Cardiovascular System: Atrial Function and Heart Muscle

Heart Function and Circulation

• The atrial wall contains stretch receptors that monitor changes in intra-atrial pressure and initiate CV reflexes.
• Atrial cells monitor changes in blood volume, leading to the release of Atrial Natriuretic Peptide, which causes the excretion of Na+ and H2O by the kidneys.

Cardiac Muscle

• Cardiac muscle fibers are divided into two types:
  • Contractile fibers (99%): form the atrial and ventricular walls and are specialized for pumping blood.
  • Autorrhythmic fibers (1%): form the conducting system of the heart and are specialized for initiation and conduction of impulses.

Circulation

• There are three types of circulation:
  • Systemic circulation (greater): starts from the left ventricle, passes through the aorta, and leads to large and small arteries, arterioles, capillaries, venules, and veins.
  • Pulmonary circulation (lesser): starts from the right ventricle, passes through the pulmonary artery, and leads to the lungs, pulmonary capillaries, and pulmonary veins.
  • Special circulations: include capillary, venous, lymph, coronary, and cerebral circulations.

Mechanism of Heart Beating

• The impulse originates at the sino-atrial node, causing contraction of the atria, then reaches the atrioventricular node, and finally passes to the ventricles through the His-Purkinje system, causing ventricular contraction.
• Heart rate: 60-90 beats/min, with an average of 72 beats/min in adults.

Functional Components of the Vascular System

• Heart: a dual pump, consisting of a compression pump (ventricles pumping blood into arteries during systole) and a suction pump (atria withdrawing blood from the venous system during diastole).

Functional Histology of Cardiac Muscle

• The heart is a functional syncytium, meaning that it is not anatomically a syncytium, but functions as a single unit.
• The pulmonary artery arises from the right ventricle and is guarded by the pulmonary valve.
• The heart valves allow blood to flow in one direction only.

Heart Valves and Endocardium

• The inner surface of the myocardium is lined with endothelial cells called endocardium, which also lines the cardiac valves and the entire vascular system.
• The atrial myocardium is much thinner than that of the ventricles, with the left ventricular wall being 3 times thicker than the right ventricular wall.

Coronary Arteries and Blood Pressure

• Coronary arteries supply the myocardium, arising from the 1st part of the aorta, and branching into small arteries, arterioles, capillaries, venules, and veins.
• The capillaries have a functional capillary pressure of 17 mmHg, dividing the system into high-pressure (arterial) and low-pressure (venous) components.

Autorhythmicity and Excitability

• The heart is characterized by being automatic and rhythmic (autorhythmic), excitable, conductive, and contractile.
• Autorhythmic cells have a low resting membrane potential, are less permeable to K+, and more permeable to Na+ and Ca++.
• The SA node is the normal pacemaker of the heart, firing impulses at a rate of 60-90 impulses/min.

Fast and Slow Response Fibers

• Fast response fibers (atria, ventricles, and Purkinje fibers) have a more negative resting membrane potential, a steeper slope of the upstroke, a greater amplitude of the action potential, and a faster conduction velocity compared to slow response fibers (SAN and AVN).

Factors Affecting Autorhythmicity

• Positive chronotropic factors: sympathetic stimulation, fever, mild alkalosis, and mild hypoxia.
• Negative chronotropic factors: parasympathetic stimulation, hypothermia, mild acidosis, and severe hypoxia.