Cardiovascular System (The Heart)

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Pulmonary and Systemic Circuits

Heart is a transport system consisting of two side-by-side pumps

Right side receives oxygen-poor blood from tissues

Pumps blood to lungs

Left side receives oxygenated blood from lungs

Pumps blood to body tissues

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Heart Chambers

Receiving chambers of heart:

Right Atrium: from systemic circuit

Left Atrium: from pulmonary circuit

Pumping chambers of heart:

Right Ventricle: pulmonary circuit

Left Ventricle: systemic circuit

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Size, Location, and Orientation of the Heart:

Approximately the size of a fist

Location:

In mediastinum between second rib and fifth intercostal space

Diaphragm

Two-thirds to left of mid-sternal line

Anterior to the vertebral column

Posterior to the sternum

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Coverings of the Heart: Pericardium

Double-walled sac that surrounds heart

Fibrous Pericardium:

Function to:

Protect

Anchor heart to surrounding structures

Prevent overfilling

Deep, Two-Layered Serous Pericardium:

Parietal layer lines internal surface of fibrous pericardium

Visceral layer (epicardium) on external surface of heart

Separated by fluid-filled pericardial cavity

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Layers of the Heart Wall:

Epicardium:

Myocardium: bundles of cardiac muscle cells

Cardiac skeleton: connective tissue

Anchors cardiac muscle fibers

Supports muscles and valves

Endocardium: continuous with endothelial lining of blood vessels

Lines heart chambers and covers cardiac skeleton of valves

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Heart Chambers and Vessels:

Interatrial septum

Fossa ovalis: remnant of foramen ovale

Interventricular septum

Surface features

Coronary Sulcus

encircles junction of atria and ventricles

Anterior interventricular sulcus

interventricular septum

Posterior interventricular sulcus

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GROSS ANATOMY OF THE HEART:

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Atria: the receiving chambers:

Small, thin-walled chambers

Auricles: increase atrial volume

Right atrium: receives deoxygenated blood

Posterior portion contains ridges formed by pectinate muscles

Three veins empty into right atrium:

Superior Vena Cava: returns blood from body regions above the diaphram

Inferior Vena Cava: returns blood from body regions below the diaphram

Coronary Sinus: returns blood from coronary veins

Left atrium: receives oxygenated blood

Pectinate muscles found only in auricles

Four pulmonary veins return blood from lungs

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Ventricles: the Discharging Chambers:

Make up most of the volume of the heart

Trabeculae carneae

Papillary muscles: project into ventricular cavity

Thicker walls than atria

Actual pumps of heart

Right ventricle

pumps blood into pulmonary trunk

Left ventricle

Pumps blood into aorta

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Heart Valves:

Two major types of valves:

Atrioventricular Valves (AV):​​​​​​

​​​​​​​Prevent back flow into atria when ventricles contract

Tricuspid valve (Right AV Valve)

Mitral Bicuspid Valve (Left AV Valve)

Chordae tendineae: anchor cusps of AV valves to papillary muscles that function to:

Hold valve flaps in closed position

Prevent flaps from everting back into atria

Semilunar Valves (SL): ​​​​​​​​​​​​​​

Prevent back flow from major arteries back into ventricles

Each valve consists of three cusps that roughly resemble a half moon

Pulmonary semilunar Valve

Aortic semilunar Valve​​​​​​​

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Pathway of Blood through the Heart:

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Coronary Circulation:

Supplies heart muscle when relaxed

Shortest circulation in body

Coronary Arteries:

Arise from base of aorta and supply arterial blood to heart

Arteries contain many anastomoses

Anastomoses: a union or joining of nerves, blood vessels, or lymphatics; a junction

Left coronary artery has two branches:

Anterior interventricular artery (left anterior descending artery): follows the anterior interventricular sulcus and supplies blood to the interventricular septum and anterior walls of both ventricles.

Circumflex artery: supplies left atrium a the posterior walls of the left ventricle

Right coronary artery has two branches:

Right marginal artery: serves the myocardium of the lateral right side of the heart

Posterior interventricular artery: runs to the heart apex and supplies the posterior ventricular walls. Near the apex of the heart, this artery merges (anastomoses) with the anterior interventricular artery.

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Coronary Veins:

Cardiac veins collect blood from capillary beds

Coronary sinus empties into right atrium; formed by merging cardiac veins

Great cardiac vein of anterior sulcus

Middle cardiac vein in posterior interventricular sulcus

Small cardiac vein from inferior margin

Several anterior cardiac veins empty directly into right atrium anteriorly

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Cardiac Muscle Cells:

Striated, short branched, fat, interconnected

One central nucleus

Many large mitochondria

Intercalated discs are connecting junctions between cardiac cells that contain:

Desmosomes: prevent adjacent cells from separating during contraction

Gap Junctions: allow ions to pass from cell to cell, transmitting current across entire heart

Intercellular space between cells has connective tissue matrix (endomysium)

Contains numerous capillaries

Connects cardiac muscle to cardiac skeleton

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STEP ONE: Generating an Action Potential

Done by intrinsic cardiac conduction system -> noncontractile cells (PACEMAKER) that create and distribute impulses for depolarization and contraction of the heart.

3 Parts of an action potential:

Pacemaker Potential: this slow depolarization is due to both opening of Na+ channels and closing of K+ channels. Notice that the membrane potential is never a flat line.

Depolarization: the action potential begins when the pacemaker potential reaches threshold. Depolarization is due to Ca2+ influx through Ca2+ channels.

