CARDIOVASCULAR SYSTEM : Heart

CARDIOVASCULAR SYSTEM

Cardiovascular system includes heart and blood  vessels. Heart pumps blood into the blood vessels.  Blood vessels circulate the blood throughout the body.  Blood transports nutrients and oxygen to the tissues  and removes carbon dioxide and waste products from  the tissues.

HEART

 Intercalated disk is a tough double membranous  structure, situated at the junction between the branches  of neighboring cardiac muscle fbers. It is formed by the  fusion of the membrane of the cardiac muscle branches

(Fig.below).

 

 Intercalated disks form adherens junctions, which  play an important role in the contraction of cardiac  muscle as a single unit.

Syncytium

Syncytium means tissue with cytoplasmic continuity  between adjacent cells. However, cardiac muscle  is like a physiological syncytium, since there is no  continuity of the cytoplasm and the muscle fbers are  separated from each other by cell membrane. At the  sides, the membranes of the adjacent muscle fbers  fuse together to form gap junctions. Gap junction is  permeable to ions and it facilitates the rapid conduction  of action potential from one fber to another. Because  of this, all the cardiac muscle fbers act like a single  unit, which is referred as syncytium.  Syncytium in human heart has two portions,  syncytium of atria and the syncytium of ventricles. Both  the portions of syncytium are connected by a thick non-
conducting fbrous ring called the atrioventricular ring.

Muscle Fibers which Form the Pacemaker

Some of the muscle fbers of heart are modifed into a  specialized structure known as pacemaker. These muscle  fbers forming the pacemaker have less striation.

Pacemaker

Pacemaker is structure in the heart that generates the  impulses for heart beat. It is formed by pacemaker cells called P cells. Sinoatrial (SA) node forms the pacemaker  in human heart. Details of pacemaker are given in next  chapter.

Muscle Fibers which Form Conductive System

Conductive system of the heart is formed by modifed  cardiac muscle fbers. Impulses from SA node  are transmitted to the atria directly. However, the  impulses are transmitted to ventricles through various  components of conducting system, which are explained  in the next chapter.

ENDOCARDIUM

Endocardium is the inner most layer of heart wall. It is a  thin, smooth and glistening membrane. It is formed by a  single layer of endothelial cells, lining the inner surface  of the heart. Endocardium continues as endothelium of  the blood vessels.

VALVES OF THE HEART

There are four valves in human heart. Two valves are in  between atria and the ventricles called atrioventricular  valves. Other two are the semilunar valves, placed at  the opening of blood vessels arising from ventricles,  namely systemic aorta and pulmonary artery. Valves of  the heart permit the fow of blood through heart in only  one direction.

Atrioventricular Valves

Left atrioventricular valve is otherwise known as mitral  valve or bicuspid valve. It is formed by two valvular cusps  or faps (Fig.below).

Right atrioventricular valve is known  as tricuspid valve and it is formed by three cusps.

Brim of the atrioventricular valves is attached to atrioventricular ring, which is the fbrous connection between the atria and ventricles. Cusps of the valves are attached to papillary muscles by means of chordae tendineae. Papillary muscles arise from inner surface of the ventricles. Papillary muscles play an important role in closure of the cusps and in preventing the back
fow of blood from ventricle to atria during ventricular contraction. 

 Atrioventricular valves open only towards ventricles  and prevent the backfow of blood into atria.

Semilunar Valves

Semilunar valves are present at the openings of systemic  aorta and pulmonary artery and are known as aortic  valve and pulmonary valve respectively. Because of the

half moon shape, these two valves are called semilunar  valves. Semilunar valves are made up of three faps.  Semilular valves open only towards the aorta and  pulmonary artery and prevent the backfow of blood into  the ventricles.

 

Fig: Heart