Circulatory System

The Circulatory system is the transport system that supplied oxygen and nutrient to the tissue + return CO2 to the lungs.

Human Circulatory System comprises the systemic circulation, pulmonary circulation + lymphatic circulation. The main function of circulatory function are given below:

1. The transport of oxygen from lung to tissue + CO2 tissue to the lungs.
2. The transport of metabolic waste product from various body tissue to the kidney for their removable by the kidney.
3. Transport of nutrient absorb from the intestine to the liver + body tissues.
4. Regulation of body temp by altering blood flow to the skin.
5. Distribution of hormone from the endocrine gland to the target tissue.

Blood vascular System:

A well-developed Blood vascular System is present in higher invertebrate from annelid + Echinodermata and also in all chordates.

It is mainly of 2 types:

1. Open type of Circulatory System: In higher invertebrate like Prawn, insect the blood doesn't remains confined to the blood vessels but flows freely in the open spaces of body cavity + channel called lacunae + sinuses in the tissue.
2. Closed types circulatory system: The blood vascular system of human consist of heart, blood vessels and the blood. This is closed that is blood always remain with in the heart + blood vessels. The blood is distribute to various tissue by the arteries and return to the heart by tissue to the vein.

Heart:

Heart is thick, muscular, contractile, automatic pumping organ of blood vascular system. Wall of heart is made of cardiac muscles fibers while internally it has number of chambers interconnected to one other.

There are 2 kind of heart:

1. Neurogenic heart: In this initiation of heart beat is under the ganglion (nerves control) so one remove from body it stop beating. Eg insect.
2. Myogenic heart: The rhythmic contraction takes place in the heart which are initiated by cardiac muscles it self. Eg: Hearts of vertebrates.

[Diagram: External Structure of Heart - showing Superior venacava, pulmonary artery, pulmonary vein, Mitral valve, Aortic valve, Interventricular Septum, Apex, Inferior Venacava, Tricuspid Valve, right atrium, pulmonary valve]

Human heart is a hollow conical, triangular, muscular organ under the breast bone betn the lung in the thoracic cavity. They are located mediastinum and rests in the cardiac notch of the left lung.

It usually lies little to the left of the mid-line of the thoracic cavity with it apex resting on diaphragm.

It looks like a blunt cone which is 12cm in length and 9cm in width.

It weight of human heart varies from 230 to 280 gm in female + 280 to 340 gm in male.

The heart has three layer namely endocardium, myocardium, pericardium.

• The endocardium lines inner cavity of heart + is made up of thin squamous epithelium.
• The myocardium is thick muscular layer made of the cardiac muscles which is involved in the contraction.
• Pericardium is out layer + it is divided into 2 layers inner visceral and outer parietal pericardial layers forming cavity called pericardial cavity.
• The cavity betn two layers is filled with the fluid called pericardial fluid that lubricates and prevent heart from mechanical shock.

The heart has four chamber 2 auricles (atria) + 2 ventricles.

• Externally the atria can be separated from by circular group called coronary sulcus.
• Ventricle are more thick walled than atria + are separated by interventricular septum.
• Aorta arises from left auricle and it has 3 branched they are:- Right brachiocephalic, left common carotid artery + left subclavian artery.
• Pulmonary artery arises from Right Ventricle + is divided into 2 branches ie right pulmonary artery + left pulmonary artery that carry deoxygenated blood to lungs for oxygenation.
• Superior and inferior venacava open into right auricle and collects deoxygenated blood where as pulmonary vein oxygenated blood from lung to heart in the left auricle.

Internal Structure of heart:

Internally human heart is 4 chamber 2 atria and 2 ventricles. There is a complete separation of oxygenated + deoxygenated blood.

• The atria is thin wall chamber + acts as reservoir of blood entering the heart which are separated each other by interatrial septum. The right atrium receives the opening of superior + inferior venacava as well as coronary sinus. is deoxygenated blood.
• So the opening of inferior venacava is guarded by Eustachian valve + coronary sinus by Thebesian valve that prevent back flow of blood from auricles to vein.
• Left auricles receives oxygenated blood from lung to 4 pulmonary vein.
• Foramen ovale it is an opening in the interatrial septum of the foetal heart through which both the atria communicates with each other. In adult this aperture is closed + represented by a small oval depression called fossa ovalis.
• Each atrium opens into ventricle of its side through an atria ventricular aperture.
• Auricles + Ventricles are separated by auriculo-ventricular septum.
• The walls of ventricles are thicker than those of ratio the thickest wall is found in the left ventricle as it pushes the blood throughout the body with great force.
• Inner wall of ventricle consist of two types of structure called grooves + ridges that make it rough.
• So ridges are the raised muscular column on the wall of ventricles called papillary muscles (trabeculae carneae) which are attached to the flaps tricuspid or bicuspid valve with help of thread like structure called chordae tendineae.
• This help to keep the valve in position + prevent back flow of blood.
• Now each aperture is guarded by a valve that is made of sheat of tough connective tissue.
• The right Atrio-ventricular valve (RAV) consist of three triangular flaps or cusps called tricuspid valve.
• The left Atrio-ventricular valve (LAV) consist of two flaps + is called bicuspid or mitral valve.
• Semilunar valve are found at the opening of aorta called aortic semilunar + pulmonary semilunar valve at pulmonary artery.

[Diagram: Internal structure of heart - Labels: Superior venacava, fossa ovalis, Right auricle, valve of Eustachian, In ferior venacava, opening of coronary sinus, Right auriculo-ventricular valve (RAV), Tricuspid valve, Papillary muscles, chordae tendineae, left auricle, Inter auricular septum, Valve of Thebesian, left auriculo-ventricular valve (mitral/bicuspid), Inter-ventricular septum, cardiac muscles]

Circulation of blood through human heart:

Double circulation: Human heart is common double circulation so double circulation's the passage of some blood twice in the heart through separated pathway for completing one cycle.

