THE HEART

I) Anatomy of the Heart

a) Size, Location, and Orientation

---Cone shaped organ,with the bottom  of the cone (base of the heart) directed toward the left shoulder, and the point of the cone (apex of the heart) points inferiorly toward the left hip. The point of maximal intensity (PMI) is where the apex contacts the chest wall between the fifth and sixth ribs just below the left nipple and where one can feel the beating of the heart.---

---Lies anterior to the vertebral column and posterior to the sternum and enclosed within the medial cavity of the thorax called the mediastinum.---

---About the size of our fist and weighs less than a pound

b) Coverings of the Heart

---The heart is enclosed in a double walled sac called the pericardium.  The superficial part of the sac is the fibrous pericardium which protects the heart, anchors it to surrounding tissues (diaphragm and blood vessels carrying blood into and out of the heart), and prevents overfilling of the heart with blood.--

---Deep to the fibrous pericardium is the serous pericardium  composed of two layers: the parietal layer (lining the internal surface of the fibrous pericardium) and  the visceral layer (epicardium) which covers the external surface of the heart. The visceral layer is actually formed from a continuation ofthe parietal layer which folds inferiorly over the surface of the heart to form the visceral layer.----

---Between the visceral and parietal layer is the pericardial cavity which contains a thin film of serous fluid.  This fluid allows the two membranes to slide over each other and allowing the heart to work in a relatively friction free environment.--- 

---Inflammation of the pericardium is called pericarditis which can result from pneumonia.  Serous fluid production is hindered and the heart rubs against the pericardial sac producing a creaking sound (like a rusty bicycle axle). This sound is called the pericardial friction rub.  Excess inflammatory fluid may seep into the pericardial cavity compressing the heart and limiting its ability to pump blood. This condition is called cardiac tamponade (heart plug), and can be treated by draining off the excess fluid with a syringe inserted into the pericaridial cavity.---  

c) Layers of the Heart Wall

---The heart wall is composed of an outer superficial layer called the epicaridium (the visceral layer of the serous pericardium) often containing a lot of fat especially in older people.---                                                                                                              

---The middle layer of the heart is the myocardium composed of cardiac muscle forming bulk of the heart as its thickest layer. This is the layer of the heart that contracts pushing blood in and out of the four chambers. Cardiac muscle is a branching muscle fiber which link all parts of the myocardium together. Cardiac fibers are arranged and wrapped together  in spiral or circular bundles by  crisscrossing connective tissue fibers  called the fibrous skeleton of the heart. This "skeleton" of the myocardium consists of collagen and elastin fibers, some of which are thicker in some places than others such as around heart valves and around parts of the myocardium from which large blood vessels issue from the heart such as the aorta.  This connective tissue also does not carry electrical impulses, and its increased presence can slow down the spread of action potentials in specific places in the myocardium---

---The inner layer of the heart is called the endocardium. It covers the inside of the heart and is therefore and epithelial tissue. It is called endothelium and consists of squamous epithelial tissue.  It covers the valves of the heart along with the chambers and continues with the endothelial linings of the blood vessels leaving an entering the heart----

d) Chambers and Blood Vessels Entering and Leaving the Heart

--The superior two chambers of the heart are the right and left atria and the two inferior chambers are the right and left ventricles.  The heart is divided longitudinally by a partition which separates the right and left atria (the interatrial septum), and the right and left ventricles (the interventricular septum).  The right ventricle forms most of the anterior surface of the heart, and the left ventricle dominates the inferoposterior aspect of the heart and forms the apex.-----

----Two grooves are present on the anterior surface of the heart that indicate the boundaries of the four chambers  and carry the blood vessels that supply the myocardium. The atrioventricular groove or coronary  sulcus , encircles the junction of the atria and ventricles like a crown, while the anterior interventricular sulcus marks the anterior position of the septum separating the right and left ventricles and cradles the anterior interventricular artery.  The anterior interventricular sulcus continues as the posterior interventricular sulcus on the posterior surface of the heart---

---The atria each have a wrinkled protruding  appendage called an auricle (little ear) which function to increase the volume of the atria. Internally the  posterior wall of the atria are smooth, and the anterior wall are ridges of muscle bundles called pectinate muscles. The smooth and rigid areas are separated by a C-shaped ridge called the crista terminalis.  The internal septum contains a shallow depression called the fossa ovalis. The fossa ovalis is the spot where an opening called the foramen ovale existed in the fetal heart. This opening allowed blood to pass directly from the right side of the heart to the left side of the heart thereby bypassing the lungs. It is present from the 5th week of development to birth and seals up after 48 hours after birth to form the fossa ovalis.  If it doesen't close then no blood goes to the lungs and cyanosis or a blue baby results. ---

