- Myocardium
- Valve
- Conduction system
MYOCARDIUM-
The pumping function of the heart occurs through
– Coordinated contraction (during systole) and
– Relaxation (during diastole) of cardiac myocytes (the myocardium).
- Left ventricular myocytes are arranged in a spiral circumferential orientation to generate vigorous coordinated waves of contraction spreading from the cardiac apex to the base of the heart.
- Right ventricular myocytes have a less structured organization, generating overall less robust contractile forces.
- Myocardial Contraction is achieved by shortening of serial contractile elements (sarcomeres) within parallel myofibrils.
- Atrial cardiomyocytes have cytoplasmic storage granules that contain atrial natriuretic peptide(ANF)
- ventricular myocytes contain B-type natriuretic peptide(BNF).
ANF and BNF are protein hormones that are released in response to increased stretch and
-they both promote arterial vasodilation and stimulate renal salt and water elimination (natriuresis and diuresis)
Valves-
The four cardiac valves—tricuspid, pulmonary, mitral, and aortic—maintain unidirectional blood flow.
The function of the semilunar valves depends on-
- The integrity and
- Coordinated movements of the cuspal attachments.
-Thus, dilation of the aortic root can result in valvular regurgitationhttp://Medihealthserve.
The competence of the atrioventricularvalves depends on
- The proper function of the leaflets
- The tendinous cords
- The attached papillary muscles of the ventricular wall.
– Left ventricular dilation, a ruptured cord, or papillary muscle dysfunction can all interfere with mitral valve closure, causing valvular insufficiency.
Cardiac valves are lined by endothelium and share a similar, trilayered architecture:
• Fibrosa layer– A dense collagenous layer at the outflow surface, connected to the valvular supporting structures and providing mechanical integrity
• Spongiosa layer– A central core of loose connective tissue
• Ventricularis or atrialis layer (depending on which chamber it faces)- A layer rich in elastin on the inflow surface, providing leaflet recoil
Valve endothelium also does not express ABO or histocompatibility antigens, so cryopreserved valvular tissues can be transplanted with relative impunity
Conduction system
Coordinated contraction of the cardiac muscle depends on the initiation and rapid propagation of electrical impulse – accomplished through specialized myocytes in the conduction system
The frequency of electrical impulses is sensitive to
- neural inputs (e.g., vagal stimulation),
- extrinsic adrenergic agents (e.g., circulating adrenaline),
- hypoxia,
- potassium concentration (i.e., hyperkalemia can block signal transmissionaltogether).
The components of the conduction system-
• Sinoatrial (SA) node pacemaker, at the junction of the right atrial appendage and superior vena cava
• Atrioventricular (AV) node, located in the right atrium along the interatrial septum
• Bundle of His (AV bundle), connecting the right atrium to the ventricular septum
• Right and left bundle branch divisions that stimulate their respective ventricles via further arborization into the Purkinje network
The cells of the cardiac conduction system depolarize spontaneously, potentially enabling all of them to function as cardiac pacemakers.
The normal rate of spontaneous depolarization in the SA node (60 to 100 beats/min) is faster than the other components, it normally sets the pace.
If nodal tissues become dysfunctional, other cells in the conduction system can take over, generating, for example, a junctional escape rhythm (usually at a much slower rhythm).
AV node has a gatekeeper function; by delaying the transmission of signals from the atria to the ventricles, it ensures that atrial contraction precedes ventricular systole.