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Chapter 19: The Circulatory System: The Heart

1 Anatomy of the heart

About the size of a clenched fist and weighs about 250 g…it’s pretty small.

1 Location of the heart

Rests on the diaphragm, near the midline of the thoracic cavity.

1 Mediastinum

The heart lies within a mass of tissue that extends from the sternum to the vertebral column between the lungs.

About 2/3 of the heart’s mass lies to the left of the midline.

2 Apex of the heart

The apex is the pointy part of the heart at the bottom, which points down, slightly forward, and to the left.

3 Base of the heart

The base of the heart is the uppermost part, which is directed upwards, slightly towards the back, and to the right.

4 Anterior surface

Just deep to the sternum and ribs: the anterior surface faces forward.

5 Inferior surface

Between the apex and the right border. The inferior surface rests on the diaphragm.

6 Right border

Faces the right lung.

7 Left border

Also called the pulmonary border, faces the left lung.

2 Pericardium

The pericardium is the membrane that surrounds and protects the heart. It confines the heart in its position within the mediastinum, but allows sufficient movement for contractions. It consists of two parts.

1 Fibrous pericardium

Tough, inelastic, dense irregular connective tissue. The outermost pericardium layer. It prevents overstretching of the heart, provides protection, and anchors the heart to the mediastinum.

2 Serous pericardium

The deeper pericardium layer. It forms a double layer around the heart.

1 Parietal pericardium

This layer is fused to the fibrous pericardium.

2 Visceral layer or epicardium

These layer is actually part of the heart wall. It adheres tightly to the surface of the heart.

3 Pericardial cavity

There is a space between the parietal and visceral pericardium layers called the pericardial cavity. It is filled with fluid.

1 Pericardial fluid

This serous fluid reduces friction between the layers of the pericardium during contraction of the heart.

3 Layers of the heart wall

The wall of the heart is composed of three layers.

1 Epicardium

The outermost layer. Thin, transparent, gives a smooth, slippery texture to the outermost surface of the heart. Also called the visceral layer of the serous pericardium.

2 Myocardium

The middle layer. Composed of cardiac muscle tissue. Makes up the bulk of the heart and is responsible for the pumping action.

Cardiac muscle is striated, but unlike skeletal muscle, is involuntary.

3 Endocardium

The innermost layer of the heart. A thin layer of endothelium, provides a smooth lining for the chambers of the heart and covers the valves of the heart. The endocardium is continuous with the endothelium of the large blood vessels attached to the heart.

4 Chambers of the heart

The heart has four chambers.

1 Atria

The atria are the two superior chambers.

1 Right atrium

2 Left atrium

3 Auricles

On the anterior surface of each atrium is a wrinkled structure called an auricle, which increases the capacity of the auricles.

2 Sulci

Found on the surface of the heart, these grooves house blood vessels and fat. The sulci marks the external boundary between two chambers of the heart. You will not be required to identify the individual sulci. They are identified here for context.

1 Coronary sulcus

Marks the boundary between the superior atria and the inferior ventricles.

2 Anterior interventricular sulcus

On the anterior surface of the heart, marks the division between the ventricles.

3 Posterior interventricular sulcus

Marks the boundary between the ventricles on the posterior surface.

3 Ventricles

The two inferior chambers.

1 Right ventricle

2 Left ventricle

4 More detail on the chambers

1 The right atrium

1 Receives blood from three vessels

1 Superior vena cava

2 Inferior vena cava

3 Coronary sinus

2 Interatrial septum

This is the separation between right atrium and left atrium

3 Fossa ovalis

There is an oval depression in the interarterial septum that is the remnant of the foramen ovale. The foramen ovale is an opening between the atria in the fetus, which by-passes the pulmonary circuit.

4 Tricuspid valve or atrioventricular valve

This valve prevents backflow of blood from the right ventricle into the right atrium.

2 The right ventricle

Forms most of the anterior surface of the heart.

1 Trabeculae carneae

Raised bundles of cardiac muscle that are part of the conduction system (conveys the signals to contract).

2 Chordae tendineae

Tendon-like cords that connect the tricuspid valve to the right ventricle. Chordae tendineae prevent the prolapse of the AV valves during ventricular contraction.

3 Papillary muscles

The papillary muscles are the cone-shaped structures in the right ventricle to which the chordae tendineae are attached.

4 Interventricular septum

Separates the right ventricle from the left ventricle.

