Chapter 18: The Heart and Cardiovascular Function – Study Notes

Chapter 18: The Heart and Cardiovascular Function

Overview of the Cardiovascular System

The cardiovascular system consists of the heart and blood vessels, functioning together to transport blood throughout the body. The heart acts as a muscular pump, circulating blood through two main circuits: the pulmonary and systemic circuits.

• Pulmonary circuit: Carries blood between the heart and lungs for gas exchange (oxygenation).

• Systemic circuit: Delivers oxygenated blood from the heart to the rest of the body and returns deoxygenated blood back to the heart.

• Heart location: The heart is located in the mediastinum, directly behind the sternum, and is enclosed by the pericardial cavity.

Structure and Function of the Pericardium

The pericardium is a sac-like structure that surrounds and protects the heart. It consists of several layers and contains pericardial fluid to reduce friction during heartbeats.

• Fibrous pericardium: Outermost dense connective tissue layer, anchoring the heart to surrounding structures.

• Serous pericardium: Composed of parietal and visceral layers, with the visceral layer (epicardium) directly covering the heart.

• Pericardial fluid: Lubricates the heart's movement within the pericardial sac.

• Pericarditis: Inflammation of the pericardium, which can lead to cardiac tamponade (excess fluid accumulation impairing heart function).

Layers of the Heart Wall

The heart wall is composed of three main layers, each with distinct functions and structures.

• Epicardium: Outer layer (visceral pericardium), provides protection.

• Myocardium: Middle, thick muscular layer responsible for contraction; contains cardiac muscle tissue.

• Endocardium: Inner endothelial layer lining the heart chambers and valves.

Cardiac Muscle Tissue

Cardiac muscle is specialized for continuous, rhythmic contraction and is unique compared to skeletal muscle.

• Cell structure: Small, branched cells, typically uninucleate and centrally located.

• Intercalated discs: Specialized connections between cells containing desmosomes and gap junctions, allowing rapid transmission of action potentials and synchronized contraction (functional syncytium).

• Striated appearance: Due to organized myofibrils and sarcomeres.

• Metabolism: Highly aerobic, rich in mitochondria and myoglobin, requiring a constant oxygen supply.

Heart Chambers and Major Blood Vessels

The heart consists of four chambers and associated major blood vessels that direct blood flow through the pulmonary and systemic circuits.

• Right atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava.

• Right ventricle: Pumps blood to the lungs via the pulmonary trunk.

• Left atrium: Receives oxygenated blood from the lungs via the pulmonary veins.

• Left ventricle: Pumps oxygenated blood to the body via the aorta.

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, passing through chambers and valves to ensure unidirectional movement.

1. Deoxygenated blood enters the right atrium from the body.

2. Passes through the tricuspid (right AV) valve into the right ventricle.

3. Pumped through the pulmonary semilunar valve into the pulmonary trunk and lungs.

4. Oxygenated blood returns to the left atrium via pulmonary veins.

5. Passes through the bicuspid (mitral, left AV) valve into the left ventricle.

6. Pumped through the aortic semilunar valve into the aorta and systemic circulation.

Heart Valves and Their Function

Heart valves ensure one-way blood flow and prevent backflow between chambers and vessels.

• Atrioventricular (AV) valves: Tricuspid (right) and bicuspid/mitral (left) valves between atria and ventricles.

• Semilunar valves: Pulmonary and aortic valves between ventricles and major arteries.

• Valve operation: AV valves open during ventricular relaxation (diastole) and close during contraction (systole); semilunar valves do the opposite.

• Valvular heart disease (VHD): Deterioration of valve function, leading to inadequate blood flow; may require prosthetic replacement.

Comparing Right and Left Ventricles

The right and left ventricles have different workloads and wall thicknesses due to their respective circulatory roles.

• Right ventricle: Thinner wall, pumps blood to nearby lungs (low pressure).

• Left ventricle: Thicker wall, pumps blood throughout the entire body (high pressure).

