Cardiovascular System Anatomy

• The intended learning objectives include describing the anatomy of the heart, differentiating between high and low pressure systems, and identifying functional components of the vascular system.

Components of the Cardiovascular System

• The cardiovascular system consists primarily of the heart and blood vessels.

Functional Anatomy of the Heart

• The heart is a hollow muscular organ surrounded by a connective tissue sac called the pericardium.
• The pericardium protects the heart and allows its contraction with minimal friction.
• The heart wall is composed of cardiac muscle and divided into right and left halves, each consisting of one atrium and one ventricle.
• The right atrium is separated from the right ventricle by the tricuspid valve.
• The left atrium separated from the left ventricle by the bicuspid (or mitral) valve.
• Both the tricuspid and bicuspid valves are known as atrioventricular (A-V) valves
• Papillary muscle, located in the ventricles, has tendons called chordae tendineae attached to the A-V valves to prevent eversion into the atria during ventricular contraction.
• Cardiac valves allow blood passage in one direction only: atria to ventricle, and ventricle to aorta and pulmonary artery.
• The opening and closure of cardiac valves depends on the pressure difference across them - a passive process.
• Though there are no valves between the atria and veins, backflow of blood from the atria into the veins is unusual because the normal atrial pressures are not much higher than venous pressures.
• Another reason backflow is rare is that the large veins entering the atria are partially compressed during atrial contraction.
• The inner surface of the myocardium is lined with endothelial cells called endocardium.
• The endocardium also lines cardiac valves and the entire vascular system.
• The atrial myocardium is thinner than ventricular myocardium
• The left ventricular wall is about three times thicker than the right (15 mm vs. 5 mm)
• Left ventricular contraction facilitates the right ventricle and generates pressure, this is referred to as the left ventricular aid.
• Coronary arteries supply the myocardium, arising from the first part of the aorta, then branching into small arteries, arterioles, capillaries, venules, and veins.

High and Low Pressure System in CVS

• A dividing point in the capillaries separates a high-pressure component behind the arterial system and a low-pressure component in front towards the venous system.
• The functional capillary pressure at this dividing point is 17 mmHg.

Low Pressure System vs. High Pressure System

• The low-pressure system consists of systemic veins, pulmonary vessels, and both atria.
• Both ventricles during diastole are part of the low pressure system
• This system contains a large amount of blood under low pressure.
• The high-pressure system includes the systemic arterial system (aorta, arteries, arterioles).
• This system includes both ventricles during systole
• The arterial side of capillaries are part of the high pressure system
• This system contains a small amount of blood under high pressure.

Atria vs Ventricles

• The atria receive and store venous return during ventricular systole, then passively pass it on during ventricular diastole.
• Atrial systole increases ventricular filling by 20%; loss of atrial function is not fatal.
• The atrial walls have stretch receptors that monitor intra-atrial pressure changes and initiate cardiovascular reflexes.
• Atrial cells monitor changes in blood volume, leading to the release of Atrial Natriuretic Peptide, which prompts the kidneys to excrete Na+ and H2O
• Ventricles pump blood to the peripheral circulation and loss of this function is fatal.

Cardiac Muscle Fibers

• Cardiac muscle fibers are histologically divided into contractile and autorhythmic types.

Contractile Muscle Fibers vs. Autorhythmic Muscle Fibers

• Contractile muscle fibers (99%) form the atrial and ventricular walls and are responsible for pumping blood.
• Autorhythmic fibers (1%) form the conducting system of the heart and are specialized for the initiation and conduction of impulses.

Divisions of Circulation

• Systemic (greater), pulmonary (lesser) and special circulations exist
• Systemic circulation starts from the left ventricle to the aorta, large and small arteries, arterioles, capillaries, venules, veins, superior and inferior vena cava, and right atrium.
• Pulmonary circulation starts from the right ventricle to the pulmonary artery, lungs, pulmonary capillaries and veins, then to the left atrium.
• The pulmonary circulation is in series with the systemic circulation.