5 Pulmonary valve

Blood is pumped from the right ventricle, ultimately to the lungs, via the pulmonary valve. The pulmonary valve connects directly to the pulmonary trunk, which branches into the right and left pulmonary arteries.

3 The left atrium

Forms most of the base of the heart.

1 Pulmonary veins

The left atrium receives blood from the lungs via the pulmonary veins.

2 Bicuspid (mitral) valve

Blood passes from the left atrium to the left ventricle via the bicuspid or mitral valve. This valve prevents backflow into the left atrium. It is also called the atrioventricular valve.

4 The left ventricle

Forms the apex of the heart. It has trabeculae carneae and chordae tendinae that have the same structure and functions as the right ventricle.

1 Trabeculae carneae

2 Chordae tendinae

3 Aortic valve

Blood passes from the left ventricle via the aortic valve.

4 Ascending aorta

The blood leaving the left ventricle via the aortic valve pass into the ascending aorta.

1 Coronary arteries

Some of the blood that has reached the ascending aorta flows into the coronary arteries, which supply the heart wall muscle.

2 Arch of the aorta

The ascending aorta continues to the arch of the aorta. Branches of the arch and descending aorta deliver blood to the tissues of the body.

3 Descending aorta

The arch of the aorta continues to the descending aorta.

5 Myocardial thickness and function

1 Thin-walled atria

Only have to move blood into the ventricles.

2 Thick-walled ventricles

Pump blood either to the lungs (right ventricle) or entire body (left ventricle). The walls are correspondingly thick, the right less so than the left considering the shorter distance it has to pump blood.

6 Fibrous skeleton of the heart

For your information only.

2 Heart valves and circulation of blood

As each chamber contracts, it sends blood in a one-way direction out of the heart. The valves function in response to pressure changes of the blood as the heart contracts and relaxes.

1 Operation of the atrioventricular valves

Located between the atria and ventricles. When the atrioventricular (AV) valves are open, the pointed ends of the cusps point into the ventricle.

1 Relaxed ventricles

When the ventricles relax, the papillary muscles are also relaxed and the chordae tendinae are slack: as a result, blood can flow from high pressure (in the atrium) to low pressure (in the ventricle).

2 Contracted ventricles

When the ventricles contract, the pressure of the blood forces the cusps upward until their edges meet and close the opening. At the same time, the papillary muscles contract, pulling on the chordae tendinae. This prevents the cusps from opening into the atria in response to the high blood pressure in the ventricle.

2 Operation of the semilunar valves

The aortic and pulmonary valves are also known as the semilunar (SL) valves because they are made up of three crescent moon-shaped cusps. The free borders of the cusps project into the arteries. The SL valves allow ejection of blood from the ventricles into arteries and prevent backflow into the ventricles.

1 Contracted ventricles

When the ventricles contract, the SL valves open in response to pressure building up within the ventricle, permitting movement of blood into the arteries.

2 Relaxed ventricles

As the ventricle relaxes, pressure is reduced within the ventricle and blood starts to flow back into the ventricle. The backflow pushes on the valve cusps and closes them tightly, preventing blood flow into the ventricles.

3 Systemic and pulmonary circulations

1 Two closed circuits

The heart pumps blood into two closed circulatory circuits, meaning that blood remains in vessels from the time it leaves the heart until it re-enters. Note: material carried by the blood diffuses across capillary walls into tissues, but whole blood does not leave the vessels.

The two circuits are arranged in series, meaning that blood flows first through one circuit, then through the other.

The left side of the heart is the pump for the systemic circuit and the right side is the pump for the pulmonary circuit.

1 Systemic circulation

The left side of the heart receives oxygen rich blood from the lungs and pumps it into the aorta. The blood in the aorta is sent into progressively smaller systemic arteries that carry oxygenated blood to all of the tissues of the body, except the lungs. Arteries give rise to smaller arterioles, which lead to systemic capillaries. Exchange of gases and nutrients occurs across the thin capillary membranes.

Systemic venules pick up CO2 from respiring body tissues. These enter systemic venules, which merge to form systemic veins. Veins flow into the right atrium.

2 Pulmonary circulation

The de-oxygenated blood, high in carbon dioxide is pumped by the right heart to the lungs via the pulmonary trunk and pulmonary arteries. In pulmonary capillaries, blood unloads carbon dioxide and picks up inhaled oxygen. The freshly oxygenated blood is then sent to the left atrium.

4 Coronary circulation

Nutrients and gases cannot diffuse fast enough from the blood inside of the heart to the cardiac muscle. Therefore the myocardium has its own network of blood vessels called the coronary or cardiac circulation.