Cardiac Skeleton

The cardiac skeleton is a framework of dense connective tissue that stabilizes heart valves and electrically isolates atria from ventricles.

• Functions: Supports valve structure, prevents overstretching, and ensures proper conduction of electrical impulses.

Coronary Circulation

The heart muscle (myocardium) receives its own blood supply via the coronary arteries, which branch from the ascending aorta.

• Left and right coronary arteries: Supply oxygen and nutrients to cardiac tissue.

• Coronary veins: Remove deoxygenated blood from the myocardium.

• Coronary ischemia: Reduced blood flow due to blockage, leading to tissue damage.

Arteriosclerosis and Atherosclerosis

Arteriosclerosis refers to the thickening and hardening of arterial walls, while atherosclerosis is a specific form involving lipid deposits (plaques).

• Arteriosclerosis: General term for arterial wall stiffening; major cause of cardiovascular disease.

• Atherosclerosis: Formation of fatty plaques in arteries, restricting blood flow and increasing risk of heart attack and stroke.

• Risk factors: Age, sex (male), high cholesterol, high blood pressure, smoking.

• Treatments: Balloon angioplasty (compresses plaque), stent placement, vessel replacement.

Table: Comparison of Arteriosclerosis and Atherosclerosis

Condition	Main Feature	Effect on Blood Flow	Common Treatments
Arteriosclerosis	Thickening/hardening of arterial walls	Reduced elasticity, increased resistance	Medication, lifestyle changes
Atherosclerosis	Lipid plaque formation in arteries	Restricted blood flow, risk of clot	Angioplasty, stents, surgery

Cardiac Cycle and Heart Sounds

The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole) in the heart chambers, producing characteristic heart sounds.

• Systole: Contraction phase, blood is ejected from chambers.

• Diastole: Relaxation phase, chambers fill with blood.

• Heart sounds: "Lub" (AV valves closing), "Dub" (semilunar valves closing).

Action Potential in Cardiac Muscle and Role of Ca2+ Ions

Cardiac muscle cells generate action potentials that trigger contraction. Calcium ions (Ca2+) play a crucial role in this process.

• Action potential: Rapid depolarization, plateau phase (due to Ca2+ influx), and repolarization.

• Role of Ca2+: Entry during plateau phase sustains contraction and prevents tetanus.

Equation:

$\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

Conducting System of the Heart

The heart's conducting system coordinates the timing of contractions through specialized cells and structures.

• Sinoatrial (SA) node: Pacemaker, initiates action potentials.

• Atrioventricular (AV) node: Delays impulse, allowing atria to contract before ventricles.

• Bundle of His, bundle branches, Purkinje fibers: Distribute impulses throughout ventricles.

Electrocardiogram (ECG/EKG)

An electrocardiogram records the electrical activity of the heart, providing information about heart rhythm and function.

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization.

• T wave: Ventricular repolarization.

Factors Affecting Heart Rate and Stroke Volume

Heart rate and stroke volume are influenced by various physiological and external factors, determining cardiac output.

• Heart rate: Controlled by autonomic nervous system, hormones, physical activity, and temperature.

• Stroke volume: Influenced by venous return, contractility, and afterload.

• Cardiac output: The volume of blood pumped by the heart per minute.

Equation:

$\text{Stroke Volume} = \text{End-Diastolic Volume} - \text{End-Systolic Volume}$

Coordination of Stroke Volume and Cardiac Output

Stroke volume and heart rate are coordinated to meet the body's metabolic demands, especially during exercise or stress.

• Increased heart rate and stroke volume raise cardiac output.

• Regulated by neural, hormonal, and intrinsic mechanisms.

Example: During exercise, sympathetic stimulation increases both heart rate and contractility, leading to higher cardiac output to supply active muscles with oxygen.

Additional info: Some details have been expanded for clarity and completeness, including definitions, equations, and examples relevant to Anatomy & Physiology students.