It consist of pulmonary circulation and systemic circulation.

1. Pulmonary Circulation: Pulmonary circulation began by pumping of deoxygenated blood by right ventricles. It is carried to the lungs and then it returns to left atrium as oxygenated blood.
2. Systemic circulation: The systemic circulation begins with pumping of oxygenated blood by the left ventricle to aorta. It is carried to all body tissue and deoxygenated blood from these are collected and then return to the right auricle of heart.

Blood circulates twice hence it is called double circulation. Importance: The deoxygenated and oxygenated blood remains separated and they do not remain mix.

[Flowchart: Rt. Ventricle --(deoxygenated)--> lungs --(oxygenated)--> left auricle --(oxygenated)--> left ventricle --(oxygenated)--> blood aorta --> body parts --(deoxygenated)--> venacava --> right auricle --(deoxygenated)--> Rt. Ventricle]

Origin and Conducting System:

Heart keeps beating rhythmically throughout the life. The Conducting System consist of special cardiac muscle fibers so called nodal tissue it is remnant of sinus venosus.

In the heart of mammals there is a specialized area of cardiac muscle fibre called Sinoatrial node (SA node). It is about 3mm wide, 15mm long and 1mm thick and lies at the junction of superior and lateral wall of right atrium.

SA node is the point of impulse generation for heart beat. It generates impulse at the rate of 60-80/min and hence called nature pacemaker of the heart.

Process:

The Origin, conduction and regulation of heart beat involves Sino-Atrial node, atrioventricular node (AV node), the AV bundle or the bundle of His and Purkinje fibres.

1. SA node: As it lies in the junction of superior venacava and wall of rt auricle. The stimulus is conducted at the rate of 1m/sec. The impulse generated at the SA node travel through the atrial wall to reach the AV node.
2. AV nodes: The wave of excitation is now picked up by atrioventricular node or AV node. It is present close to the interatrial septum near the right AV aperture. In 0.3 sec impulse reaches AV node from SA node. Now AV node passes the impulse to ventricle at 1.5-4 m/sec.
3. AV bundle: It is the bundle of muscle fibre that descend from the AV node along the interatrial and the interventricular branches. It receives impulse from AV node and passes to bundle of His.
4. Bundle of His: These are present in the interventricular septum with left and right branches and is connected to the AV bundle. This conduct impulse to the ventricles.
5. Purkinje's fibres: Each bundle of His give out a fine network of fibre spreading all over the ventricles called purkinje's fibres. This the impulse from SA node reaches the ventricular muscles and causes contraction of the heart.

Working Mechanism of Heart:

The heart is a pumping machine. The working of heart consist of two phases ie systole and diastole.

Rhythmic contraction and relaxation of cardiac chamber ie atria and the ventricle in a specific manner during one heart beat constitute a Cardiac cycle on an average heart beat is 72 times/minute. Heart pumps about 5l of blood per minute.

Both atria contract simultaneously and the blood flow into the ventricles and both the ventricles contracts together forcing the blood into pulmonary artery and aorta.

• Systole: It refers to the contraction of cardiac chamber.
• Diastole: It refers to the relaxation of cardiac chamber.
• Joint Diastole: It refers to the relaxed state of both atria and ventricles.

Sequence of changes in Cardiac chamber during one Cardiac cycle:

• During auricular diastole, filling of right atrium with the deoxygenated blood from superior and inferior venacava of left atrium with oxygenated blood from pulmonary veins occurs.
• As the pressure increase in atria the bicuspid and tricuspid valve open and the blood flows into the respective relaxed ventricles.
• Now auricular systole occurs forcing most of blood to ventricles and closing of superior and inferior venacava. It last for about 0.15 second.
• Now, the ventricle are filled with the blood it undergoes ventricular systole that pushes upward dose the AV aperture (ie tricuspid and bicuspid) that prevent the black flow of blood. This produces the first heart sound "lub".
• So, deoxygenated blood enters into pulmonary artery which is carried to lungs where as oxygenated blood enters to aorta and is distributed throughout the body. It last for about 0.25 second.
• Now the ventricular diastole occurs the relaxes the ventricles and pressure falls.
• So the closing of semilunar valve in pulmonary artery and aorta occurs. This produces second heart sound "dub". Thus it prevents the back flow of blood into ventricles. It last about 0.4 sec.
• One complete systole and diastole forms a Cardiac cycle which takes about 0.8 sec. The new cardiac cycle begins with the atrial systole.

To each cardiac cycle there is a louder lub heart sound during ventricular systole and a fainter dub sound during ventricular diastole.

[Diagram: Working mechanism of heart - Labels: Superior venacava, pulmonary semilunar valve, Opening of Superior venacava, Opening of inferior venacava, Opening of coronary sinus, Tricuspid valve, Inferior venacava, Aorta, Aortic semilunar valve, Opening of pulmonary vein, Bicuspid / mitral valve, Interventricular septum, RV, LV]

1. Heart rate: It refers to the number of times the heart beat per minute in humans heart beats 68-72 times/min of rest.
2. Stroke volume: During cardiac cycle or one heart beat the volume of blood pumped the heart is called stroke volume. This is normally 70 ml/beat (from diff ventricles in aorta).
3. Cardiac output: The amount of blood pumped by heart per minute is colled cardiac output or heart output. It is calculated as:
   heart beat x stroke volume = cardiac output
   72 x 70 = 5040 ml/min
   ie about 5 lit per minute.