Volume 3 Chapter 2a

Cardiology

 

Cardiovascular Anatomy and Physiology Review

 

I.                    Anatomy of the Heart - Location

a.       Cardiovascular system is composed of 2 major parts – the heart and peripheral blood vessels.

b.      Heart is a muscular organ approximately the size of a closed fist.

c.       The heart is located in the center of the chest in the mediastinum, anterior to the spine and posterior to the sternum (fig 2-1).

d.      2/3 of the hearts mass is to the left midline.

e.       Apex is the bottom of the heart just above the diaphragm, left of midline

f.        Base is the top of the heart located approximately at the level of the second.

 

II.                 Anatomy of the Heart – Tissue Layer

a.       The  has three tissue layers

b.      Endocardium – is the innermost layer

c.       Myocardium – is the middle layer. 

                                                               i.      Thickest layer

                                                             ii.      Cells are unique in that they resemble skeletal muscle but have electrical properties like smooth muscle.

d.      Pericardium – is the protective sac that covers the heart

                                                               i.      Visceral pericardium – (a.k.a. epicardium) is the inner layer that is in contact with the heart itself.

                                                             ii.      Parietal pericardium – outer fiberous layer of the heart that serves as a protective barrier.

 

III.               Anatomy of the Heart – Chambers

a.       The mammalian heart has 4 chambers.

b.      Two Atria – superior chambers that receive incoming blood.

c.       Two Ventricles – Inferior, larger chambers that receive blood from the atria.

d.      Interatrial septum – divides the left and right atrias.

e.       Interventricular septum – divides the left and right ventricles.

 

IV.              Anatomy of the Heart – Valves

a.       Two types of valves – atrioventricular valves and semilunar valves that prevent the back flow of blood.

b.      Atrioventricular valves – control blood flow between the atria and ventricles.

                                                               i.      Tricuspid valve – located between the right atrium and right ventricle.

                                                             ii.      Bicuspid valve (a.k.a. Mitral valve) – located between the left atrium and left ventricle.

                                                            iii.      Papillary muscle – located in the ventricles. Connected to the leaflets of the valve.  When relaxed the valves are open and allow blood to leave the atria and enter the ventricles.  The valves are closed when these muscles contract.

                                                           iv.      Chordae tendoneae – physically connect the valve leaflets to the papillary muscle.

c.       Semilunar valves – control the flow of blood between ventricles and the arteries into which they empty.– control the flow of blood between ventricles and the arteries into which they empty.

                                                               i.      Pulmonic (right) semilunar valve – located between the right ventricle and the pulmonary artery

                                                             ii.      Aortic (left) semilunar valve – located between the left ventricle and the aorta.

 

V.                 Blood flow through the heart

a.       Blood enters the heart from the inferior and superior Vena Cava where it is dumped into the right ventricle.

b.      Upon atrial contraction the blood is forced through the tricuspid valve into the right ventricle.

c.       From the right ventricle blood is forced through the pulmonary semilunar valve into the pulmonary artery.

d.      The pulmonary artery carries the deoxygenated blood to the lungs where oxygen and carbon dioxide are exchanged and the blood then returns to the heart via the pulmonary vein.

e.       The pulmonary vein delivers the blood to the left atrium.  When the left atrium contracts the blood is forced through the Mitral valve in to left ventricle.

f.        Upon ventricular contraction, blood leaves the left ventrical through the aorta semilunar valve to the aorta and systrmic circulation

 

VI.              Coronary Circulation

a.       The heart does not receive blood from the endocardium, but from the coronary circulation.

b.      Coronary arteries originate in the aorta just above the leaflets of the aortic valve.

c.       Left coronary artery – one of the main coronary arteries.  It supplies blood to the left ventricle, the interventricular septum, part of the right ventricle and the hearts conduction system

d.      The left coronary artery has 2 major branches

                                                              i.      Anterior descending artery

                                                            ii.      Circumflex artery

e.       Right coronary artery – the other main coronary artery.  It provides blood to the right atrium, right ventricle, and part of the conduction system.