1 Coronary arteries

The coronary arteries branch from the ascending aorta and encircle the heart. Coronary arteries supply nutrients and oxygen to respiring cardiac muscle cells via capillaries. You are not required to identify the individual coronary arteries. They are identified here for context.

1 Left coronary artery

2 Right coronary artery

3 Anastomoses

Anastomosis is a general term describing the situation when two or more arteries supply the same tissue. This is a method for ensuring the supply of blood to critical tissue – like heart myocardium. The heart muscle may receive sufficient blood even if one coronary artery is blocked.

2 Coronary veins

Blood that is low in oxygen and high in carbon dioxide is returned to the heart via the coronary veins. You are not required to identify the individual coronary veins. They are identified here for context.

1 Coronary sinus

Most of the deoxygenated blood from heart myocardium is dumped into a vascular large sinus located in the coronary sulcus (see way above). From the coronary sinus, the deoxygenated blood dumps into the right atrium. The veins listed below empties empty into the coronary sinus.

2 Great cardiac vein

3 Middle cardiac vein

4 Small cardiac vein

5 Anterior cardiac vein

5 Histology of cardiac muscle tissue

1 Cardiac muscle cells compared with skeletal muscle cells

1 Shorter in length

2 Less circular in diameter

3 Branched

4 Single nucleus (usually)

5 Larger and more numerous mitochondria

6 Same arrangement of myofilaments into sarcomeres

7 Intercalated discs

The ends of cardiac muscle cells are joined to neighboring cells by a thickening of the sarcolemma called intercalated discs.

1 Desmosomes

The intercalated discs contain Desmosomes, which bind the cells together.

2 Gap junctions

Gap junctions are essentially holes in the intercalated discs that allow the action potential from one cardiac muscle cell to be communicated directly to another cell.

6 Autorhythmic fibers: The conduction system

1 Autorhythmic fibers

The heart is able to set the rhythm of cardiac muscle contraction and relaxation – the cycle of contractions that pumps blood – without external stimulation. Specialized cells called autorhythmic fibers are considered self-excitable. These fibers repeatedly generate action potentials that trigger heart contractions. The autorhythmic fibers have two functions:

1 Pacemaker

The autorhythmic cells set the rhythm of electrical excitation that causes contraction of the heart.

2 Conduction system

There is a cycle of cardiac excitation that starts in the right atrium and progresses through the heart, resulting in sequential contracting of the atria and ventricles. Action potentials are conducted along autorhythmic fibers in a precisely timed fashion.

1 Sequential propagation of cardiac action potentials through the conduction system

1 Sinoatrial (SA) node

This is the starting point of cardiac excitation. The SA node is located within the right atrial wall. These cells do not have a stable resting membrane potential. These cells repeatedly depolarize to a threshold spontaneously. The spontaneous depolarization is called the pacemaker potential. When the pacemaker potential reaches the threshold, it triggers an action potential, which propagates through the intercalated discs of all of the muscle fibers in the atria.

2 Atrioventricular (AV) node

The action potential initiated by the SA node spread throughout the atria and eventually reaches the AV node located in the septum between the two atria.

3 Atrioventricular (AV bundle) or bundle of His

From the AV node, the action potentials are conducted along a bundle of muscle cells to the ventricles – the AV bundle. This is the only pathway for action potentials from the atria to the ventricles.

4 Right and left bundle branches

The AV bundle splits and extends towards the apex of the heart as the right and left bundle branches.

5 Purkinje fibers

The Purkinje fibers conduct the action potentials from the apex of the heart back upward through the ventricular myocardium, which induces ventricular contraction.

3 Pacemaker rate

Without external modification, the SA node would initiate action potentials about every 0.6 second or 100 times a minute. This rate is modulated by the autonomic nervous system and endocrine system via hormones. To emphasize, they modify the rate but do not set the rate of heart contraction.

7 Action potential and contraction of contractile fibers

This section (everything up to Section C) is for your information only.

This section deals with the nature of the action potential in the contracting cardiac muscle cells. Remember that the action potentials are initiated in modified cardiac muscle cells of the autorhythmic fibers and spreads into contractile myocardium.

1 Depolarization

Although the autorhythmic fibers have an unstable membrane potential, the contractile fibers have a stable resting membrane potential that is disrupted by neighboring cells. The point of this section is that cardiac muscle cells depolarize fast.

