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describe the size of the heart and its location in the thorax

14cm long
9cm wide

Lies inside the thoracic cavity, resting on the diaphragm. Its within the mediastinum, the medial cavity of the thorax, in between the lungs. Its posterior border is near the vertebral column, its anterior border is near the sternum. Partially obscured laterally by the lungs.

The base, beneath the 2nd rib, is the upper portion, where it is attached to several large blood vessels.

The distal end of the heart extends downwards, to the left, ending in a blunt point called the apex, which is even with the 5th intercostal space.

Hollow cone shaped, varying in size.

not completely central. Two thirds of the heart lies to the left of the midsternal line.


Coverings of the heart

Fibrous pericardium - dense connective tissue attached to the central diaphragm, posterior sternum, vertebral column and large blood vessels connected to the heart.

Deeper inside is serous pericardium. A double Layer of serous membrane - parietal and visceral pericardium. Between them is the pericardial cavity containing lubricating serous pericardial fluid (15-50ml).

At the base of the heart the visceral pericardium folds back to become the parietal pericardium, which lines the the inner surface of the fibrous pericardium.


Heart layers

3 layers compose the wall of the heart

Epicardium - is the visceral portion of the pericardium. Connective and adipose tissue.

Myocardium - cardiac muscle tissue. Richly supplied by blood capillaries, lymph capillaries and nerve fibres.

Endocardium - squamous epithelium and connective tissue, with many elastic and collagenous fibres. Contains blood vessels and specialised cardiac muscle fibres called perkinje fibres. Endocardium rests on the inner myocardial surface, lining the chambers of the heart and cover the fibrous tissue that forms the heart valves. Its continuous with endothelial linings of the hearts blood vessels.


Chordae tendineae

The chordae tendineae (tendinous cords), colloquially known as the heart strings, are tendon-resembling fibrous cords of connective tissue that connect the papillary muscles to the tricuspid valve and the bicuspid valve in the heart.


Chambers of the heart and associated great vessels: Right Atrium

The right atrium receives deoxygenated blood from systemic circulation through the:
- Superior vena cava, returning blood from areas superior to the diaphragm
- Inferior vena cava, returning blood from areas inferior to the diaphragm.
- The coronary sinus is a small vein which drains blood into the right atrium from the myocardium.

The right atrium sends blood to the right ventricle through the tricuspid valve.

When each atrium is not filled with blood, its outer portion deflates to resemble a wrinkled flap. This expandable atrial appendage is also called an auricle.


Chambers of the heart and associated great vessels: Left Atrium

Makes up most of the base of the heart.

Four pulmonary veins enter the left atrium, carrying oxygenated blood from the lungs.

Blood passes form the left atrium into the left ventricle through the 'left atrioventricular valve'/AV valve/ mitral valve. Which prevents it from flowing back into the left atrium from the ventricle.

Papillary muscles and Chordae tendineae prevent the mitral valve's cusps from swinging back into the left atrium when the ventricle contracts.

The mitral valve closes passively, directing blood through the large artery known as the Aorta


papillary muscles

contract as the hearts ventricles contract, pulling on the chordae tendinaea to prevent the cusps from swinging back into the atrium.


Chambers of the heart and associated great vessels: Atria

The Atria. The atria are chambers in which blood enters the heart. They are located on the anterior end of the heart, with one atrium on each side.


Chambers of the heart and associated great vessels: Left Ventricle

Forms most of the apex and inferoposterior area of the heart.

The left ventricle is thicker because it must force blood through the aorta to all body parts, which have a much higher resistance to blood flow.

the cavity of the left ventricle is almost circular.


interventricular sulcus

The area where the septum separates the right and left ventricles is marked on the hearts surface by the 'anterior interventricular sulcus'. This groove cradles the 'anterior interventricular artery' and continues to become the 'posterior interventricular sulcus'. This sulcus is visible on the posteroinferior surface of the heart.


Chambers of the heart and associated great vessels: Right Ventricle

Forms most of the anterior surface of the heart.

muscular wall 3x thinner then the left ventricle's, because the lungs have low resistance to blood flow.

The inner surface of the right ventricle contains a series of muscular ridges called trabeculae carnae.

The cavity of the right ventricle is flatter that that of the left ventricle, with a crescent shape and partially enclosing the left ventricle.

As the right ventricle contracts, its blood increases in pressure to passively close the tricuspid valve. Therefore, this blood can only exit through the pulmonary trunk, which divides into left and right pulmonary arteries that supply the lungs.

At the trunks base is a pulmonary valve that allows blood to leave the right ventricle while preventing backflow into the ventricular chamber.

The pulmonary valve contains three cusps.

Before the pulmonary valve, the superior end of the right ventricle becomes tapered, forming a cone shaped pouch called the 'conus arteriosus'


A-V valves

the tricuspid and bicuspid/mitral valve are known as A-V valves because they lie between the atria and ventricles.

They prevent back flow or regurgitation of blood from the ventricles to the atria while the ventricles contract.


Tricuspid valve

also know as the right A-V valve.

during ventricular contraction, it prevents blood from moving from the right ventricle into the right atrium.

has 3 flexible projections called cusps and lies between the right atrium and right ventricle.

the cusps are flaps of endothelium that are reinforced by cores of connective tissue.

the cusps of the tricuspid valve are attached to strong fibres called chordae tendineae, which originate from small papillary muscles that project inward from the ventricle walls.

papillary muscles contract when the ventricle contracts. when the tricuspid valve closes, the papillary muscles pull on the chordae tendineae to prevent the cusps form swinging back into the atrium.


Bicuspid/mitral valve

Located between the left atrium and left ventricle.