f.        The right coronary artery has 2 major branches

                                                              i.      Posterior descending artery

                                                            ii.      Marginal artery

g.       The coronary arteries receive blood during diastole.  This is because the aortic semilunar valve leaflets cover the coronary artery ostia during systole.

h.       Blood drains from the left coronary artery into the anterior great cardiac vein and the lateral marginal veins.  These veins empty into the coronary sinus then into the right atrium.

i.         Blood drains from the right coronary arteries directly into the right atrium by smaller cardiac veins.

j.        Anastomosis – communication between 2 or more blood vessels allows for collateral circulation

k.      Collateral circulation -  is a protective mechanism that provides an alternative path for blood flow in case of a blockage somewhere in the system

 

VII.            Anatomy of Peripheral Circulation

a.       Arteries and veins 3 layers of tissue

                                                               i.      Tunica intima – inner most layer consisting of a single layer of cells

                                                             ii.      Tunica media – middle layer that consist of elastic fibers and muscle.  It gives blood vessels their strength and recoil.  Much thicker in arteries than veins.

                                                            iii.      Tunica adventitia – outer layer that is a fiberous tissue covering that gives the vessels the ability to withstand high pressures

                                                           iv.      Luman – is the cavity inside the vessel

b.      Poiseulle’s Law – states that the blood flow through a vessel is directly proportional to the radius of the vessel to the fourth power.

                                                               i.      For example – a vessel with a radius of 1mm would transport 1 ml per minute of blood (14) at a pressure of 100 mm Hg

                                                             ii.      For example – if the radius was increased to 4mm, using Poiseulle’s Law: 44 = 256ml per minute to maintain a pressure of 100 mm Hg

VIII.         Cardiac Cycle

a.       Cardiac cycle is the sequence of events that occurs between the end of one heart contraction to the beginning of another.

b.      Diastole – the first phase of the cardiac cycle or relaxation phase.  Occurs while the heart is at rest.

c.       Systole – second phase. The atria contract first and force blood into the ventricles.  The ventricles then contract and eject blood out of the heart.

d.      Ejection fraction – the ration of blood pumped from the ventricle to the amount remaining at the end of diastole.  The normal ejection fraction is 2/3 of the blood volume or 45% – 70%.

e.       Stroke volume – the amount of blood ejected from the heart with one contraction.  The average adult stroke volume is 60ml to 100ml with and average of 70ml.

f.        Preload – the pressure within the ventricles at the end of diastole

                                                               i.      Starling’s Law – law of the rubber band.  The more the heart is stretched the greater the force of the contraction.

                                                             ii.      The greater the volume of blood filling the heart, the greater the preload, the more forceful the cardiac contraction

g.       Afterload – the resistance against which the heart must pump

h.       Cardiac output – the volume of blood that the heart ejects per minute

                                                               i.      Stroke Volume (ml) x Heart Rate (bpm)= Cardiac Output (ml/min)

                                                             ii.      70 ml x 70 bpm = 4900 ml/min (or 4.9 L/min)

i.         Blood pressure = cardiac output x systemic vascular resistance

 

IX.              Nervous Control of the Heart

a.       The sympathetic and parasympathetic components of the autonomic nervous system work in direct opposition to one another to regulate the heart

b.      Sympathetic Nervous System – innervates the heart through the cardiac plexus (a network of nerves at the base of the heart)

                                                               i.      The SNS primary neurotransmitter is norepinephrine

                                                             ii.      Norepinephrine increases heart rate and contractility force

                                                            iii.      Norepinephrine works on beta receptors

1.      Alpha receptors – located in the peripheral blood vessels and are responsible for vasoconstriction

2.      Beta1 receptors – located on the heart increase heart rate and contractility

3.      Beta2 receptors – located in the lungs and peripheral blood vessels cause bronchodilation and vasodilation

c.       Parasympathetic Nervous System – innervates the heart through the vagus nerve (the 10th cranial nerve).

