as lubricant, thus facilitating heart movement by reducing friction to a minimum during heart beats. 

Figure 1: location of human heart in thoracic cavity

Heart walls

Heart walls are composed of special types of muscles known as cardiac muscles. These muscles have the ability of rhythmic contraction and relaxation under the control of autonomic nervous system. 

The layer of muscular tissue called the septum. Which  divides the heart walls into the left and right sides. It includes:

• Interracial septum: it separated two upper chambers.
• Interventricular septum: it separate two lower chambers
• Atrioventricular septum: separate  articles from ventricles.

Heart walls have three layers:

• Endocardium: Inner layer.
• Myocardium: Muscular middle layer.
• Epicardium: Protective outer layer.

The epicardium is one layer of pericardium. 

Heart chambers 

The human heart is made up of four chambers: two atria or auricles and two ventricles.

Auricles 

Auricles are thin-walled, smaller, and weaker chambers. They act as receiving chambers. They have to pump blood into the ventricles. Both auricles are separated from one another by an inter-auricular septum. 

The left auricle receives oxygenated blood from the lungs by the pulmonary vein. The right auricle receives deoxygenated blood from the anterior part (head region) of the body by the superior vena cava and from the posterior part of the body by the inferior vena cava. 

Ventricles

Ventricles are thick-walled, larger, stronger, and more muscular because they have to pump blood to the whole body. Right ventricle receives deoxygenated blood from right atrium and then pumps it to the lungs via a pulmonary artery for oxygenation. The left ventricle receives oxygenated blood from the left auricle and pumps it into the aorta. This aorta then divides into many arteries which supply blood to all parts of the body. 

Figure 2: dorsal ventral view of human heart.

Valves of heart

• Tricudpid: the right auricle opens into the right ventricle by an aperture called right auricloventricular apertures. This aperture is guarded by a tricuspid valve. The tricuspid valve is composed of three flaps of connective tissue. The tricuspid valve allows the flow of blood from the right auricle into the right ventricle but not in the opposite direction. 
• Bicuspid: the left auricle opens into the left ventricle via a left auricloventricular aperture. This aperture is guarded is guarded by a bicuspid valve. Bicuspid valve is made up of two flaps of connective tissue. Bicuspids allow blood to enter from the left auricle into the left ventricle in one way only. 

      Both tricuspid and bicuspid valves are connected to the muscles on the inner surfaces of ventricles by a thread-like structure called chordae tendineae. Chordae tendineae support and prevent the inverting of valves. 

• Mitral valve: it acts as a door between the left atrium and the left ventricle, preventing the backward flow of blood. 
• Semilunar valve: the opening of the right ventricle into the pulmonary artery and the left ventricle into the aorta is also guarded by the semilunar valve. It includes an aortic semilunar valve between the aorta and the left ventricle and the pulmonary or Pulmonic valve lies between the right ventricle and the pulmonary artery. 

These valves are shaped like half moons. Semilunar valves allow one-way blood flow from ventricles into vessels. 

Blood supply to heart 

Blood is supplied to the heart wall myocardium by coronary arteries. The right and left coronary arteries are branches of the ascending aorta. The coronary arteries divide and re-divide and form a network of capillaries in the myocardium. After blood supply to the heart wall, the capillaries collect into cardiac (coronary) veins. The majority of cardiac veins drain into the coronary sinus, which opens into the right atrium. 

Heart beat 

Contractions of the heart chambers are called systole, and relaxation is known as diastole. The two auricles contract together for about 0.1 second, and the ventricles contract for about 0.3 seconds. Heart chambers relax for about 0.4 seconds. Thus, the heart cycle takes about 0.8 seconds. For the rest of its life, the heart beats continuously at a rate of 72 to 75 beats per minute. Each contraction of the heart pumps out about 75 ml of blood. In severe exercise, the rate may increase to over 120 beats per minute. 

Control of heart function 

The main function of the heart is to transport blood to all parts of the body with a supply of oxygen and nutrients. 

This function (heart beat) is controlled by the nervous system. 

• Nervous control: the nervous system sends signals to the heart to beat. The rate of heart beat varies during rest, work, and stress. It is also under the control of the nervous system. 
• Endocrine control: hormones in the blood vessels control the constriction or relaxation of blood vessels, which affects your blood pressure. 

Figure 3: inside view of human heart.

Blood flow path through heart

oxygenated blood enters the left side of the heart through the pulmonary veins, emptying directly into the left atrium. From the left atrium, blood flows down into the left ventricle. Most of this flow occurs before a contraction starts. When the heart starts to contract, the atrium contracts first, pushing its remaining blood into the ventricle. 

After a slight delay that permits the atrium to empty fully, the ventricle contracts. The walls of the ventricle are far more muscular than those of the atrium, so the ventricle’s contraction is much more forceful than that of the atrium. As the left ventricle contracts, the blood is prevented from going back into the left atrium by a one-way valve called the mitral valve. All four valves in the heart are one-way valves. Each acts as a flap door that opens in only one direction, ensuring that the blood that passes through them will not flow back. The blood within the contracting left ventricle enters the largest artery of the body, the aorta. Once inside the aorta, the blood is prevented from reentering the left ventricle by the aortic valve. Many major arteries branch from the aorta and carry oxygen-rich blood to all parts of the body. 

The first arteries to branch from the aorta are the coronary arteries, which carry freshly oxygenated blood to the heart muscle itself. Many other arteries also branch from the aorta. For example, two renal arteries leave the aorta and carry blood to the kidneys, where nitrogen wastes are filtered from the blood. 

After supplying oxygen to the cells of the body, the blood makes its way back to the heart through the body's many veins. Two large veins collect all the deoxygenated blood from the systemic circulatory system. The superior vena cava collects blood from the upper body, while the inferior vena cava collects blood from the lower body. These two veins empty directly into the right atrium of the heart. Blood passes from the right atrium into the right ventricle through the tricuspid valve. As the right ventricle contracts, it sends the blood through the pulmonary valve and into the pulmonary arteries, which carry the blood to the lungs. The oxygenated blood then returns from the lungs to the left atrium and the cycle continues.

Figure 4: representation of blood flow path.

Sources

Weinhaus, A. J., & Roberts, K. P. (2005). Anatomy of the human heart. In Handbook of cardiac anatomy, physiology, and devices (pp. 51-79). Humana Press.

Mahadevan, V. (2018). Anatomy of the heart. Surgery (Oxford), 36(2), 43-47.