ANATOMY AND PHYSIOLOGY OF THE HEART

ANATOMY AND PHYSIOLOGY OF THE HUMAN HEART

Adeboye Oluwajuyitan

The HeartThe heart is a hollow muscular cone-shaped organ, lying between the lungs, with the pointed end (Apex) directed downwards, forwards and to the left; the Base upwards and to the right. The heart is about the size of a closed fist. Over a typical life span of 76 years, the heart will beat nearly 2.8 billion times and move 169 million liters of blood.The average heart is 12cm long, 9cm wide across its broadest point, and about 6cm thick. Its wall consists largely of cardiac muscle (myocardium), lined and surrounded by membranes; endocardium on the inside, and pericardium on the outside.

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The pericardium consists of two portions with a small amount of fluid in between them. The fluid lubricates the outer wall of the heart so it can beat without causing friction, it holds the heart in place, it forms a barrier against infections and keeps the heart from over expanding.

The Cardiovascular system of the body consists of:HeartBlood vesselsBlood

Chambers of the HeartThe heart has two sides, the right side and the left side. The heart has four chambers. The left and right side each have two chambers, a top chamber and a bottom chamber. The two top chambers are known as the left and right atria. The left atrium receives blood from the lungs and the right atrium receives blood from the rest of the body.

The bottom two chambers are known as the left and right ventricles. The ventricles pump blood out to different parts of the body. The right ventricle pumps blood to the lungs while the left ventricle pumps out blood to the rest of the body. The ventricles have much thicker walls than the atria which allows them to perform more work by pumping out blood to the whole body.

Function and Location of the HeartThe heart's job is to pump blood around the body. The heart is located in between the two lungs. It lies left of the middle of the chest.

Blood vesselsBlood vessels are tubes which carry blood. Veins are blood vessels which carry blood from the body back to the heart. Arteries are blood vessels which carry blood from the heart to the body. There are also microscopic blood vessels which connect arteries and veins together called capillaries. Blood vessels include:

AORTAlargest artery in the bodyapproximately the diameter of your thumbcarries oxygenated blood from left ventricle to other parts of the body

PULMONARY TRUNKsplits into 2 pulmonary arteries (to each lung)collects blood pumped from right ventricle

PULMONARY VEINStake blood from lungs to the left atrium

SUPERIOR AND INFERIOR VENA CAVASuperior vena cava collects veins from head and upper body and Inferior vena cava collects veins from legs and lower torso.

VALVESValves are fibrous flaps of tissue found between the heart chambers and in the blood vessels. They are rather like gates which prevent blood from flowing in the wrong direction. Valves between the atria and ventricles are known as the right and left atrioventricular valves. Right is called Tricuspid Valve; left is Mitral or Bicuspid valve. The Tricuspid valve opens to allow deoxygenated blood collected in right atrium flow into the right ventricle. It closes as RV contracts, preventing blood from returning to RA, thereby forcing it through the pulmonary trunk. It is formed of 3 flaps or cusps of tissue.

The Mitral valve opens to allow oxygenated blood collected in the LA flow into the LV. It closes as LV contracts, preventing from returning to LA, thereby forcing the blood to exit through the aortic valve into the aorta. It is formed of 2 flaps of tissue.

The Chordae tendinae (heart strings) are tiny white cords that anchor the cusps to the walls of the ventricle. They attach margins of the atrio-ventricular valves leaflets to projections of the wall of the ventricles (Cone-shaped Papillary muscles). Rupture of the chordae, through injury, endocarditis or degeneration results in mitral regurgitation.

Valves between the ventricles and greater arteries are known as Semilunar valves. These include the Aortic valve and the Pulmonary valve;

They open when the ventricles contractThey close when the ventricles relax, preventing blood from returning to the heartEach consists of 3 cusps that curve upward to form small pocket.

CIRCULATION OF BLOODThe heart involves in two main cycles of circulation:

PULMONARY CIRCULATIONThis is the flow of impure deoxygenated blood (containing CO2 and wastes products) from the right ventricle of the heart to the lungs for purification and return of pure oxygenated blood (containing Oxygen and nutrients) from the lungs to the left atrium. It is also called lesser circulation. The pulmonary trunk arises from the right ventricle and divides into 2 pulmonary arteries which carry blood to both lungs. Pulmonary veins bring back the oxygenated blood from the lungs to the left atrium.

SYSTEMIC CIRCULATIONSystemic circulation is the flow of pure oxygenated blood from the left ventricle through the aorta to all parts of the body (except lungs) and the return of impure deoxygenated blood back to the right atrium. The functions of systemic circulation are to supply oxygen and nutrients to body tissues and to remove carbon dioxide, heat and other waste material from the organs and tissues.