---Blood vessels carrying blood  to the right atria include three veins.  The superior vena cava returns blood from the body regions superior to the diaphragm, the inferior vena cava returns blood from body areas below the diaphragm, and the coronary sinus  collects blood draining from the myocardium.  Four pulmonary veins enter the left atrium transporting blood from the lungs back to the heart. The atria are more receiving chambers rather than pumping chambers. They contract minimally to push blood into the ventricles with the aid of gravity.----

--- The trabeculae carnae are irregular ridges of muscle found on the internal walls of the ventricular chambers.  Conelike muscle bundles called the papillary muscles project into the ventricular cavities which play a role in valve function. Both the ventricles pump blood out of the heart. The right ventricle pumps the blood into the pulmonary trunk which further subdivides into the right and left pulmonary arteries which carry the blood to the lungs to pick up oxygen and to release carbon dioxide. The blood vessels that carry blood to and from the lungs is called the pulmonary circuit and the right side of the heart is called the pulmonary circuit pump.  The left ventricle pumps the blood rich in oxygen  returning from the pulmonary circuit  into the aorta (the largest artery in the body) and to upper and lower regions of the body where oxygen is exchanged for carbon dioxide.  The blood vessels that carry the functional blood supply to and from all body tissues is called the systemic circuit, and the right side of the heart is the pulmonary circuit pump.  The walls of the left ventricle is three times as thick as those of the right ventricle and its cavity is nearly circular.  The left ventricle therefore is a much stronger pump since it has to push blood over a longer distance than the right ventricle.----

e) Coronary Circulation

---Arterial supply of coronary circulation is provided by the right and left coronary arteries. They both arise from the base of the aorta and encircle the heart in the atrioventricular groove.---                                                                                                

---The left coronary artery runs toward the left side of the heart and then divides into two major branches: the anterior interventricular artery which followis the anterior interventricular sulcus and supplying blood to the interventricular septum  and anterior walls of both ventricles, and the circumflex artery which supplies the left atrium and the posterior walls of the left ventricle.---

---The right coronary artery carries blood to the right side of the heart where it divides into two branches: the marginal branch serves the myocardium of the lateral part of the right side of heart, and the more important posterior interventricular artery carrying blood to the heart apex and supplies the posterior ventricular walls.  Near the apex of the heart it merges (anastomoses) with the anterior interventricular artery. Nearly all of the right ventricle and  the right atrium is supplied by the right coronary artery.  Delivery of blood to the myocardium only occurs during ventricular relaxation; during contraction of the myocardium the coronary arteries are compressed, and the entrances are partially blocked by the flaps of the aortic valves.---

---Blood is collected by the cardiac veins (the great cardiac vein, the middle cardiac vein , the small cardiac vein)  after the blood has passed through the capillary beds of the myocardium.  The cardiac veins join together to form an enlarged vessel called the coronary sinus which empties blood into the right atrium. Anterior cardiac veins empty blood directly into the right atrium anteriorly.---

---Stress induced spasms of the coronary arteries or  increased physical demands of the heart can result in thoracic pain caused by a decrease in blood delivery to the mycardium called angina pectoris. In such cases the myocardial cells are weakened, but they do not die.  More serious would be a coronary blockage which can lead to a myocardial infarction (MI) (heart attack  or a coronary).  Myocardial cells die during a heart attack.  Because adult cardiac cells are amitotic any areas of cell death are repaired with noncontractile scar tissue. Damage to the left side (the systmic pump) is the most serious.--- 

f) Heart Valves

--Two atrioventricular (AV) valves located in each atrial-ventricular junction, prevent backflow of blood into the atria while the ventricles are contracting. The right AV valve is called the tricuspid valve and has three flaps of endocardium that are reinforced by connective tissue cores made of collagen fibers called the chordae tendonae which anchor the cusps to the papillary muscles.  The left AV valve is called the bicuspid valve more commonly called the mitral valve because of its resemblance to a bishop's miter or hat. The flaps of the bicuspid valve are also reinforced with the chordae tendonae. The papillary muscle contract before the rest of the ventricles taking up the slack in the chordae tendonae and after ventricular contraction, they keep the flaps (cusps) from being pushed back up into the atria like and umbrella turned inside out by the wind.---

---Two semilunar valves (aortic and pulmonary semilunar valves) prevent the backflow of blood from the aorta and the pulmonary trunk into the ventricles. Three pocketlike cusps each shaped like a crescent moon form each valve. No valves are present at the entrance into each atria and some blood does pass back out of the atria upon contraction, but as the atrial myocardium contracts the atria collapse and block the entrance back into the blood vessels. --

II) Heart Physiology

a) Microscopic Comparisons with Skeletal Muscle

1) Cardiac muscle cells (fiber) are shorter, thicker, branched and interconnected in contrast to the long cylindrical multinucleated skeletal muscle fibers.