2 Plateau

Once depolarization occurs in contractile fibers, it lasts a long time (compared with skeletal muscle fibers and neurons). About a quarter of a second for cardiac muscle fibers compared with about a thousandth of a second for skeletal muscle.

3 Repolarization

Same as any other excitable cell.

4 Mechanism of contraction in cardiac muscle cells

The molecular mechanism is essentially the same as in skeletal muscle fibers: action potential releases Ca2+; Ca2+ releases regulatory proteins; actin and myosin cross-bridge and slide.

5 Refractory period

The refractory period of cardiac muscle is the same idea as in skeletal muscle: there is a period of time directly after stimulation/contraction in which another stimulus cannot cause contraction. This period of time is longer in cardiac muscle compared with skeletal muscle. This is a good thing in cardiac muscle because there must be enough time for the contraction of an entire chamber (actually both chambers) to happen before another can start.

8 ATP production in cardiac muscle

For your information only.

9 Electrocardiogram

For your information only.

10 Correlation of ECG waves with atrial and ventricular systole

3 The cardiac cycle

A single cardiac cycle includes all of the events involved with one heartbeat. It includes systole and diastole of the atria plus systole and diastole of the ventricle.

1 Systole

Systole means contraction of a heart chamber. Blood pressure is highest during systole.

2 Diastole

Diastole means relaxation of the heart chamber. Blood pressure is lowest during diastole.

3 Pressure and volume changes during the cardiac cycle

Note that the blood volumes given in this figure are for only the left ventricle: the volume of blood ejected from the right ventricle is the same as that ejected from the left ventricle.

In each cardiac cycle, the atria and ventricles contract and relax in an alternating fashion i.e. first both atria contract, then both ventricles contract.

During contraction of a chamber, the blood pressure within that chamber increases and blood will flow from the area of high pressure to any area of low pressure to which it has access.

1 Atrial systole

During atrial systole, the atria are contracting.

1 Depolarization of the SA node causes atrial depolarization

2 Atrial depolarization causes atrial systole

Blood is forced through the AV valves into the ventricles.

3 The end of atrial systole is also the end of ventricular diastole

The ventricle has filled mostly during its diastolic (relaxed) phase prior to atrial systole. Atrial systole contributes the last 25 ml of blood for a total volume in the ventricles of 130 ml (in each ventricle).

4 Onset of ventricular depolarization

2 Ventricular systole

1 Ventricular depolarization causes ventricular systole

1 Isovolumetric contraction

Pressure rises in the ventricles during ventricular systole, but for a short time, neither the SL nor the AV valves are open: there is nowhere for the blood to go. This is called isovolumetric contraction. The cardiac muscle contraction is considered isometric and because all ventricular valves are closed, the ventricular volume remains the same.

2 Ventricular ejection initiation

Continued contraction of ventricular muscles cause increased blood pressure and eventually, both SL valves open and blood is ejected into the aorta and pulmonary artery from the ventricles.

3 Ventricular ejection completion

Both ventricles eject about 70 ml of blood each and about 60 ml remains in each ventricles. The amount of blood ejected from each ventricle per beat is called the stroke volume.

4 Onset of ventricular repolarization

3 Relaxation period

During the relaxation phase, the atria and ventricles are both relaxed.

1 Ventricular diastole

Ventricular repolarization causes ventricular diastole. Blood pressure drops during this phase and the blood pressure in the aorta and pulmonary artery and closes both of the SL valves.

1 Isovolumetric relaxation

After the SL valves close there is a brief time in which the volume of blood in the ventricles does not change. This is called isovolumetric relaxation.

2 Ventricular filling

Eventually, the blood pressure in the ventricles drops below the atrial pressure and the AV valves open and ventricular filling begins. The major part of ventricular filling occurs immediately after AV valve opening (i.e. prior to atrial systole.) Depolarization of the SA node initiates a new cardiac cycle.

4 Heart sounds

1 Lubb

A.K.A. S1. This sound is associated with the closure of the AV valves soon after ventricular systole begins.

2 Dupp

A.K.A. S2. Cause by closure of the SL valves at the beginning of ventricular diastole.

4 Cardiac output

1 Cardiac output

2 Regulation of stroke volume

1 Preload: effect of stretching

2 Contractility

3 Afterload

3 Regulation of heart rate

1 Autonomic regulation of heart rate

2 Chemical regulation of heart rate

1 Hormones

2 Cations

3 Other factors in heart rate regulation

5 Development of the heart

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