Book Notes | The Heart

19.1 Overview

The cardiovascular system consists of the heart and blood vessels

Arteries carries blood away from the heart-efferent vessels

Veins carry it toward the heart-afferent vessels

Capillaries are microscopic vessels that connect the smallest arteries to the smallest veins

• The Pulmonary and Systemic Circuits

  • Pulmonary circuit carries blood to lungs for gas exchange and returns it to the heart

  • The right half of the heart supplies the pulmonary circuit

    • Pumps oxygen poor (deoxygenated) blood into a large artery, the pulmonary trunk, which immediately divides into right and left pulmonary arteries

      • These transport blood to air sacs (alveoli) of the lungs, where CO2 is unloaded and O2 is picked up

      • The oxygenated blood flows by way of pulmonary veins to the left side of the heart

  • Systemic circuit supplies blood to every organ of the body, including parts of the lungs and the wall of the heart

    • The left side of the heart supplies the systemic circuit, blood leaves by the Aorta.

      • The aorta turns like an inverted U, the aortic arch, and passes downward posterior to the heart

      • The aorta travels through the thoracic and abdominal cavities and issues smaller arteries to the other organs before branching into the lower limbs

    • After circulating through the body & unloading O2, deoxygenated systemic blood returns to the right side of the heart mainly by two large veins: The superior vena cava (draining the upper body) and inferior vena cava (draining everything below the diaphragm)

  • Normally the pressure in the pulmonary circuit is less than the systemic circuit

  • A shunt us an abnormal, non capillary connection btw the two circuits

  • Position, Size, & Shape of Heart

    • The heart lies btw the two thoracic cavities in an area called the mediastinum

      • Buffalo’s do not have a mediastinum, therefore easy to kill

    • It is triangular shaped, with the broad base lying superiorly

    • The apex is pointed inferiorly and somewhat to the left

    • It is roughly the size of a fist

    • it weight about 300 g in an adult

• The Pericardium

  • The heart lies in a sac called the pericardium

  • It is a two layered structure, an outer fibrous layer and an inner serous layer

  • The fibrous layer is attached to the diaphragm

  • The serous side is a single squamous layer of cells, that secrete and absorb pericardial fluid. It also covers the surface of the heart called the epicardium

  • The space btw the parietal and visceral layers of the serous pericardium

  • is called the pericardial cavity

  • The layer is called the visceral layer of the pericardium, the layer under the fibrous pericardium is the parietal layer

  • The space btw the two layers is the pericardial sac, containing 5 to 30 ml of pericardial fluid

    • too much fluid is cardiac tamponade

19.2 Gross Anatomy of the Heart

The Heart Wall

• Consists of three layers: epicardium, myocardium, and endocardium

• Epicardium

  • outermost layer, visceral layer of serous pericardium

  • some places, it includes a thick layer of adipose tissue that encloses the major coronary blood vessels and protects them from compression

• Endocardium

  • lines the interior of the heart chambers

  • simple squamous epithelium overlying a thin areolar tissue layer

  • covers the valve surfaces and is continuous with the endothelium of the blood vessles

• Myocardium

  • composed of cardiac muscle

  • thickest layer and performs the work of the heart

  • forms the vortex of the heart

    • when the ventricles contract, they exhibit a twisting or wringing motion that enhances the ejection of blood

• Has a framework of collagenous elastic fibers that make up the fibrous skeleton

  • developed in fibrous rings around valves and in the sheets that connect the rings

The Chambers

• 4 chambers of the heart, 2 atria superiorly, and 2 ventricles below them inferiorly

• Each atrium has a small flap called an auricle

• The walls of the atria are thin due to their low-pressure workload

• Each atrium is separated from the other by an interatrial septum

• The right and left ventricles are thicker than the atrial walls, the left ventricular walls are thicker than the right

• The LV is the circular in cross section whereas the right ventricle has a crescent shape

• The two inferior chambers, right & left, are the pumps that eject blood into the arteries and keep it flowing around the body

