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Cardiology: Basic Physiology Of The Heart And Mechanisms Of Its Actions
What is the Heart?
The normal Adult Human Heart Weighs 250 to 350g. It has 4 chambers (Two Atria and Two Ventricles-both left and right). How does it function? Why is it so important to the Human physiology?
The left ventricle is two to three times the thickness of the right ventricle. The former constitutes 60% of the total weight. Histologically, the Cardiac muscle forms a syncytium though the myocardial cells are electrically isolated by high resistance membranes. Cardiac muscle possesses intrinsic properties such as excitability, contractility, rhythmicity, conductivity and distensibility.
As the Cardiac Muscle is stretched within physiological limits, the force of contraction increases (starling's law). Though the sinoatrial node (S-A node), atrioventricular node (A-V node), the conducting tissues and Cardiac muscle are all capable of impulse production, in health, the heart heat is initiated by impulses from the S-A node.
Vagal impulses depress the rate of the S-A node and increase the refractory period of the cardiac muscle. Sympathetic fibers arising from the cervical and upper thoracic ganglia supply the heart and they accelerate the S-A node and decrease the refractory period of the myocardium.
The heart as a Pump
Atrial contraction is followed by ventricular contraction. Atrial contraction helps in pumping blood remaining in the atria towards the end of diastole and this helps to augment ventricular filling, thus preparing the ventricles for more effective contraction. Atrial contraction gives rise to the a-wave in the jugular veins. With the onset of ventricular systole, A-V valves close the A-V septum bulge towards the atria giving rise to the c-wave in the jugular veins. As the ventricular pressure increases rapidly, the semi-lunar valves open and blood is ejected into the aorta and the pulmonary artery.
The ventricles eject 60-70% of their contents during each systole. Initial phase is one of rapid ejection. This is followed by the period of slowed ejection. During each contraction, the ventricles eject 70ml of blood which is the stroke volume. The atrial filling continues during ventricular systole and the rise of pressure in the right atria gives rise to the V-wave of the jugular venous tracing. At the end of ejection, the semilunar valves close and the isometric relaxation follows. Though, in a broad sense, the ventricular contraction occurs synchronously on the right and left sides of the heart, closer examination will reveal that the right ventricular ejection starts earlier and is completed slightly later than left ventricular ejection.
Atroventricular valves (A-V Valves) open when the ventricular pressure falls below that of the atria during diastole. Atrial blood flows into the ventricle rapidly in the early part of diastole and this may give rise to the third heart sound (S3).
© 2013 Funom Theophilus Makama