2)  Cardiac muscle fibers contain one or at the most two centrally located nuclei in contrast to skeletal muscle which contain numerous nuclei per fiber.

3) Skeletal muscle fibers  are independent of one another but all cardiac muscle fibers are interconnected by intercalated discs.  Each intercalated disk marks the union of two plasma membranes of adjacent cardiac cells. Each plasma membrane is folded in a manner in which they interlock like two sheets of corrugated cardboard.  Each disc contains desmosomes which anchor the plasma membranes together preventing the cardiac cells from separating during contraction, and gap junctions which allow ions to pass freely from one cell to another allowing the depolarization current to pass over the entire heart. The cardiac cells are electrically coupled so that the entire myocardium behaves as a single unit or functional syncytium. 

4) Cardiac cells contain more mitochondria (25 % of cell volume) than skeletal muscle (2% of cell volume) giving the cardiac muscle a high resistance to fatigue. The heart does ,however, exclusively rely upon aerobic respiration, and cannot incur much of an oxygen debt like skeletal muscle and still operate properly.  Nutritionally, both types of muscle tissue use glucose, fatty acids, and other organic molecules.  Cardiac muscle is much more adaptable and can switch metabolic pathways to whatever nutrient supply is available including lactic acid generated by muscle cells.  The real danger of an inadequate blood supply to the myocardium is lack of oxygen, not of nutrient fuels. 

5) Cardiac cells contain myofibrils within the sarcoplasm containing the Z discs, A bands, and I bands.  Actin and myosin proteins also form the myofibrils like skeletal muscle.  The myofibrils are thicker in cardiac muscle and branch repeatedly to accommodate for all the extra mitochondria.

6) There are no triads found in cardiac muscle; the sarcoplasmic reticulum is simpler in cardiac muscle and lacks the terminal cisternae.  The system for delivering  Ca2+ for muscular contraction in heart muscle is less elaborate than in skeletal muscle and only involves T tubules.

7) The fibers are surrounded by an endomysium like skeletal muscles, and the endomysium is continuous with the fibrous skeleton of the heart mentioned earlier. Fibers are not arranged in fascicles, and there is no perimysium, or epimysium in cardiac muscle as it is in skeletal muscle 

8) In skeletal muscle, all cells of a given motor unit (a motor unit in skeletal muscle is a motor neuron and all the muscle fibers it supplies--page 293) are stimulated and contract at the same time. All motor units of  a skeletal muscle are not necessarily stimulated and contract at the same time, since each group of fibers are innervated by a different neuron. In contrast in cardiac muscle the entire heart contracts as a unit or contraction doesn't occur at all (All heart muscles are connected by gap junctions through which depolarization spreads through the entire myocardium.

9) All skeletal muscle must be stimulated by a nerve ending to contract.  Cardiac muscle possesses  a property called automaticity, or autorhythmicity (an intrinsic beat) in which cardiac cells can initiate their own depolarization.  About 1% of the heart's fibers do this and are described to be autorhythmic. The other 99% of the muscle fibers are contractile muscle fibers and have to be innervated to contract.

10) The length of the refractory period during skeletal muscular contraction  is 1 to 2 ms in contrast to the longer cardiac refractory period of 250 ms which is nearly as long as the contraction period of cardiac muscle.  This longer refractory period prevents tetanic (sustained) contractions which would stop the heart's pumping action.

