HEART

HEART
In the embryo, the heart begins to beat at 4 weeks of age, even before its nerve supply has been established.
If a person lives to be 80 years old, his or her heart continues to beat an average of 100,000 times a day,
every day for each of those 80 years. Imagine trying to squeeze a tennis ball 70 times a minute. After a few
minutes, your arm muscles would begin to tire. Then imagine increasing your squeezing rate to 120 times a
minute. Most of us could not keep that up very long,but that is what the heart does during exercise. A
healthy heart can increase its rate and force of contraction to meet the body’s need for more oxygen,
then return to its resting rate and keep on beating as if nothing very extraordinary had happened. In fact, it
isn’t extraordinary at all; this is the job the heart is meant to do.
The primary function of the heart is to pump blood through the arteries, capillaries, and veins. As you
learned in the previous chapter, blood transports oxygen and nutrients and has other important functions
as well. The heart is the pump that keeps blood circulating
properly.
LOCATION AND
PERICARDIAL MEMBRANES
The heart is located in the thoracic cavity between the lungs. This area is called the mediastinum. The base
of the cone-shaped heart is uppermost, behind the sternum, and the great vessels enter or leave here. The
apex (tip) of the heart points downward and is just above the diaphragm to the left of the midline. This is
why we may think of the heart as being on the left side, because the strongest beat can be heard or felt
here. The heart is enclosed in the pericardial membranes,
of which there are three layers.The outermost is the fibrous pericardium, a loosefitting
sac of strong fibrous connective tissue that extends inferiorly over the diaphragm and superiorly
over the bases of the large vessels that enter and leave the heart. The serous pericardium is a folded membrane;
the fold gives it two layers, parietal and visceral.Lining the fibrous pericardium is the parietal pericardium.
On the surface of the heart muscle is the visceral pericardium, often called the epicardium.
Between the parietal and visceral pericardial membranes is serous fluid, which prevents friction as the
heart beats.
The heart is a hollow cone shaped muscular organ that pumps blood through the blood vessels to all parts of the body to reach all cells in the body
Size, Shape and location
The size of the human heart varies with the body size but it is generally about 14cm long and around 9cm wide and it is about the size of ones crunched fist
It is located obliquely within the mediastinum of the thorax and resting upon the diaphram
It is bordered lateraly by the lungs posteriorly by the backbone and anteriorly by the sternum
Coverings of the heart
The heart is enclosed within the pericardium which a triple layered sac that sorrounds and protects the heart
It confines the heart to its position in the mediastinum yet allows it sufficient movement for vigorous and rapid contraction
The pericardium comprises of two principle portions i) The fibrous pericardium ii) The serious pericardium
The outer fibrous pericardium is a tough inelastic, dense irregular connective tissue
The fibrous pericardium prevents overstreching of the heart,provides protection and anchors the heart in the mediastinum
The inner serous pericardium is a thinner and more delicate membrane that forms a double layer around the heart
The inner layer of this sac is the visceral pericardium also known as epicardium.This covers the heart
At the base of the heart, the visceral pericardium turns back upon itself to become the outer layer which is known as the parietal pericardium
The outer parietal layer of the serous pericardium is fused to the fibrous pericardium
Between the two layers of the serous pericardium is a thin film of serous fluid know as the pericardial fluid.
The pericardial fluid is a slippery secretion of the pericardial cells and the reduces friction between the membrane as the heart moves

Layers of the Heart wall
Three layers form the walls of the heart
i) The epicardium- External layer ii) The myocardium- The middle layer iii) The endocardium- The inner layer

The epidcardium
The visceral layer of the serous pericardium which is composed of the mesothelium and the delicate connective tissue that imparts a smooth, slippery texture to the outermost surface of the heart.

The myocardium
This is the cardiac muscle tissue which makes up the bulk of the heart and is responsible for its pumping action
Cardiac muscle fibers are involuntary,striated and branched

The endocardium
Is a thin layer of the endothelium overlying a thin of connective tissue
Chambers and Associated Great Vessels
The heart has two superior atrai and two inferior ventricles. Functionally the heat is a double pump- Rt and lt sides
Entering the Right atrium are the superior vena cava, the inferior vena cava and the coronary sinus. Four pulmonary veins enter the left
The right ventricle discharges bood into the pulmonary trunk; The left ventricle pumps blood into the aorta

Pathway of Blood Through the Heart
The right heart is the pulmonary circuit pump. Oxygen poor systematic blood enters the right atrium, passes into the right ventricle, Through the pulmonary trunk to the lungs and back to the left atrium via the pulmonary veins
The left heart is the systematic circuit pump. Oxygen laden blood entering the left atrium from the lungs flow into the left ventricle and then into the aorta which provides the functionaly supply of all organs .systematic veins return the oxygen depleted blood blood to the right atrium
Heart Valves
The atrioventicular valves( Tricuspid and bicuspid) prevent the back flow into the atrai when the ventricles are contracting the pulmonary and aortic semilunar valves prevent backflow into the ventricles when they are relaxing

Blood supply to the heart: Coronary Circulation
The right and left coronary arteries branch from the aorta and via their main branches (anterior and posterior interventricular,marginal, and circumflex arteries) supply the heart itself
Venous blood, collected by the cardiac veins (great, middle and small), is emptied into the coronary sinus
Blood delivery to the myocardium occurs during heart relaxation.
Properties of Cardiac Muscle Fibers
Microscopic Anatomy
Cardiac muscle cells are branching striated, generally uninucleated cells
They contain myofibrils consisting of typical sarcomeres
Adjacent cardiac cells are connected by intercalated discs containing desmosomes
The myocardium behaves a functionally synctium because of electrical coupling provided by gap junctions
Energy Requirements
Cardiac muscle has abundant mitrochondria and depends primarily on aerobic respiration to form ATP
Mechanism and Events of Contraction
The means of action potential generation in the contractile cardiac muscle mimic that of skeletal muscle
Depolarization of the membrane causes opening of sodium channels and sodium entry which is responsible for the rising phase of the action potential curve
Depolarization also opens slow calcium channels and entry of Ca 2+ prolongs the period of depolarization (creates the plateau)
The action potential is coupled to sliding of the myofilaments by ionic calcium released by the SR and entering from the extracellular space
Compared to skeletal muscle, Cardiac muscle has a prolonged refractory period that prevents
tantalization

REFERENCES
•Bloom, M.D., and D.W. Fawcett (1975) A Textbook of Histology, W.B. Saunders

•Campbell, N.A., J.B. Reece, L.G. Mitchell, and M.R. Taylor (2003) Biology: Concepts and Connections, Benjamin/Cummings

•Hoar, W.S. (1983) General and Comparative Physiology, Prentice-Hall

•Snell, R.S.(2003) Clinical Neuroanatomy for Medical Students. Lippincott Williams &Wilkins

•Rosenzweig, M.R., S.M. Breedlove, and A.L. Leiman (2002) Biological Psychology: An Introduction to Behavioural, Cognitive and Clinical Neuroscience. Sinauer Associates