Cardiovascular System: The Heart and Blood Vessels – Exam 2 Study Guide

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Chapter 18: The Cardiovascular System – The Heart

Heart Coverings and Wall Structure

The heart is protected and supported by several coverings and consists of three main layers in its wall, each with distinct structure and function.

Pericardium: The double-walled sac surrounding the heart. It consists of:
- Fibrous pericardium: Tough, outer layer that protects, anchors, and prevents overfilling.
- Serous pericardium: Thin, inner layer with two parts:
  - Parietal layer: Lines the internal surface of the fibrous pericardium.
  - Visceral layer (epicardium): Covers the heart itself.
Heart Wall Layers:
- Epicardium: Outer layer, also the visceral pericardium.
- Myocardium: Middle, muscular layer responsible for contraction; composed of cardiac muscle.
- Endocardium: Inner layer lining the heart chambers and valves; made of endothelium.

Heart Chambers and Associated Great Vessels

The heart has four chambers, each with specific structure and function, and is associated with major blood vessels.

Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava and the coronary sinus.
Right Ventricle: Pumps blood to the lungs through the pulmonary trunk.
Left Atrium: Receives oxygenated blood from the lungs via four pulmonary veins.
Left Ventricle: Pumps oxygenated blood to the body through the aorta.

Blood Flow Pathways: Pulmonary, Systemic, and Coronary Circulation

Blood flows through the heart in a specific sequence, involving pulmonary, systemic, and coronary circuits.

Pulmonary Circuit: Right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium.
Systemic Circuit: Left ventricle → aorta → body tissues → vena cava → right atrium.
Coronary Circulation: Supplies the heart muscle itself via coronary arteries; blood returns via cardiac veins to the coronary sinus.

Heart Valves: Location, Function, and Operation

Valves ensure unidirectional blood flow and prevent backflow.

Atrioventricular (AV) Valves:
- Tricuspid valve: Between right atrium and right ventricle.
- Bicuspid (mitral) valve: Between left atrium and left ventricle.
Semilunar Valves:
- Pulmonary valve: Between right ventricle and pulmonary trunk.
- Aortic valve: Between left ventricle and aorta.
Operation: AV valves open during ventricular filling; close during contraction. Semilunar valves open during ventricular ejection; close during relaxation.

Cardiac Muscle: Structure and Function

Cardiac muscle is specialized for continuous rhythmic contraction and differs from skeletal muscle.

Structure: Striated, branched cells connected by intercalated discs (gap junctions and desmosomes).
Function: Involuntary contraction, coordinated by intrinsic conduction system.
Differences from Skeletal Muscle: Cardiac muscle has longer refractory period, relies on calcium from extracellular fluid, and contracts as a unit.

Action Potentials in Cardiac Cells

Cardiac pacemaker and contractile cells generate action potentials with unique features.

Pacemaker Cells: Exhibit prepotential (slow depolarization), leading to spontaneous action potentials.
Contractile Cells: Action potential includes a plateau phase due to calcium influx.
Absolute Refractory Period: Longer in cardiac cells, prevents tetanus.
Plateau Phase: Maintained by Ca2+ influx.

Intrinsic Conduction System and Heart Block

The heart's intrinsic conduction system coordinates contraction.

Components: SA node → AV node → AV bundle (Bundle of His) → bundle branches → Purkinje fibers.
AV Node Delay: 0.1 second delay allows atria to contract before ventricles.
Heart Block: Disruption in conduction, often at AV node, leading to uncoordinated contraction.

Electrocardiogram (ECG): Waves, Intervals, and Abnormalities

An ECG records electrical activity of the heart.

P wave: Atrial depolarization.
QRS complex: Ventricular depolarization.
T wave: Ventricular repolarization.
Intervals: PR, QT, and others represent timing between events.
Abnormalities: Fibrillation (uncoordinated contraction), junctional rhythm (SA node failure).

Cardiac Cycle: Timing and Events

The cardiac cycle consists of systole (contraction) and diastole (relaxation), with distinct phases.

Ventricular Filling: AV valves open, blood flows into ventricles.
Isovolumetric Contraction: All valves closed, ventricles contract.
Ventricular Ejection: Semilunar valves open, blood expelled.
Isovolumetric Relaxation: All valves closed, ventricles relax.
End Diastolic Volume (EDV): Volume in ventricle at end of filling.
End Systolic Volume (ESV): Volume remaining after contraction.

Heart Sounds

Heart sounds are produced by valve closure during the cardiac cycle.

First sound ("lub"): AV valves close at start of systole.
Second sound ("dup"): Semilunar valves close at start of diastole.

Cardiac Output, Stroke Volume, and Cardiac Reserve

Cardiac output is the volume of blood pumped per minute, determined by stroke volume and heart rate.

Cardiac Output (CO):
Stroke Volume (SV):
Cardiac Reserve: Difference between maximal and resting CO.

