Cardiovascular System: The Heart and Blood Vessels – Study Guide

Chapter 18: The Cardiovascular System – The Heart

Heart Coverings and Wall Structure

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

• Pericardium: A double-walled sac surrounding the heart, consisting of:

  • Fibrous pericardium: Tough, dense connective tissue; protects, anchors, and prevents overfilling.

  • Serous pericardium: Thin, slippery, two-layer membrane (parietal and visceral layers) with pericardial cavity in between, filled with lubricating fluid.

• Heart Wall Layers:

  • Epicardium: Outer layer; visceral layer of serous pericardium.

  • Myocardium: Middle, thick muscular layer; responsible for contraction.

  • Endocardium: Inner layer; smooth endothelial lining of heart chambers and valves.

Heart Chambers and Associated Vessels

The heart has four chambers: two atria (upper) and two ventricles (lower), each with specific functions and associated vessels.

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

• Right Ventricle: Pumps blood to the lungs via the pulmonary trunk (pulmonary circulation).

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

• Left Ventricle: Pumps oxygenated blood to the body via the aorta (systemic circulation).

Blood Flow Pathways

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

• Pulmonary Circuit: Right ventricle → pulmonary trunk → lungs → left atrium.

• Systemic Circuit: Left ventricle → aorta → body tissues → right atrium.

• Coronary Circulation: Supplies blood to the heart muscle itself via coronary arteries and veins.

Heart Valves: Location, Function, and Operation

Valves ensure unidirectional blood flow through the heart.

• Atrioventricular (AV) Valves:

  • Tricuspid valve: Between right atrium and right ventricle.

  • Bicuspid (mitral) valve: Between left atrium and left ventricle.

  • Function: Prevent backflow into atria during ventricular contraction.

• Semilunar (SL) Valves:

  • Pulmonary valve: Between right ventricle and pulmonary trunk.

  • Aortic valve: Between left ventricle and aorta.

  • Function: Prevent backflow into ventricles after contraction.

Cardiac Muscle: Structure and Function

Cardiac muscle is specialized for continuous, rhythmic contraction.

• Striated, branched cells connected by intercalated discs (containing gap junctions and desmosomes).

• Involuntary control (unlike skeletal muscle).

• Autorhythmicity: Some cells can generate their own action potentials.

• Longer refractory period than skeletal muscle, preventing tetanus.

Action Potentials in Cardiac Cells

Cardiac pacemaker and contractile cells have distinct action potential profiles.

• Pacemaker Cells: Exhibit prepotential (slow depolarization), rapid depolarization (Ca2+ influx), and repolarization (K+ efflux).

• Contractile Cells: Rapid depolarization (Na+ influx), plateau phase (Ca2+ influx balances K+ efflux), repolarization (K+ efflux).

• Absolute refractory period: Prevents summation and tetanus.

Intrinsic Conduction System

This system coordinates the heart's electrical activity and contraction sequence.

• Components: SA node → AV node → AV bundle (Bundle of His) → bundle branches → Purkinje fibers.

• AV node delay (0.1 s): Allows atria to contract before ventricles.

• Heart block: Impaired conduction between atria and ventricles.

Electrocardiogram (ECG) Waves and Intervals

An ECG records the electrical activity of the heart.

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization (and atrial repolarization).

• T wave: Ventricular repolarization.

• Abnormalities: Fibrillation (uncoordinated contraction), junctional rhythm (SA node failure).

Cardiac Cycle: Timing and Events

The cardiac cycle consists of all events associated with one heartbeat.

• Systole: Contraction phase.

• Diastole: Relaxation phase.

• Phases:

  • Ventricular filling: AV valves open, blood flows into ventricles.

  • Isovolumetric contraction: All valves closed, ventricles contract.

  • Ventricular ejection: SL valves open, blood ejected.

  • Isovolumetric relaxation: All valves closed, ventricles relax.

• End diastolic volume (EDV): Volume in ventricles at end of filling.

• End systolic volume (ESV): Volume remaining after contraction.

Heart Sounds

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

• "Lub" (S1): AV valves close at start of ventricular systole.

• "Dub" (S2): SL valves close at start of ventricular diastole.

Cardiac Output, Stroke Volume, and Heart Rate

Cardiac output is the amount of blood pumped by each ventricle per minute.

