THE HEART

Dr. AYISHA QURESHIProfessor

MBBS, MPhil

Why does the heart develop so early?

Location of the Heart:

Heart walls are composed of 3 distinct layers:

1. ENDOCARDIUM (inner): thin layer of endothelium that lines the entire circulatory system (endo means “within”).

2. MYOCARDIUM (middle): composed of cardiac muscle that forms the bulk of heart wall (myo means “muscle”).

3. EPICARDIUM (outer): thin external membrane covering the heart (epi means “on”).

The Pericardium & the Pericardial Sac:

The heart is enclosed in a double-walled, membranous pericardial sac. The sac consists of two layers—a tough, fibrous outer covering and a secretory inner 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 secretory lining secretes the 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.

SOME KEY POINTS ABOUT HEART

Arrangement of the heart musclesThe myocardium consistsof interlacing bundles ofcardiac muscle fibersarranged spirally aroundthe circumference of theheart.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.

Comparison of the Right & Left Side Pumps:

Pulmonary circulation(low pressure/ low resistance system

performing less work) vs

Systemic circulation (high pressure/ high

resistance system performing more work)

↓Left side of the heart

thicker (Stronger Pump) as compared to the Right side

(Weaker Pump) of the heart

Pressure-operated heart valves ensure that blood flows in the right direction through the heart.

The Four Heart Valves

Significance of Heart Valves

Semilunar Valves Atrioventricular Valves

There are NO valves between Atria and Veins

Backflow of blood is usually not a problem b/c: 1. Atrial pressures are usually not much higher

than venous pressures 2. The sites where the vena cavae enters the

atria are compressed due to atrial contraction

Fibrous skeleton of the Heart The fibrous skeleton provides a fairly rigid structure for attachment of the cardiac muscle. The atrial muscle is anchored above the rings, while the ventricular muscle mass is attached to the bottom of the ring. The AV valves and the semilunar valves all lie in the same plane in the fibrous skeleton. Due to the fibrous skeleton, no impulse can travel from ventricular muscle to the cardiac muscle and requires a special pathway.

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 & interatrial septum. It ensures that the blood from the left and right side of the heart does not mix. This also separates the PULMONARY & SYSTEMIC CIRCULATION. 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….WHY DO WE CALL HEART A DUAL PUMP?

The Complete Circuit of Blood Flow

Trace a drop of blood from the point where it enters the heart to the point it

exits it…

O2 poor blood returns from the body thru the Superior & Inferior Vena Cava

↓Enters the Right atrium

↓Right ventricle

↓Pulmonary artery

↓Lungs- PULMONARY CIRCULATION

↓Blood is Oxygenated

↓Pulmonary Veins

↓Left Atrium

↓Left Ventricle

↓Aorta

↓Body- SYSTEMIC CIRCULATION

STRUCTURE OF CARDIAC MUSCLE

Cardiac Muscle Structure(Myocardium)

The structure of Cardiac muscle is identical to the skeletal muscle except the following

differences are seen: 1. Intercalated discs are seen

2. Cardiac muscle is a syncytium

The Cardiac Muscle

• Smaller size • Cardiac muscle arranged as

latticework (fibers dividing, recombining and then spreading again).

• Centrally located nucleus• Outside membrane is called

sarcolemma. • They have LESS sarcoplasmic

reticulum than skeletal muscles and requires Calcium from EXTRA CELLULAR FLUID.

• They have an abundance of MITOCHONDRIA, as they work continuously throughout the life.

• They have a very rich BLOOD SUPPLY.

Intercalated Discs

• Cardiac muscles are branched, have a single nucleus and are interconnected to each other, end to end by INTERCALATED DISCS.

• Intercalated discs are actually cell membranes that separate individual cardiac muscle cells from one another.

• The intercalated discs are made up of:1. Gap Junctions2. Desmosomes

STRUCTURE OF A DESMOSOME:

A desmosome has 2 components: 1. A pair of dense, button-

like cytoplasmic thickenings known as plaque located on the inner surface of each of the two adjacent cells.

2. Strong glycoprotein filaments extend across the space between the two cells and attach to the plaque on both sides (they come together like buttons or zippers).

Thus, because of the intercalated discs, the cardiac muscle is a syncytium in which, when one cardiac cell becomes excited, the action potential spreads from cell to cell

throughout the heart.

Actually, the heart is composed of 02 syncytiums: 1. Atrial Syncytium – which constitutes the walls of

the 2 atria 2. Ventricular syncytium – which constitute the walls

of the 2 ventricles

Because of the syncytial nature of the cardiac muscle, an impulse generated in one part of the

heart spreads through the entire heart.

Thus, unlike skeletal muscle, where graded contractions can be produced (recruitment of motor

units), either all the cardiac muscle contracts or none do. There is no “half-hearted” contraction.

HOW DOES THE CARDIAC MUSCLE CONTRACT?

The cardiac muscle contracts in much the same way as skeletal muscle through the process called

Sliding filament theory, comprising of Power strokes.

The only exception is that the duration of contraction of cardiac muscle is much longer.

Excitation Contraction Coupling in Cardiac Muscle

The mechanism by which the action potential causes the myofibrils of cardiac

muscle to contract.

The strength of contraction of the cardiac muscle depends to a great extent on the

concentration of calcium in the ECF.