Functional Components of the Vascular System

• The heart is described as a dual pump, performing both compression and suction functions.
• As a compression pump, ventricles provide intermittent blood flow during systole into the arteries
• As a suction pump, atria withdraw blood from the venous system during diastole.

Functional Histology of Cardiac Muscle

• The heart is a functional syncitium.
• Gap junctions provide low resistance passages between cells for electrical signals.
• Intercalated discs provide mechanical cohesion between cells.

Characteristics of Heart (Cardiac Muscle)

• Automatic and Rhythmic (i.e., autorhythmic)
• Excitable
• Conductive
• Contractile (27)

Autorhythmicity

• Is the ability of the heart to generate its own electric impulses and beat regularly.
• The SA node is the cardiac electric generator, firing impulses regularly at a rate of 60-90 impulse / minute.
• The AV node is a potential pace maker, and if the AV node fails, Purkinje fibers become a tertiary pace maker.

Autorhythmic cells are characterized by

• Low RMP
• Less permeability to K+
• More permeability to Na+ and Ca++
• SA node is the normal pace maker of the heart,

Fast Response Fibers vs. Slow Response Fibers

• In fast response fibers the resting membrane potential is more negative; in slow response fibers it is less negative.
• In fast response fibers the slope is more steep, in slow response fibers it is less steep.
• In fast response fibers the overshoot is greater; in slow response fibers it is smaller.
• In fast response fibers the amplitude of the action potential is greater; in slow response fibers it is smaller.
• Fast Na channels are used compared to slow Ca channels.
• For fast response fibers, RRP ends at phase 4; for slow response fibers, it extends into phase 4.
• Fast response fibers have fasters conduction velocity compared to slow response fibers.

Factors Affecting Autorhythmicity

• Positive chronotropic factors (increase heart rate) include sympathetic stimulation and fever.
• Positive chronotropic factors also include mild alkalosis and mild hypoxia.
• Negative chronotropic factors (decrease heart rate) include parasympathetic stimulation and hypothermia.
• Negative chronotropic factors also include mild acidosis and severe hypoxia.

Excitability

• Is the ability of cardiac muscle to generate an action potential in response to adequate stimulation.
• During the absolute refractory period, excitability of cardiac muscle is zero.
• During the relative refractory period, excitability of cardiac muscle is improved but still subnormal.
• Cardiac muscle has a long refractory period, meaning it cannot be tetanized.

Conductivity

• Is the ability of cardiac muscle to transmit an action potential from one cell to the next.

Contractility

• Is the ability of cardiac muscle to convert chemical energy into mechanical energy in the form of tension, work, and pressure
• Contraction is triggered by increased intracellular Ca++, sourced from extracellular fluid and the sarcoplasmic reticulum (SR)

Factors affecting contractility

• Positive inotropic factors (increase contractility) include sympathetic stimulation, catecholamines, digitalis, and mild heat with increased Ca++ in ECF.
• Negative inotropic factors (decrease contractility) include parasympathetic stimulation decrease atrial contractility, ether chloroform and bacterial toxins, ischemia, and mild cold with increased K+ in ECF.

Blood Vessels

• Capillaries (5%) are exchange vessels between blood and tissue
• Veins (54%) and pulmonary vessels (18%) capacitance vessels able to accommodate large volume of blood under low pressure; are volume reservoirs.
• Roughly 80-85% of the blood is contained in the low-pressure system.
• The arterial system makes up 11% of blood vessels
• Aorta, pulmonary artery, and large arteries have a windkessel function, elastic, and able to distend because of the ejection of blood. They then recoil during diastole.
• Medium sized as well as small arteries are distributing, muscular and have low resistance.
• These vessels deliver blood at a considerable pressure (without loss of volume or pressure or time)
• Arterioles are muscular, high resistance (constrict and dilate) and regulate rate of blood flow to the tissues and arterial blood pressure by adjusting the peripheral resistance.
• About 12% of blood is in the heart.