                                                               i.      The PNS primary neurotransmitter is acetylcholine

                                                             ii.      Acetylcholine slows the heart rate and atrioventricular conduction

                                                            iii.      PNS can be stimulated several ways

1.      Valalva maneuver

2.      Carotid Sinus Massage

3.      Distention of the urinary bladder

4.      Mammalian dive reflex

d.      Other terms that refer to autonomic control of the heart are chronotrophy, inotrophy, and dromotrophy.

                                                               i.      Chronotrophy – pertains to heart rate

                                                             ii.      Inotrophy – pertains to cardiac contractility force

                                                            iii.      Dromotrophy – pertains to the speed of impulse transmission

 

X.                 Electrophysiology

a.       Intercalated disk – specialized bands of tissue inserted between myocardial cells that increase the rate in which the action potential is spread from cell to cell.

b.      Intercalated disk are located between cardiac muscle fibers and can transmit the electrical impulse 400 times faster.

c.       Syncytium – group of cardiac muscle cells that physiologically function as one unit.

d.      The heart has 2 syncytium

                                                               i.      Atrial syncytium – contracts from the superior to inferior so that blood moves to the ventricles

                                                             ii.      Ventricular syncytium – contracts from inferior to superior so that blood moves  from the ventricles to the aorta and the pulmonary artery

e.       The synctia are separated by the fiberous structure that supports the valves and physically separates the atria from the ventricles.

f.        Atrioventricular (AV) bundle is the only passage between the atria and ventricles for the action potential to pass

g.       This provides the “all-or-none” principle

 

XI.              Cardiac Depolarization

a.       The cell’s Na+/K+ pump expels Na+ ions from the cell, thus giving the inside of the cell a more negative charge.

b.      Resting potential - the state of more anions on the inside of the cell and more cations on the outside of the cell. “The normal electrical state of cardiac cells.”

c.       When stimulated, the cell instantaneously opens its Na+/K+ pumps and Na+ cations move into the interior of the cell increasing the charge to +20mV greater than the outside of the cell.  Ca++ channels open and slowly allow Ca++ to leak into the cell.

d.      Action potential – is the stimulation of myocardial cells as evidenced by a change in the membrane electrical charge, that subsequently spreads across the myocardium

e.       This depolarization is spread throughout the heart via synctium and intercalated disk until the entire mass is depolarized – the muscle contracts.

f.        Repolarization - K+ then escapes from the inside of the cell until the electrical difference is back to about -90 mV.  Na+ is then actively pumped out of the cell and K+ back into the cell via the Na+/K+ pump.

 

XII.            Cardiac conduction system –

a.       To accomplish their task, the cells of the cardiac conduction system have the properties of excitability, conductivity, automaticity, and contractility

                                                               i.      Excitability – ability of the cells to respond to an electrical stimulus

                                                             ii.      Conductivity – ability of the cells to propagate the electrical impulse from one cell to another

                                                            iii.      Automaticity – Pacemaker cell’s capability of self depolarization

                                                           iv.      Contractility – the ability of muscle cells to contract or shorten.

b.      Internodal pathways – conduct the initial impulse in the heart from the SA node to the rest of the heart

                                                               i.      Sinoatrial (SA) node – pacemaker of the heart that fires at a rate of 60 -100 bpm in the adult

                                                             ii.      Atrioventricular (AV) node – gathers the impulse from the atrial synctium and acts as a gate keeper and slows the impulse giving the ventricles time to fill (intrinsic firing rate 40-60 bpm).

                                                            iii.      Bundle of His – portion of the ventricle were the atrial fibers gather and focus the impulse.

                                                           iv.      Right/Left Bundle branches – point at which the Bundle of His divides and travels down the septal portion of the left and right ventrical.

                                                             v.      Purkinje system – gathers the impulse from the bundle branches and sends the impulse back up into the myocardium from the apex upward. (Intrinsic firing rate 15 -40 bpm).