• Formula: $CO = SV \times HR$

• Stroke Volume (SV): Amount of blood pumped per beat.

  • Formula: $SV = EDV - ESV$

• Cardiac Reserve: Difference between resting and maximal 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 and Heart Rate

The autonomic nervous system modulates heart rate through sympathetic and parasympathetic input.

• Sympathetic stimulation: Increases heart rate and contractility.

• Parasympathetic (vagal) stimulation: Decreases heart rate (vagal tone).

• Homeostatic imbalances: Tachycardia, bradycardia, arrhythmias.

Chapter 19: The Cardiovascular System – Blood Vessels

Blood Vessel Wall Structure

Most blood vessels have three layers (tunics), each with specific functions.

• Tunica intima: Endothelial lining; reduces friction.

• Tunica media: Smooth muscle and elastic fibers; controls vasoconstriction and vasodilation.

• Tunica externa (adventitia): Connective tissue; protects and anchors vessel.

Vasoconstriction and Vasodilation

These processes regulate vessel diameter and blood flow.

• Vasoconstriction: Decrease in vessel diameter due to smooth muscle contraction.

• Vasodilation: Increase in vessel diameter due to smooth muscle relaxation.

Types of Arteries

Arteries are classified by size and function.

• Elastic arteries: Largest; act as pressure reservoirs (e.g., aorta).

• Muscular arteries: Medium-sized; distribute blood to organs (e.g., brachial artery).

• Arterioles: Smallest; control resistance and blood flow into capillary beds.

Capillary Beds and Capillary Types

Capillaries are the sites of exchange between blood and tissues.

• Structure: Thin walls (one cell layer), form networks (beds).

• Types:

  • Continuous: Most common; tight junctions, found in skin, muscle, brain.

  • Fenestrated: Pores for increased permeability; found in kidneys, intestines.

  • Sinusoidal: Large gaps; found in liver, bone marrow, spleen.

• Blood flow regulation: By arterioles and precapillary sphincters.

Veins: Structure and Function

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

• Thinner walls, larger lumens than arteries.

• Contain valves to prevent backflow.

• Function as blood reservoirs.

Vascular Anastomoses

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

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

Blood Flow, Blood Pressure, and Resistance

These factors determine how blood moves through the circulatory system.

• Blood flow (F): Volume of blood flowing per unit time.

• Blood pressure (P): Force per unit area exerted on vessel wall.

• Resistance (R): Opposition to flow, mainly from vessel diameter.

• Relationship: $F = \frac{\Delta P}{R}$

Sources of Peripheral Resistance

Three main factors contribute to resistance in blood vessels.

• Blood viscosity (thickness)

• Vessel length

• Vessel diameter (most influential)

Blood Pressure in Different Vessels

Blood pressure varies throughout the vascular system.

• Highest in arteries, lowest in veins.

• Steepest drop in arterioles.

• Systolic pressure: Peak during ventricular contraction.

• Diastolic pressure: Lowest during ventricular relaxation.

• Pulse pressure: Difference between systolic and diastolic.

• Mean arterial pressure (MAP): Average pressure driving blood to tissues.

  • Formula: $MAP = \text{Diastolic Pressure} + \frac{1}{3}(\text{Pulse Pressure})$

• Muscular and respiratory pumps: Aid venous return.

Velocity of Blood Flow and Cross-Sectional Area

Blood flow velocity is inversely related to total cross-sectional area.

• Fastest in arteries, slowest in capillaries, increases in veins.

• Vasomotion: Intermittent flow through capillaries due to sphincter contraction/relaxation.

• Capillaries have the highest total cross-sectional area.

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, hormones, neural controls.

Capillary Exchange and Bulk Flow

Exchange of substances across capillary walls occurs by diffusion and bulk flow.

• Hydrostatic pressure: Pushes fluid out of capillaries (filtration).

• Colloid osmotic pressure: Pulls fluid into capillaries (reabsorption).

• Filtration predominates at arterial end; reabsorption at venous end.

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

Example: In the kidneys, fenestrated capillaries allow rapid filtration of blood plasma, while in the brain, continuous capillaries form the blood-brain barrier.

Additional info: For a more detailed understanding, refer to Figures 19.10 and 19.13 in your textbook for factors influencing mean arterial pressure and blood pressure regulation mechanisms.