It is also called the bicuspid valve because it has two cusps.

When the left atrium is filled with oxygenated blood, it pushes the mitral valve open, sending the blood into the left ventricle.

During ventricular contraction, it prevents blood form moving from the left ventricle into the left atrium.


Semilunar valves

the pulmonary and aortic semilunar valves have half moon shapes and therefore are referred to as semilunar valves.

the pulmonary and aortic valves prevent backflow of blood form the aorta and pulmonary trunk back into their associated ventricles.

They respond to pressure differences, similar to the A-V valves.

The semilunar valves are forced open, causing their cusps to flatten against the artery walls as blood moves past them. This occurs when ventricles contract, increasing intraventricular pressure above the pressure in the aorta and pulmonary trunk.

When the ventricles are relaxed, the blood can then flow back toward the heart, filling the cusps and closing the valves.


Aortic sinuses

near each cusp of the aortic valve are sac like structures called aortic sinuses.

They prevent the cusps from sticking to the aortic wall as the valve opens.


coronary circulation

is the functional blood supply that nourishes the heart.

shortest circulation in the body.


Coronary Arteries

The heart receives its own supply of blood from the coronary arteries.

Two major coronary arteries branch off from the aorta near the point where the aorta and the left ventricle meet. These are the first two aortic branches.

These arteries and their branches supply all parts of the heart muscle with blood.

They deliver blood when the heart is relaxed and have less function when the heart contracts due to compression by the myocardium.

Both enclose the heart in the 'coronary sulcus' and provide the arterial supply of the coronary circulation.


Left Coronary Artery

Runs toward the left side of the heart.

Divided into 'anterior interventricular artery'/'left anterior descending artery' and 'circumflex artery'.

LAD Supplies blood to the anterior walls of both ventricles and to the interventricular septum.

LAD follows the anterior interventricular sulcus.

The circumflex artery supplies the posterior walls of the left ventricle and left atrium.


Right Coronary Artery

branches into the 'posterior interventricular artery' and the 'right marginal artery'.

The right marginal artery serves the myocardium of the lateral right side. There also may be more then one.

The posterior interventricular artery supplies the posterior ventricular walls and runs to the heart apex. It merges or 'Anastomoses' near the apex of the heart with the 'anterior interventricular artery'/LAD

In addition to supplying blood to the right ventricle (RV), the RCA supplies 25% to 35% of the left ventricle (LV).


Coronary arteries: Anastomoses

connections between blood vessels that provide alternate pathways known as 'collateral circulation'

These pathways may supply oxygen and nutrients to the myocardium when blockage of a coronary artery occurs.


electrical events associated with a normal ECG tracing

Each normal heart beat should follow the same sequence of electrical events:

The P wave represents depolarisation of the atria in response to signalling from the sinoatrial node (i.e. atrial contraction)

The QRS complex represents depolarisation of the ventricles (i.e. ventricular contraction), triggered by signals from the AV node

The T wave represents repolarisation of the ventricles (i.e. ventricular relaxation) and the completion of a standard heart beat

Between these periods of electrical activity are intervals allowing for blood flow (PR interval and ST segment)


Conduction pathway

Sinoatrial node/SA node - pacemaker

Atrioventricular node/AV node

Atrioventricular bundle/bundle of his

Right and Left branches of AV bundle

Conduction myofibrils/Purkinje fibers


Cardiac Output

The cardiac output is simply the amount of blood pumped by the heart per minute.

Necessarily, the cardiac output is the product of the heart rate, which is the number of beats per minute, and the stroke volume, which is amount pumped per beat.

CO = HR X SV. The cardiac output is usually expressed in liters/minute.


factors that influence Cardiac Output

factors effecting HR:
Autonomic innervation, hormones, fitness levels, age

factors effecting SV:
heart size, fitness levels, gender, contractility, duration of contraction, preload, afterload


Preload/end-diastolic volume EDV

Preload is the degree the heart muscle can stretch just before contraction.

In cardiac physiology, preload is the end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demand.


Afterload/end-systolic volume ESV

The back pressure exerted by the arterial blood on the pulmonary and aortic valves. The ventricles must overcome this pressure to eject blood.

80mm Hg in the aorta and 10 mm Hg in the pulmonary trunk.

in hypertensive people, afterload reduces the ventricles ability to eject blood. As a result the heart retains more blood after systole, which increases end-systolic volume and decreases stroke volume.

When arterial pressure is reduced, the ventricle can eject blood more rapidly, which increases the stroke volume (difference between EDV and ESV) and thereby decreases the ESV. Because less blood remains in the ventricle after systole, the ventricle does not fill to the same EDV found before the afterload reduction.



is the contractile strength achieved at a certain muscle length.

factors increases contractility can increase SV because more blood gets ejected form the heart. This increases SV and lowers end-systolic volume.

contractility is increased when sympathetic stimulation is increased.

various substances also effect contractility. Positive inotropic agents are substances that increase contractility: epinephrine, glucagon, thyroxine, digitalis and extracellular calcium ions.

Negative inotropic agents decrease contractility: hydrogen ions, increased extracellular potassium, calcium channel blockers.


role of the autonomic nervous system in controlling cardiac output

the most important extrinsic controls on heart rate occur because of the autonomic nervous system.

Anxiety, exercise, or fright activate the SNS. Related nerve fibres release norepinephrine at their cardiac synapses. This binds to B-adrenergic receptors in the heart and threshold can be reached faster. Therefore the SA node fires more quickly, and the heart beats faster.

sympathetic stimulation also speeds relaxation by enhancing contractility. It accomplishes this by enhancing calcium ion movements in contractile cells.



Irregularities in the force of rhythm of the heart beat