1. Dr. Ayisha Qureshi Assistant Professor, MBBS, MPhil
CARDIAC MUSCLE
Dr. Ayisha Qureshi
Assistant Professor,
MBBS, MPhil

2. The heart is one of the most easily recognized yet the most mysterious organs of the human body. Heart has been described as the core of love, spirituality, intellect and emotion. The heart was originally thought to be the source of conscious thought. For people whose loved ones have suffered from cardiovascular diseases, heart is a source of stress. For others it is a source of song and poetry…..For a physiologist or a physician, it is primarily a pump performing the function of circulating blood throughout the cardiovascular system. The heart has one basic function: to receive oxygen poor blood from all the tissues of the body, pump it to the lungs where it is oxygenated and then pump it back to the rest of the body.

3. From just a matter of days following conception until death, the beat goes on. In fact, throughout an average human life span, the heart contracts about 3 billion times, never stopping except for a fraction of a second to fill between beats. Within about 3 weeks after conception the heart starts to function and it goes on beating till a person dies whether it is at the age of 35 or 75. The heart is the first organ to become functional. It is a hollow, muscular organ, which has a mass of about 250 to 300 gms and is the size of one’s clenched fist. The heart has a base and an apex. Because we associate base with bottom, just remember that the heart is like a cone; the base of the cone is broad and the apex is pointed. The heart can be thought of as an inverted cone with apex down and base up…..

4. Location of the Heart:
It lies in the thoracic (chest) cavity about the midline between the sternum (breastbone) anteriorly and the vertebrae (backbone) posteriorly, in the ventral quadrant medial to the two lungs (or between the 2 lungs). Place your hand over ur heart: People usually place their hand on the left side of the chest, even thought the heart is actually in the middle of the chest. The heart has a broad base at the top and tapers to a pointed tip, the apex, at the bottom. It is situated at an angle under the sternum so that its base lies predominantly to the right and the apex to the left of the sternum. When the heart beats forcefully, the apex actually thumps against the inside of the chest wall on the left side. Thus, it is wrongly thought that the heart is present on the left side.
The heart’s position between 2 bony structures, the vertebrae and the sternum, makes it possible to manually drive the blood out of the heart when it is not pumping effectively, during the process of cardiopulmonary resuscitation (CPR).

5.  If seen from the outside, the bulk of the heart is the thick muscular walls of the ventricles, the 2 lower chambers. The thinner-walled atria lie above the ventricles. Emerging from the top of the heart are the major blood vessels:
1. The Aorta & the Pulmonary trunk (arteries) take blood from the heart to the lungs (pulm. Art.) & tissues (aorta).
2. Superior & Inferior venae cavae & Pulmonary veins return blood to the heart from the lungs (pulm. v.) & the tissues (venae cavae).

6. You should be able to identify the following parts of the heart: right and left atrium, right and left ventricles, pulmonary artery and vein, superior and inferior vena cava and aorta…..Also pay attention to the fact that all the arteries emerge from the top of the heart so that when the heart contracts, the blood is squeezed from the ventricles into the arteries…..

7. The Pericardium & the Pericardial Sac:
The heart is enclosed in the double-walled, membranous pericardial sac (peri means “around”). The sac consists of two layers—a tough, fibrous covering and a secretory lining.
The outer fibrous covering of the sac attaches to the connective tissue partition that separates the lungs. This attachment anchors the heart so that it remains properly positioned within the chest.
The sac’s secretory lining secretes a thin pericardial fluid, which provides lubrication to prevent friction between the pericardial layers as they glide over each other with every beat of the heart.
Pericarditis, an inflammation of the pericardial sac that results in a painful friction rub between the two pericardial
layers, occurs occasionally because of viral or bacterial infection.

8. Heart walls are composed of 3 distinct layers:
ENDOCARDIUM (inner): thin layer of endothelium that lines the entire circulatory system (endo means “within”)
MYOCARDIUM (middle): composed of cardiac muscle that forms the bulk of heart wall (myo means “muscle”)
EPICARDIUM (outer): thin external membrane covering the heart (epi means “on”)

9. The Cardiac Muscle Branched muscle cells (myofibrils)
Centrally located nucleus
Outside membrane is called sarcolemma.
Cardiac muscles have the same arrangement of actin and myosin, and the same bands, zones and Z discs as skeletal muscles forming sarcomeres.
They do have a poorly developed sarcoplasmic reticulum than skeletal muscles and require Calcium from ECF for contraction as T-tubules are well organized.

10. Arrangement of the heart muscles
The myocardium consists of interlacing bundles of cardiac muscle fibers arranged spirally around the circumference of the heart.
What is the advantage of the spiral arrangement?
When the cardiac muscle contracts & shortens, a wringing effect is produced, efficiently pushing blood upwards towards the exit of the major arteries of the heart.
The effect is exactly like the when u press a tooth paste tube from bottom up…..

11. Intercalated Discs
Although the cardiac muscles interdigitate & branch, there is NO anatomical continuity b/w the individual muscle fibers.
Cardiac muscles are branched, have a single nucleus and are interconnected to each other, end to end by specialized structures called INTERCALATED DISCS. The intercalated discs are further composed of:
Gap Junctions
Desmosomes

13. HEART AS A DUAL PUMP:
Even thought the heart is a single organ, the left and the right side of the heart is anatomically and functionally separate.
This is done with the help of the interventricular spetum.
It ensures that the blood from the left and right side of the heart does not mix.
Although the left and the right sides are separated, the heart contracts in a co-ordinated fashion: the atria contract together and the ventricles contract together….
What we are going to do is to trace a drop of blood as it leaves from the right atrium of the heart……

15. Enters the Right atrium
O2 poor blood returns from the body thru the Superior & Inferior Vena Cava ↓
Enters the Right atrium
Right ventricle
Pulmonary artery
Lungs
Blood is Oxygenated
Pulmonary Veins
Left Atrium
Left Ventricle
Aorta
Circulated to the body