11) The sequence of electrical events are similar in both cardiac muscle fibers and skeletal muscle fibers (page 695-696). A major difference, however, is that the duration of the action potential and the contractile phase is much greater in cardiac muscle than in skeletal muscle. In skeletal muscle, the action potential typically lasts between 1 and 5 ms and the period of muscle contraction ranges from 15 to 100 ms. In cardiac muscle, the action potential lasts 200 ms or more (because of the plateau), and tension development persists for 200 ms or more.  A calcium surge across the membrane occurs in cardiac muscle and prolongs the plateau and peaks just after the plateau ends.   The dramatic increase of intracellular calcium concentration results when extracellular Ca2+ enters the cell through slow channels (20% of the calcium needed for contraction) in the plasma membrane. The extracellular  Ca2+ then stimulates the release of the other 80% (intracellular calcium) from the sarcoplasmic tubules  which liberate "bursts" of calcium ions (calcium sparks).    

b) The Intrinsic Conduction System

1) The autorhythmic cells do not maintain a stable resting membrane potential, but instead have an unstable resting potential that continuously depolarizes drifting slowly toward threshold for firing. These spontaneously changing membrane potentials are called pacemakers potentials, or prepotentials which initiates the action potentials that spread throughout the heart to trigger its rhythmic contractions. The reversal of membrane potential and the rising phase of the action potential is actually caused by an influx of Ca2+ through fast Ca2+ channels rather than an influx of Na+.

2) Autorhythmic cardiac cells are localized in the following areas (figure 19;14): the sinoatrial node (SA node or pacemaker-located in the right atrium inferior to the entrance of the superior vena cava), the atrioventricular (AV) node, atrioventricular (AV) bundle (bundle of HIS), right and left bundle branches, and the Purkinje fibers of the ventricular walls.  Impulses pass across the heart in the same order.

 The SA node or pacemaker located in the right atrium inferior to the entrance of the superior vena cava. The pacemaker receives extrinsic neural and hormonal factors from two nerves (vagus nerve and  spinal nerve) and generates impulses from the two nerves about 75 times every minute (average heart rate). Without the extrinisic factors, it is closer to 100 times per minute. The cells in the pacemaker have the fastest depolarization rate of any cells in the conduction system or the myocardium, and therefore "sets the pace" for the heart as a whole and has a characteristic rhythm called sinus rhythm.  

 The atrioventricular (AV) node is located in the inferior portion  of the interatrial septum immediately above the tricuspid valve. From the SA node a depolarization wave spreads via gap junctions and through the internodal pathway  to the AV node. At this point both the right and left atria have contracted together pushing blood into the ventricles.  At the AV node, the impulse is delayed momentarily (0.1sec)  allowing the atria to respond and to complete their contraction before the ventricles contract.  The delay is accomplished because of the smaller fibers present in the AV node and the presence of fewer gap junctions in the AV node for current flow.  The AV nodes conduct impulses more slowly than other parts of the system, as cars slowing as they mere from two lanes into four. Once through the AV node the signal passes rapidly through the rest of the system. 

 From the AV node, the impulse sweeps to the atrioventricular bundle (bundle of HIS)  located in the inferior part of the interatrial septum.  There are no gap junctions between the atria and ventricles, so the AV bundle is the only electrical connection between them.  The balance of the AV junction is insulated by the nonconducting fibrous skeleton of the heart.

 The AV bundle persists only briefly before splitting into two pathways--the right and left bundle branches which course along the interventricular septum toward the apex of the heart.

 The Purkinje fibers  are essentially long strands of barrel- shape cells with few myofibrils complete the pathway through the interventricular septum  penetrating the heart apex and then turn superiorly into the ventricular walls.  The perkinje system is much larger and more elaborate in the left ventricle.  The perkinje fibers also innervate the papillary muscles  which contract before the rest of the ventricular muscles. Contraction of the ventricles begin at the heart apex and spreads to the ventricles. 

***The total time between initiation of the impulse by the SA node and depolarization of the last of the ventricular muscles is approximately 0.22 sec (220ms) in a healthy human heart.

****The intrinsic beat of the SA nod is 75/min/, the AV node depolarizes at an average rate of 50/min, and the Perkinje fibers at a rate of 30/min.  These "other pacemakers" cannot dominate the heart unless everything faster than them (the pacemaker) becomes nonfunctional.

c) Electrocardiography

1) The electrocardiograph  can monitor the electrical currents that are generated from the heart, because they are transmitted over the whole body. The graphic recording of heart activity is called an electrocardiogram (ECG or EKG). Twelve electrical leads are placed on various sites on the body surface. Three of the leads are bipolar leads that measure the voltage difference between the arms, or an arm and a leg, and nine are unipolar leads. Together the leads give a comprehensive picture of the electrical activity of the heart.  

2) An ECG consists of three distinguishable waves called deflection waves:\

The first wave is the P wave lasting about 0.08 seconds and results from the depolarization of the atria (movement of the depolarization wave from the SA note through the atria). Approximately 0.1 seconds after the P wave begins, the atria contract.

