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Human Circulatory System - I

Human circulatory system, also called the blood vascular system consists of a muscular chambered heart, a network of closed branching blood vessels and blood, the fluid which is circulated.

Human heart
Heart is a mesodermally derived organ and is situated in the thoracic cavity, in between the two lungs, slightly tilted to the left. It has the size of a clenched fist. It is protected by a double walled membranous bag, pericardium. The outer is parietal pericardium and inner is visceral pericardium. The cavity between two pericardial membranes is filled with pericardial fluid that protects the heart from shocks, mechanical injury and allows free movements of the heart during heart beat.


Structure of Heart

Our heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called ventricles. A thin, muscular wall called the inter-atrial septum separates the right and the left atria, whereas a thick-walled, the inter-ventricular septum, separates the left and the right ventricles. The atrium and the ventricle of the same side are also separated by a thick fibrous tissue called the atrio-ventricular septum.However, each of these septa are provided with an opening through which the two chambers of the same side are connected. The opening between the right atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve, whereas a bicuspid or mitral valve guards the opening between the left atrium and the left ventricle. The openings of the right and the left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves. The valves in the heart allows the flow of blood only in one direction, i.e., from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta. These valves prevent any backward flow.

Muscles of Heart

Histologically, the heart wall has three muscular layers.

1. Outer layer - Visceral pericardium or epicardium (smooth coelomic epithelium)

2. Middle layer-  Myocardium (Thick muscular layer)

3. Inner layer - Endocardium (simple squamous epithelium)

Blood supply to the Heart

The heart itself requires the oxygenated blood. The muscles of auricles and ventricles are supplied with oxygenated blood through coronary arteries with a branch from aorta. A series of coronary veins collect impure blood from heart and joins to form coronary sinus which opens into the right auricle. Heart receives about 4% of cardiac output.



Blood Circulation Through the Heart

There are two separate chambers for the oxygenated and de oxygenated blood. The right atrium and right ventricle are the chambers for de oxygenated blood while left atrium and left ventricle are for oxygenated blood. There is no chance of mixing of blood. The circulation of the blood in the body is as follows:

1. Circulation begins with the right atrium that receives de oxygenated blood from the body through two main vessels:

a. Superior venacava - It collects the impure blood from head and upper parts of the body.

b. Inferior venacava - It collects impure blood from lower body parts.

2. Due to the contraction of right atrium, the impure blood is pumped into right ventricle.

3. When the right ventricle contracts, de oxygenated blood is forced through a set of semi lunar valves into the pulmonary artery which carries the blood to the lungs for purification.

4.After the blood gets oxygenated in the lungs, it is returned to the left autrium through the left and right pulmonary veins. From there the oxygenated blood is passed to the left ventricle.

5. As the left ventricle contracts, oxygenated blood is pumped forcefully through the systemic aorta and distributed to all parts of the body.



Heart Beat

The rhythmic contraction and relaxation of the heart muscles is known as heartbeat. It includes two phases:

a. a contraction phase - Systole

b. a relaxation phase - Diastole

The contraction and relaxation of different parts of heart takes place in a definite order. Atria and ventricles never contract at the same time. Atria contract first which is followed by the ventricular contraction. The heart beat is completed in the following stages:

1. Atrial systole

It is the simultaneous contraction of both atria. As a result the blood is passed from the atria to the ventricles. Atrial systole lasts for 0.1 second.

2. Ventricular systole

It is the simultaneous contraction of both ventricles. As a result, blood is forced into the aorta and the pulmonary trunks. During this, atria are relaxing. Ventricular   systole lasts for 0.3 second.

3. Joint diastole

In this stage, both atria and ventricles are relaxing. This stage is called general pause. It lasts for 0.4 second. During this stage blood enters the atria from various blood vessels.

So, total time taken for cardiac cycle or heartbeat of an average person at rest is about 70 to 80 times per minute.

Origin and conduction of heart beat

The human heart is myogenic type, in which the heart beat originates from a muscle and regulated by the nerves. The heart beat is controlled by a mass of neuromuscular tissue called node. It produces electrical impulses. There are two types of nodes in the heart:


a. Sinoatrial node (SA node) or Pacemaker

It is located in the wall of right atrium near the opening of superior venacava. SA node generates wave of contraction which radiate throughout the atria, These waves are picked up by the entire atrial wall which results in the contraction of both atria at one time. SA node is also known as natural pacemaker because it initiates the heart beat.

b. Atrioventricular node(AV node) or Pace setter

It is located near the right atria at atrioventricular septum. It gives a mass of specialized nerve fibers called Bundle of His present in the inter ventricular septum. It passes down the septum to reach the apex and then divides into right and left branches called Purkinje fibers. Each branch extends through the wall of ventricle on its corresponding side. These fibers form a network which spread on the entire walls of the ventricle.

After picking up the wave of contraction propagated by the SA node, AV node produces contraction waves which spread along the Bundle of His and Purkinje fibers to all around the ventricles and thus excites the muscles of the ventricles to contract.

Control of Heart beat

The rate of heart beat is controlled by the nervous and endocrine systems:

1. Nervous control

The sinoatrial node receives impulses from the brain by two sets of  nerve fibers.

i. Sympathetic that stimulates the SA node and accelerate the heart beat.

ii. Parasympathetic or Vagal that inhibits the SA node and retard the heart beat.

Nerve center: Vasomotor  center lies in the medulla oblongata.

2. Hormonal control

Two hormones from the medulla part of adrenal glands epinephrine and nor epinephrine accelerate the heart beat at the time of emergency. These hormones directly influence the SA node.