Regulation of Stroke Volume and Heart Rate

Multiple factors influence stroke volume and heart rate.

Stroke Volume: Influenced by preload, contractility, and afterload.
Heart Rate: Influenced by autonomic nervous system, hormones, and other factors.

Autonomic Nervous System Regulation

The autonomic nervous system modulates heart rate.

Sympathetic stimulation: Increases heart rate.
Parasympathetic (vagal tone): Decreases heart rate; vagal tone is the dominant influence at rest.
Homeostatic Imbalances: Tachycardia (fast HR), bradycardia (slow HR), arrhythmias.

Chapter 19: The Cardiovascular System – Blood Vessels

Blood Vessel Wall Structure

Blood vessels have three layers, each with specific functions.

Tunica intima: Inner layer, endothelium, reduces friction.
Tunica media: Middle layer, smooth muscle and elastic fibers, controls vasoconstriction and vasodilation.
Tunica externa: Outer layer, connective tissue, protects and anchors vessel.

Vasoconstriction and Vasodilation

These processes regulate vessel diameter and blood flow.

Vasoconstriction: Narrowing of blood vessels, increases resistance and blood pressure.
Vasodilation: Widening of blood vessels, decreases resistance and blood pressure.

Types of Arteries: Structure and Function

Arteries are classified by size and function.

Elastic arteries: Largest, e.g., aorta; pressure reservoirs.
Muscular arteries: Medium-sized, e.g., femoral artery; distributing arteries.
Arterioles: Smallest; resistance arteries, regulate blood flow to capillaries.

Capillary Beds and Types of Capillaries

Capillaries are sites of exchange between blood and tissues.

Capillary Bed: Network of capillaries supplied by arterioles, drained by venules.
Continuous capillaries: Most common, tight junctions, found in skin and muscle.
Fenestrated capillaries: Pores for increased permeability, found in kidneys and intestines.
Sinusoidal capillaries: Large gaps, found in liver, spleen, bone marrow.
Regulation: Arterioles control blood flow to capillary beds via vasoconstriction/dilation.

Veins: Structure and Function

Veins return blood to the heart and differ structurally from arteries.

Structure: Thinner walls, larger lumens, less muscle and elastic tissue.
Function: Low-pressure vessels, contain valves to prevent backflow.

Vascular Anastomoses

Anastomoses are connections between blood vessels, providing alternate routes for blood flow.

Importance: Ensure continuous blood supply even if one pathway is blocked.

Blood Flow, Blood Pressure, and Resistance

These factors determine circulation dynamics.

Blood Flow: Volume of blood moving through vessels per unit time.
Blood Pressure: Force exerted by blood on vessel walls.
Resistance: Opposition to flow, mainly due to vessel diameter.
Relationship:

Sources of Peripheral Resistance

Three main factors contribute to resistance.

Blood viscosity
Blood vessel length
Blood vessel diameter (most influential)

Blood Pressure Differences in Vessels

Blood pressure varies across vessel types.

Arteries: Highest pressure.
Capillaries: Moderate pressure.
Veins: Lowest pressure.
Steepest drop: Occurs in arterioles.
Systolic pressure: Peak during ventricular contraction.
Diastolic pressure: Lowest during relaxation.
Pulse pressure: Difference between systolic and diastolic.
Mean arterial pressure (MAP): Average pressure driving blood to tissues.
Muscular and respiratory pumps: Aid venous return by compressing veins and creating pressure gradients.

Velocity of Blood Flow and Vasomotion

Blood flow velocity changes with vessel type and cross-sectional area.

Arteries: Fastest flow.
Capillaries: Slowest flow, highest cross-sectional area.
Veins: Intermediate flow.
Vasomotion: Rhythmic changes in vessel diameter.
Relationship: Inverse between cross-sectional area and velocity.

Regulation of Blood Pressure

Blood pressure is regulated by cardiac output, peripheral resistance, and blood volume.

Factors influencing MAP: Heart rate, stroke volume, vessel diameter, blood viscosity, blood volume.

Capillary Exchange and Bulk Flow

Exchange of substances across capillary walls is driven by hydrostatic and osmotic pressures.

Hydrostatic pressure: Pushes fluid out of capillaries.
Colloid osmotic pressure: Pulls fluid into capillaries.
Filtration: Predominant at arterial end.
Reabsorption: Predominant at venous end.

Vessel Type	Pressure	Velocity	Cross-sectional Area
Arteries	High	Fast	Low
Capillaries	Moderate	Slow	High
Veins	Low	Intermediate	Low

Example: The aorta is an elastic artery (pressure reservoir), while the femoral artery is a muscular artery (distributing artery).

Additional info: The study guide references figures for MAP regulation; typical factors include sympathetic stimulation, hormones (e.g., epinephrine), and renal regulation of blood volume.