Chapter 20: The Cardiovascular System - The Heart

Where exactly is the heart located in the body?

The heart is situated in the mediastinum, an anatomical region extending from the sternum to the vertebral column and between the lungs.

• Approximately two-thirds of the heart's mass lies to the left of the body's midline.

• It rests on the diaphragm, with its pointed apex directed anteriorly and inferiorly toward the left, and its broad base directed posteriorly

Describe the layers of the pericardium and the heart wall

The pericardium is a triple-layered sac consisting of an outer fibrous pericardium that anchors the heart and an inner serous pericardium, which is divided into a parietal layer and a visceral layer (epicardium).

The heart wall itself has three layers:

• the outer epicardium (visceral serous pericardium)

• the thick middle myocardium (cardiac muscle responsible for pumping)

• the inner endocardium (smooth endothelial lining)

What are the primary internal and external features of the heart's chambers?

Externally

• Each atrium features an auricle to increase capacity, and the chambers are demarcated by sulci (grooves) containing blood vessels and fat

Internally:

• The right atrium receives deoxygenated blood and contains pectinate muscles and the fossa ovalis

• The right ventricle contains trabeculae carneae and papillary muscles connected to the tricuspid valve by chordae tendineae

• The left atrium receives oxygenated blood from four pulmonary veins

• The left ventricle forms the apex and pumps blood into the aorta.

How does the thickness of the chamber walls relate to their function?

Wall thickness is directly proportional to the workload of the chamber

• The atria are thin-walled because they only deliver blood to the adjacent ventricles

• The left ventricle is much thicker (10–15 mm) than the right ventricle (4–5 mm) because it must pump blood at higher pressure over the long distance of the systemic circulation, whereas the right ventricle only pumps a short distance to the lungs under low pressure.

Describe the structure and function of the four heart valves.

The atrioventricular (AV) valves (tricuspid and bicuspid/mitral) are located between the atria and ventricles

• they prevent backflow into the atria when the ventricles contract

The semilunar (SL) valves (aortic and pulmonary) consist of three moon-shaped cusps and prevent blood from flowing back into the ventricles from the arteries

• All valves open and close in response to pressure changes to ensure one-way blood flow.

Outline the path of blood through the pulmonary and systemic circulations

In the pulmonary circulation, the right ventricle pumps deoxygenated blood through the pulmonary trunk and arteries to the lungs, where it gains O2 and returns via pulmonary veins to the left atrium.

In the systemic circulation, the left ventricle pumps oxygenated blood into the aorta and systemic arteries to all body tissues (except the lung air sacs), where it loses O2 and returns via the venae cavae and coronary sinus to the right atrium

What is the purpose and primary components of the coronary circulation?

Because the heart wall is too thick for nutrients to diffuse from the chambers, the coronary circulation provides the myocardium with its own blood supply

• The left and right coronary arteries branch from the ascending aorta to deliver oxygenated blood.

• Deoxygenated blood is collected by cardiac veins and drains into the coronary sinus, which empties into the right atrium

Describe cardiac muscle tissue and the path of the conduction system

Cardiac muscle fibers are branched, striated, and involuntary, connected by intercalated discs that contain gap junctions for rapid electrical communication.

The conduction system follows this sequence:

• the sinoatrial (SA) node (pacemaker) initiates an action potential, which travels to the atrioventricular (AV) node, through the AV bundle (bundle of His), down the bundle branches, and finally to the Purkinje fibers to trigger ventricular contraction

Explain the three phases of an action potential in cardiac contractile fibers

1. Depolarization: Voltage-gated fast Na+ channels open, causing a rapid inflow of sodium ions.

2. Plateau: Voltage-gated slow Ca2+ channels open, allowing calcium inflow that balances potassium outflow, sustaining depolarization for about 0.2 seconds.

3. Repolarization: Calcium channels close and additional voltage-gated K+ channels open, allowing potassium outflow to restore the resting membrane potential

What do the waves of a normal electrocardiogram (ECG) represent?

• The P wave represents atrial depolarization

• The QRS complex represents rapid ventricular depolarization.

• The T wave indicates ventricular repolarization, occurring as the ventricles begin to relax

Describe the pressure and volume changes during a cardiac cycle

During atrial systole, atria contract to add 25 mL of blood to the ventricles, reaching an end-diastolic volume (EDV) of ~130 mL.

In ventricular systole, pressure rises, closing AV valves (isovolumetric contraction) until SL valves open for ventricular ejection

Each ventricle ejects about 70 mL (stroke volume), leaving ~60 mL as the end-systolic volume (ESV)

In the relaxation period, SL valves close (isovolumetric relaxation) until ventricular pressure drops below atrial pressure, allowing ventricular filling

How do heart sounds relate to ECG waves and mechanical events?

The first heart sound (S1, lubb) is caused by blood turbulence from the closure of AV valves shortly after the QRS complex begins (ventricular systole)

The second sound (S2, dupp) is caused by turbulence from the closure of SL valves at the start of ventricular diastole, shortly after the T wave begins.

Define cardiac output and provide the formula

Cardiac output (CO) is the volume of blood ejected from a ventricle into its respective artery each minute

• It is calculated by multiplying stroke volume (SV) by heart rate (HR): CO=SV×HR

What three factors regulate stroke volume?

1. Preload: The degree of stretch on the heart before it contracts; the Frank–Starling law states that more stretch leads to a more forceful contraction.

2. Contractility: The force of contraction at any given preload, increased by positive inotropic agents like epinephrine.

3. Afterload: The pressure that must be exceeded (e.g., aortic pressure) before the semilunar valves can open

Outline the factors that regulate heart rate

The cardiovascular center in the medulla oblongata regulates HR via the autonomic nervous system:

• Sympathetic impulses increase HR via cardiac accelerator nerves,

• Parasympathetic impulses decrease HR via the vagus (X) nerves

• Other factors include hormones (epinephrine, norepinephrine, thyroid hormones), cations (K+, Na+, Ca2+), age, gender, physical fitness, and body temperature.