Repolariation: is due to Ca2+ channels inactivating K+ channels opening. This allows K+ efflux, which brings the membrane potential back to its most negative voltage.

Sequence of excitation --> impulses passed in this order:

Sinoatrial Node (SA Node) -->

Pacemaker of heart in right atrial wall; depolarizes fastest in heart

Atrioventricular Node -->

Delays impulse due to small fibers (fewer gap junctions)

Atrioventricular Bundle -->

connects the atria to the ventricles

Right and Left bundle branches -->

conduct the impulses through the interventricular septum

Purkinje fibers -->

Complete pathway through interventricular septum into apex and ventricular walls

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How does the brain control heart rate?

Autonomic nervous system

Cardiac centers in medulla oblongata

Cardioacceleratory center: sends signals to increase both rate and force

Stimulates SA and AV nodes, heart muscle, and coronary arteries

Cardioinhibitory center: parasympathetic signals to decrease rate

Inhibits SA and AV nodes via vagus nerves

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STEP TWO: Action potential through the heart:

Contractile muscle fibers make up bulk of heart and are responsible for pumping action

Depolarization: is due to Na+ influx through fast voltage-gated Na+ channels. A positive feedback cycle rapidly opens many Na+ channels, reversing the membrane potential. Channel inactivation ends this phase.

Plateau phase: is due to Ca2+ influx through slow Ca2+ channels. This keeps the cell depolarized because most K+ channels are closed.

Repolarization: is due to Ca2+ channels inactivating d K+ channels opening. This allows K+ efflux, which brings the membrane potential back to it's resting voltage.

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Electrocardiography:

Electrocardiograph can detect electrical currents generated by heart

Electrocardiogram (ECG or EKG) is a recording of electrical activity

Shows all action potentials at given time

Electrodes are placed on body to measure voltage differences

Main features:

P wave: depolarization of SA node and atria

QRS complex: ventricular depolarization and atrial repolarization

T wave: ventricular repolarization

P-R interval: beginning of atrial excitation to beginning of ventricular excitation

S-T segment: entire ventricular myocardium depolarized

Q-T interval: beginning of ventricular depolarization through ventricular repolarization

Sequence of depolarization a repolarization of the heart related to the deflection waves of an EKG tracing:

Atrial depolarization, initiated by the SA node, causes the P wave

With atrial depolarization complete, the impulse is delayed at the AV node

Ventricular depolarization begins at apex, causing the QRS complex. Atrial repolarization occurs.

Ventricular depolarization is complete

Ventricular repolarization begins at apex, causing the T wave.

Ventricular repolarization is complete.

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Mechanical Events of the Heart:

Systole: heart contraction

Diastole: heart relaxation

Cardiac Cycle: blood flow through heart during one complete heartbeat

Atrial systole and diastole are followed by ventricular systole and diastole

Three phases:

ventricular filling: mid to late diastole

Pressure is low; most blood passively flows from atria through open AV valves into ventricles (SL valves closed)

Atrial depolarization triggers atrial systole, atria contract, pushing rest of blood into ventricle

Depolarization spreads to ventricles

Atria finish contracting and return to diastole while ventricles begin systole

ventricular systole:

Atria relax; ventricles begin to contract

Rising ventricular pressure causes closing of AV valves

Two phases:

2a. Isovolumetric contraction phase: all valves are closed

2b. Ejection phase: ventricular pressure exceeds pressure in large arteries, forcing SL valves open

isovolumetric relaxation: early diastole

Following ventricular depolarization, ventricles are relaxed; atria are relaxed and filling

Backflow of blood in aorta and pulmonary truck closes SL valves

Ventricles are totally closed chambers (isovolumetric)

When atrial pressure exceeds ventricular pressure, AV valves open; cycle begins again

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Heart Sounds:

Two sounds (Lub-dub) associated with closing of heart valves

First sound is closing of AV valves at beginning of ventricular systole

Second sound is closing of SL valves at beginning of ventricular diastole

Pause between ub-dubs indicates heart relaxation

Mitral valve closes slightly before tricuspid, and aortic closes slightly before pulmonary vavle

Differences allow asculatation of each valve when stethoscope is placed in four different regions

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Factors that Influence Heart Rate:

Age

Fetus has fastest HR; declines with age

Gender

Females have faster HR than males

Exercise

Increases HR

Trained athletes can have slow HR

Body Temperature

HR increases with increased body temperature

Clinical Application:

Taycardia: abnormally fast heart rate ( > 100 beats/min)

If persistent, may lead to fibrillation

Bradycardia: heart rate slower than 60 beats/min

may result in inadequate blood circulation in nonathletes

may be desirable result of endurance training

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Congestive Heart Failure:

Progressive condition; CO is so low that blood circulation is inadequate to meet tissue needs

Reflects weakened myocardium caused by....

Coronary atherosclerosis: clogged arteries cause by fat build up; impairs oxygen delivery to cardiac cells

Persistent high blood pressure: aortic pressure >90 mmHg causes myocardium to exert more force -> causes myocardium hypertrophy and weakness

Multiple myocardial infarcts: heart becomes weak as contractile cells are replaced with scar tissue

Dilated cardiomyopathy (DCM): ventricles stretch and become flabby, and myocardium deteriorates

Drug toxicity or chronic inflammation may play a role

Either side of heart can be affected:

Left-sided failure results in pulmonary congestion (blood backs up in lungs)

Right-sided failure results in peripheral congestion (edema)

Failure of either side ultimately weakens other side

Leads to decompensated , seriously weakened heart

Treatment: removal of fluid, drugs to reduce after load and increase contractility