• Ventricles separated by a thick muscular wall, the interventricular septum

• Both ventricles exhibit ridges called trabeculae carneae

• Three sulci of the heart

  • The coronary sulcus encircles the heart near the base and separates the atria above from the ventricles below

  • The other two extend obliquely down the heart from the coronary sulcus toward the apex - one on the front called the anterior interventricular sulcus and one on the back called the posterior interventricular sulcus

The Valves

• Each valve consists of fibrous flaps of tissues called cusps or leaflets, covered with endocardium

• The Tricuspid valve (will be used more often) or right AV regulate openings btw the atria and ventricles

• The mitral valve or left AV is considered inaccurate and obsolete

• Tendinous cords prevent the the AV valves from flipping inside out or bulging into the atria when the ventricles contract

• The semilunar valves regulate the flow of blood from the ventricles into the great arteries

• The pulmonary valve controls the opening from the Tricuspid valve into the pulmonary trunk

• The aortic valve controls the opening from the left ventricle into the aorta

Blood Flow Through the Chambers

Coronary Circulation

• Blood vessels of the heart wall constitute the coronary circulation

• the coronary

19.4 Electrical and Contractile Activity of the Heat

• Contraction is called systole

• Relaxation is a diastole

The Cardiac Rhythm

• Triggered by the SA node

  • Sinus Rhythm - normal heartbeat

• Spontaneous firing of the SA node is called ectopic focus

• slower heart beat of 40 to 50 bp, is called a nodal (junctional) rhythm

Pacemaker Physiology

• pacemaker potential - showing a gradual depolarization

Impulse Conduction to the Myocardium

• firing of the SA node excitres atrial cardiomyocytes

• AV slows the signal down to about 0.05 m/s bc cardiomyocytes here are thinner

• Delays AV node for about 100 ms - allowing the ventricles time to fill with blood before they begin to contract

• Ventricular systole begins at the apex of the heart

The EKG

• P wave - atrial depolarization - produced when a signal from the SA node spreads

• QRS complex - Ventricular depolarization - when the signal from the AV node spreads through the ventricular myocardium ad depolarized the muscle

• T wave - Ventricular repolarization -

• QT interval - duration of ventricular depolarization; shorter during exercise

• QRS interval - atrial repolarization and diastole; repolarization concealed by QRS wave

• PQ segment - signal conduction from SA node to AV node; atrial systole begins

• ST segment ventricular systole and ejection of blood; corresponds to plateau of cardiomyocyte action potential

• Any deviation from the regular is called an arrhythmia

• V-fib is the major sign of a heart attack

• Heart block, lack of QRS behind the P wave.

• Bundle branch block is a heart block resulting from damage to one or both branches of the AV bundle

• PVC - premature ventricular contraction - ventricular ectopic focus firing and setting off an extra beat (extrasystole)

• Sinus Rhythm - normal

• V-fib - heart rhythm is going nuts - makes ‘v"‘

• A fib - irregular, weak ripping contraction in the atria

• Heart Block - lack of QRS following p waves

• PVC - irregular “v” in the EKG

19.5 Blood Flow, Heart Sounds, and the Cardiac Cycle

Measurement Of Pressure

• Blood pressure specifically has been traditionally measured with sphygmomanometer

• A fluid flows only if it is subjected to more pressure at on appoint than another - the difference is a pressure gradient

• Pressure gradient - fluids always flow down their pressure gradients, from high pressure point to low pressure point

• Volume and pressure have an inverse relationship

• AV valve is open, blood flows into the ventricle from the above atrium

• The ventricle contracts, its internal pressure arises

• When the ventricles are relaxed and their pressure is low, the AV valve cusps hang down limply and both valves are open

• Lub-dub

  • Lub S1 and Dubb S2

    • S1 louder and longer

    • S2 a little softer and sharper

    • Third Heart Sound

      rarely audible, triple rhythm or gallop

Phases of the Cardiac Cycle

• Wiggers Diagram

  • major events that occur simultaneously at each moment throughout the cardiac cycle

Ejection Fraction: the percentage of blood ejected during ventricular systole, the percentage of the vend-diastolic volume in the ventricles

