The HeartThe heart or cardiac muscle is a hollow cone shaped muscular organ that is divided into four

chambers. The heart straddles the midline within the thoracic cavity

just dorsal to the sternum

The base of the heart is located craniodorsally and the apex

points ventrally and to the left. The apex is free within the

pericardium. The heart is a pump that circulates blood throughout the body to nourish and remove

waste

The Structure of the HeartThe heart is covered by a saclike

membrane which has 3 layers*pericardium

tough external layer*parietal

the lining of the pericardium*visceral

covers the surface of the heart

The space between the inner layers is the pericardial space which contain

pericardial fluid The heart is composed of three layers

The outer layer (epicardium)The middle layer (myocardium, the

muscle itself)The inner most layer (endocardium, lines

the chambers and covers the valves)

The Chambers of the HeartThe heart is divided into a right and left side. The hollow of the

heart is divided into four chambers. The cranial chambers

are called the atrium and the ventral chamber is called the

ventricle

A wall (interatrial septum) divides the atria into right and left sides. A wall (the interventricular septum) divides the ventricles into right

and left sides. There is no communication between the two

sides.

The atrium have thin walls and are receiving chambers. The

ventricles do the pumping and have thick walls

The right side of the heart receives blood from the body and sends it to the lungs to be oxygenated.

The left side of the heart receives oxygenated blood from the lungs

and sends it to all tissues. Because the left ventricle pumps the blood to all of the body the

walls are much thicker.

ValvesBetween the atria and ventricles are valves that ensures that blood

flows in only one direction.The right atrium and ventricle are separated by the tricuspid valve and the left atrium and ventricle is separated by the

mitral or bicuspid valve.

The semilunar valves prevent backflow from the arteries into the ventricles and are located at the base of the pulmonary and

aortic arteries.

Conduction SystemThe heart begins pumping in

utero. This action consists of:sinoatrial node (SA node): called the pacemaker. This

impulse contracts the atrium and forces blood into the ventricles

The atrioventricular node (AV node)

This consists of cells that conduct the electrical impulses down to:

The atrioventricular bundle (bundle of his)

This continues down as the Purkinje fibers which stimulate

ventricular contraction

Purkinje fibers

Nerve Function:The autonomic nervous system has two divisions with opposite

actions within the heart.

The parasympathetic division:Mainly supplies the SA and AV nodes. This slows the heart rate, reduces impulse conduction and constricts the coronary arteries

The sympathetic division:Through cardiac nerves and also acts on the SA and AV nodes to increase heart rate and impulse

conduction and dilate the coronary arteries

Cardiac Cycle:The cycle includes the contraction

(sysole) and relaxation (diastole) of the atria and ventricles. The heart chambers do not contract all at once. The two atria contract in unison and as they relax the two ventricles contract, as the ventricles

relax the atria contract.

When the atria contract they force blood through the bicuspid

and tricuspid valves into the ventricles, the semilunar valves

close to prevent blood from entering the arteries

When the atria relax blood enters the atrial chambers from the

pulmonary veins (left) and the vena cava (right) and the

ventricles contract. When the ventricles contract the bicuspid

and tricuspid valves close to prevent backflow of blood into

the atria

Blood vessels:arteries

capillaries veins

Arteries:Arteries carry oxygenated blood

to all structures of the body. Ateries are elastic tubes with thick walls composed of three

layers:*tunica intima (inner)

*tunica media (middle)tunica adventitia (outer)

Arterioles, capillaries and venulesAteries become smaller and

smaller becoming arterioles. The arterioles feed the blood into the

capillaries. The capillaries distribute blood to all tissue and return to the venules which pass the blood into the veins which

return blood to the heart

Veins:These are tubes similar to

arteries but have thinner and less elastic walls. These transport blood back to the heart. The

smallest veins (venules) collect the blood from the capillaries

and connect to larger veins and finally join the vena cava that returns the blood to the heart