The second wave which is the largest is the QRS wave (complex) results from ventricular depolarization and precedes ventricular contraction. The duration of the QRS wave is 0.08 seconds.  The  shape of the QRS wave  results from the complicated path taken by the impulse through the walls of the ventricles, corresponding to differences in current direction.

The last wave called the T wave is caused by ventricular repolarization and lasts about 0.16 seconds.  Repolarization is slower than depolarization so the T wave is more spread out and has a lower amplitude than the QRS wave.  Atrial repolarization occurs during ventricular contraction so its occurrence is obscured by the larger QRS wave. 

****The P-Q interval is the time from the beginning of atrial contraction to the beginning of ventricular contraction  (0.16 sec). It includes atrial depolarization (and contraction) as well as the passage of the depolarization wave through the rest of the conducting system.

****The S-T segment represents represents the point when the action potential is in its plateau phase and the entire ventricular myocardium is depolarized.

****The Q-T interval lasting about 0.38 sec is the period from the beginning of ventricular depolarization through their repolarization.  It is often called the"electrical systole of the ventricles" because it includes the time of ventricular contraction.

**** An enlarged R wave could mean that one has enlarged ventricles, a flattened T wave cardiac ischemia, and a prolonged Q-T wave interval a repolarization abnormality that increase the risk of ventricular arrhythmias.  Also abnormalities can be seen such as heart blocks, PACs and  the more serious PVCs

d) The Cardiac Cycle

--The cardiac cycle includes all events associated with the flow of blood through the heart during one complete heartbeat: atrial systole (contraction) and diastole (relaxation) followed by ventricular systole (contraction and the high point of blood pressure) and diastole (relaxation and the low point of blood pressure) and can be represented by the ECG.   There are pressure and blood volume changes within the heart. Pressure changes on the left side of the heart is five times greater than the left side, but the volume of the blood per beat is the same for both chambers. The length of the cardiac cycle is about 0.8 second (atrial systole accounts for 0.1 second, ventricular systole accounts for 0.3 seconds, and the 0.4 seconds represents the quiescent period which is the total time of heart relaxation)  Figure 19.19 shows the summary of events occurring in a cardiac cycle

e) Heart Sounds

---Two sounds are heard  lup-dup, pause, lup-dup, pause and so on.   The pause represents the quiescent period.  The lup sound is produced when the AV valves close during ventricular systole and tends to be louder and longer and the dup sound shorter a softer as the semilunar valves snap shut.    The mitral valve colses slightly before the tricuspid valve, and the aortic semilunar valve closes just before the semilunar valve and it is possible to distinguish each individual valve by ausculating (listening to---with a stethoscope) four different regions of the thorax (figure 19.20) . Different sounds from each valve can lead to a diagnosis of such things as heart murmers and  stenotic valves (page 704)

f) Cardiac Output

---Cardiac output is the amount of blood pumped out by each ventricle in 1 minute and is the product of heart rate (HR) and stroke volume (SV). Stroke volume is the volume of blood pumped out by a ventricle with each beat.  Normal heart rate is 75 beats/minute and stroke volume is equal to 70ml/beat, therefore the average cardiac output (CO) is equal to 5250 ml/min (5.25 liters/minute). The entire blood supply passes through each side of the heart once every minute!  Note that CO can vary as the SV varies and/or the HR varies.   The CO above (5.25 liters/minute) is what we call the resting CO (based upon resting heart rate).  We also have a maximum CO which increases if our heart beats faster during special demands such as running so we will not be late for class.  The difference between resting CO and maximal CO is called our cardiac reserve.  In nonathletic people, cardiac reserve is typically four to five times their normal CO (20-25 L/min as opposed to trained athletes whose CO may increase by a much as seven time their CO (35 L/min).

g) Regulation of Heart Rate: Extrinsic Controls

---The heart rate is regulated by the autonomic nervous system.  There are two divisions of the autonomic nervous system which includes the sympathetic and the parasympathetic nervous systems.  There are two nerves which regulate heartbeat at the sight of the SA node or pacemaker. The parasympathetic nerve is one of the cranial nerves known as the vagus nerve.  The vagus nerve releases a neurotransmitter known as acetylcholine (a hormone) at the SA node. Acetylcholine acts to slow down the heart beat. Another hormone known as norepinephrine  is secreted by another nerve (a spinal nerve) which is part of the sympathetic nervous system.  Epinephrine acts to  speed up the heartbeat. During emergency situations the kidney will release adrenalin into the blood stream, one of its targets being the SA node of the heart which also increases heartrate.