Stroke Volume: the actual volume of the ejection fraction

End Diastolic volume: total volume of blood in the ventricle at the end of the ventricular filing..the end of the diastole

Normal ejection fraction-around 55-60%

End systolic volume                         60 ml
                        volume during atrial diastole   +  30 ml
                        volume during atrial kick            + 40 ml
             Total end-diastolic volume                    130 ml
                        stroke volume                                 -   70 ml
                        End systolic volume                          60 ml

• Both ventricles have equal volumes

• BP in the right ventricle is relatively low

• Equal ooutput by the two ventricles is essential for homeostasis

• If the right ventricle pumps more blood into the lungs then the left ventricle can can handle on return, blood accumulates in the lungs, causing pulmonary hypertension, edema, and risk of drowning in one’s own body fluid

• CHF - congestive heart failure - insufficiency of ventricular pumping

19.6 Regulation of Cardiac Output

• Autonomic Innervation of the heart

  • Sympathetic stimulation increases heart rate, contraction strength, and dilates coronary arteries

  • There is little to no parasympathetic innervation of the myocardium or ventricles

• Cardiac Output

  • CO = HR x SV

  • Difference btw the maximum and resting cardiac output is called cardiac reserve

  • Magic 5 liters - max at rest

• Heart Rate and Chronotropic Agents

  • Avg. adult female HR is 72-80bpm

  • Avg. adult male HR is 64-72bpm

• tacycardia & bardycardia

• Positive chronotropic - factors outside of the heart itself that raise the heart rate

• Negative chronotropic agents lower the heart rate

• Adrenergic stimulation increases heart rate by increases SA node firing through stimulation with catecholamines, epi and norepi

• Cholergenic stimulation stimulation from the Vagus nerve slows the heartbeat, secrete acetylcholine at the SA and AV nodes, opens k+ channels, hyper-polarize these nodes and the heart slows down

Central Nervous System

• propioceptors in the muscles and joints provide information changes in physical activity. Thus, the heart can increase its output even before the metabolic demands of the muscles rise.

• Baroceptors

  • pressure sensors in the aorta and internal carotid arteries

  • HR increases, cardiac output increases and raises bp

  • signals to medulla to raise or lower BP

• Chemoreceptors

  • occur in the aortic arch, carotid arteries, and the medulla oblongata

  • sensitive to pH, CO2, and O2 lvls

  • Hypercapnia - excess CO2

  • Acidosis - pH greater than 7.35

  • Hypoxemia - O2 deficiency

hormones, drugs, and other chronotropic chemicals

• Increased HR

  • positive chronotropic agents

  • sympathetic nervous system

  • epi, norepi

  • TH

  • glucagon

  • Nicotiene, caffeine

  • Hypocalcemia

• Increased stroke volume

  • Increased preload (myocardial stretch)

  • Positive inotropic agents

  • Sympathetic nervous system

  • glucagon digitalis

  • nicotine, caffeine

  • Hypercalcemia

• Reduced Heart Rate

  • Negative chronotropic agetns

  • parasympathetic nervous system

  • acteycholine

  • hypercalcemia

  • HyperK

  • Beta blockers

• Reduced stroke volume

  • reduced preload

  • reduced contracility

  • increased afterload

  • negative inotropic agents - changes force of contraction

  • Hypocalcemia

  • HyperK

• More Definitions

  • preload: amount of stretch prior to contraction, increased preload, up to a point

  • frank-starling relationship - stroke volume is proportional to the end diastolic volume

  • contractility - how hard the myocardium contracts for any given level of preload

  • Positive inotropic agents increase contractility

  • negative inotropic agents decrease contractility

  • calcium is a positive inotropic agents

  • hyperK is a negative inotropic agent

  • digitalis increases intercellular calcium, making it a positive inotropic agents decrease contractility

  • afterload - the sum of the forces the ventricle must overcome to to eject blood

  • increased blood pressure is an increase in the afterload, anything that impedes the flow of blood increases the afterload