Cardiac Conduction SystemLooking at the analogy of the central heating system, the pump, pipes and radiators are of no use unless connected to a power supply. The pump needs electricity to work. The human heart has a similar need for a power source and also uses electricity. Thankfully we don't need to plug ourselves in to the mains, the heart is able to create it's own electrical impulses and control the route the impulses take via a specialized conduction pathway.This pathway is made up of 5 elements:The Sino-atrial (SA) nodeThe Atrio-ventricular (AV) nodeThe bundle of HisThe left and right bundle branchesThe Purkinje fibers

The Sino-atrial nodeIt is the primary pacemaker of the heart located in the upper border of the right atriumIt sends impulses in all directions (down & to the left atrium as well)Impulses cannot go directly to the ventricles because there is a dense fibrous tissue around AV groove that act as insulators.The Atrio-ventricular nodeIt is located between atrium and ventricle of the right sideIt receives some of the SA node impulses, then sends impulses to hiss bundle into right and left bundle branches.

Both right and left bundle branches give off Purkinje fibers that innervate ventricles. From Purkinje fibers, pulses go upwards; the heart contracts in accordance with the way the electrical impulses move

Bachmanns bundle (Anterior inter-atrial band)It is a broad band of atrial muscle that runs just behind ascending aorta and connects the top of the right atrium with the top of the left atrium.

ACTION POTENTIAL OF THE HEARTBlood is pumped as chambers contract and relax in turn. The beating of a healthy heart is regulated by electrical impulses. The mechanism of the electrical conduction of the heart follows a patternThe sequence begins as the atria fill with blood with deoxygenated blood on the right and oxygenated blood on the left.An electrical signal from the SA node then causes the atria to contract forcing blood into both ventricles

The electrical signal is then picked up by the AV node and directed into the purkinje fibers in the ventricle walls causing the ventricles to contract. The blood is then pumped through the pulmonary valve on the right to the lungs and the aortic valve on the left to the rest of the body.

These valves close and the cycle then restarts

HOW THE HEART WORKS

Pacemakers of the HeartDefinition: A pacemaker is simply that part of the heart that regulates the rate at which it beats.The primary pacemaker of the heart is the Sino-atrial node. The SA node controls the whole heart because its action potentials are released the most often (highest frequency) to the cardiomyocytes, and arrives before other cells have had a chance to generate their own action potential. When the SA node does not function, ectopic pacemakers (cells further down the heart) will become the hearts pacemaker. i.e. the AV node is the secondary pacemaker. Bundle branches and purkinje fibers will also produce spontaneous action potential if SA and AV node both fail.

The Sino-atrial node: This is a microscopic area of specialized cardiac muscle located in the upper wall of right atrium near the entry of the superior vena cava. It gives the heart a frequency of 60-80bpm.

The Atrio-ventricular node: This is a mass of modified heart muscle situated in the lower middle part of the right atrium. It gives 40-60bpm.

Atirio-ventricular Bundle: A band of modified heart muscle fibers (purkinje fibers) passing from AV node forward to septum between ventricles, where it divides into right and left branches, one for each ventricle; the fibers transmit contraction waves from the Atria via the AV node to the ventricles. Hiss bundle have a rate is 30-40bpm; Bundle branches 20-30bpm.

Purkinje Fibers: They form interweaving networks on endocardial surface of both ventricles, which transmit impulses to the entire right and left endocardium. Impulse from these fibers is about 15-20bpm.

CARDIAC BLOOD SUPPLYARTERIAL BLOOD SUPPLYThe heart is supplied with pure oxygenated blood by its own coronary arterial network. There are 2 coronary arteries, right and left main coronary arteries. Both originate just superior or above the aortic semilunar valve (this area is called the aortic sinus); so they get very fresh blood that is being pumped from the left ventricle.The right coronary artery travels along or parallel to the right atrio-ventricular groove. The most important of its branches is the Right Marginal branch which travels almost all the way down to the apex of the heart. It supplies anterior and lateral aspects of the right ventricle. It wraps around and travels to the posterior aspect of the heart where it forms a descending branch.

The left coronary artery originates more in the posterior side of the aorta just above the valve (left aortic sinus) and travels along the posterior aspect of the pulmonary artery. When it gets to the base of the aorta, it gives off some branches;

Anterior descending artery which supplies a large part of the left ventricle. It continues over the apex to the posterior part of the heart

Left Circumflex artery: this travels from underneath the AV groove around to posterior aspect of the heart

Left Marginal artery: It travels down the left antero-lateral side of the heart.

VENOUS DRAINAGE OF THE HEARTThe venous drainage of the heart is accomplished by veins which accompany the coronary arteries and drain into the Coronary sinus which in turn opens into the right atrium. These veins are:1. The great cardiac vein in the anterior inter-ventricular groove; 2. The middle cardiac vein in the inferior inter-ventricular groove3. The small cardiac vein accompanying the marginal artery along the lower border of the heart4. The oblique vein which descends obliquely on the posterior aspect of the left atrium.

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