Cardiovascular System: Structure, Function, and Pathophysiology

The Cardiovascular System

Overview

The cardiovascular system is responsible for the transport of blood, nutrients, gases, and wastes throughout the body. It consists of the heart, blood vessels, and blood, and is essential for maintaining homeostasis and supporting cellular metabolism.

The Blood Vessels

Types of Blood Vessels

• Arteries and Arterioles: Carry blood away from the heart.

• Capillaries: Smallest vessels; permit exchange of fluid, nutrients, and gases with tissues.

• Veins and Venules: Return blood to the heart.

Arteries

Structure and Function

• Arteries carry blood away from the heart under high pressure.

• Arterial walls have three distinct layers:

  • Tunica intima: Made of endothelium (simple squamous epithelium).

  • Tunica media: Thickest layer, composed of smooth muscle.

  • Tunica externa: Outer layer of connective tissue.

• Lumen: Hollow interior of the vessel.

• Arterioles: Smallest arteries, regulate blood flow into capillaries.

Arterioles

Role in Circulation

• Arteries branch into arterioles, which feed blood into capillaries.

• Diameter of arterioles is controlled by the sympathetic nervous system and is crucial for blood pressure regulation.

• Arterioles also control blood flow through capillary beds.

• After passing through capillaries, blood is collected by venules for return to the heart.

Capillaries

Structure and Function

• Capillaries are microscopic vessels with walls only one cell thick (endothelium with basement membrane).

• No smooth muscle in capillary wall.

• Red blood cells pass through in single file due to narrow diameter.

• Capillary beds are networks of branching capillaries.

• Blood flow through capillary beds is regulated by arteriolar diameter.

• Capillaries permit exchange of nutrients, gases, and wastes between blood and tissues.

Exchange Mechanisms

• Thin, porous walls allow exchange with tissue fluid (extracellular fluid).

• Substances move by diffusion, filtration, and osmosis.

• Slow flow of red blood cells allows time for exchange.

Routes Through Endothelial Cells

• Direct diffusion through cell membrane.

• Diffusion through intercellular clefts.

• Diffusion through pores (fenestrations).

• Transport via vesicles.

Pressure in Capillaries

• Arterial end: 35–40 mmHg

• Venous end: 15–20 mmHg

• Low pressure is important for efficient exchange and prevents tissue damage.

• Adult has ~10 billion capillaries, totaling over 40,000 miles in length.

Bulk Fluid Flow

Mechanisms

• Fluid moves out of capillaries at arterial end due to higher blood pressure (filtration).

• Fluid moves back into capillaries at venous end due to lower blood pressure and osmotic pressure (reabsorption).

Avascular Tissues

• Some tissues lack capillaries (avascular): cornea, lens, all epithelial tissues.

• Tendons and ligaments have few capillaries but are not truly avascular.

Veins

Structure and Function

• Veins return blood to the heart.

• Same three layers as arteries, but less muscle and more stretchable walls.

• Valves prevent backflow, especially in lower extremities.

• Large diameter lumens; 65–70% of blood is contained in veins.

Venous Return

Mechanisms

• Low pressure and thin walls can lead to problems with blood return.

• Three mechanisms aid venous return:

  • Skeletal muscle contractions

  • One-way valves

  • Respiratory pump

The Heart

Basic Function

• Beats about 75 times per minute (100,000 times/day).

• Pumps approximately 5 liters of blood per minute at rest.

• Exercise can increase cardiac output to 25–35 L/min in athletes.

Anatomy of the Heart

Location and Structure

• Located between the lungs; apex is the pointed end.

• Pericardium: Serous membrane around heart; protects, anchors, and prevents overfilling.

• Pericardial cavity contains lubricating serous fluid.

Layers of the Heart Wall

• Endocardium: Simple squamous epithelium, continuous with blood vessel lining.

• Myocardium: Thick middle layer of cardiac muscle.

• Epicardium: Outer layer, visceral pericardium.

Chambers and Valves

• Interventricular septum: Separates right and left sides.

• Atria: Upper receiving chambers.

• Ventricles: Lower pumping chambers.

• Four heart valves:

  • Two atrioventricular (AV) valves

  • Two semilunar valves

  • Valves open/close in response to pressure changes.

  • Chordae tendineae: Fibrous cords supporting AV valves.

Heart Sounds

Normal and Abnormal Sounds

• Normal: "lub-dub" (AV valves close, then semilunar valves close).

• Abnormal (murmurs): Caused by turbulent blood flow, valve problems, anemia, etc.

Path of Blood Through the Heart

Circulation Pathways

• Deoxygenated and oxygenated blood do not mix.

• Right side pumps to lungs (pulmonary circuit); left side pumps to body (systemic circuit).

• Left ventricle has thicker walls to generate higher pressure.

Pulmonary Circuit

• Right side pumps blood to lungs for oxygenation.

• Pulmonary trunk splits into right and left pulmonary arteries.

• Pulmonary veins return oxygenated blood to left atrium.

Systemic Circuit

• Left side pumps oxygenated blood to body.

• Aorta: Largest artery.

• Superior/Inferior Vena Cava: Largest veins.

• Coronary circulation: Vessels supplying the myocardium.

The Cardiac Cycle

Phases

• Systole: Contraction (atrial and ventricular).

• Diastole: Relaxation.

• One cardiac cycle = 0.8 seconds (atrial systole: 0.1s, ventricular systole: 0.3s, diastole: 0.4s).

Periods of Cardiac Cycle

• Atrial diastole and passive ventricular filling.

• Atrial systole: Signal from SA node causes atria to contract.

• Ventricular systole: Ventricles contract, blood ejected through semilunar valves.

Cardiac Output

Definition and Formula

• Cardiac output (CO) = heart rate (HR) × stroke volume (SV)

$\text{CO} = \text{HR} \times \text{SV}$

• Stroke volume: Volume of blood pumped by a ventricle per beat.

• Example: 75 beats/min × 70 ml/beat = 5,250 ml/min

Factors Affecting Cardiac Output

• Decreased HR or contraction force lowers CO.

• Increased HR or contraction force raises CO.

• Exercise increases HR and SV via sympathetic stimulation.

• Blood pressure is directly affected by CO.

Control of Heart Rate

Intrinsic and Extrinsic Control

• Intrinsic: SA node and AV node within the heart.

• Extrinsic: Autonomic nervous system and adrenal medulla.

Autonomic Nervous System Effects

• Parasympathetic: Decreases HR (rest state).

• Sympathetic: Increases HR (stress/exercise).

• Adrenal medulla releases epinephrine/norepinephrine, prolonging sympathetic response.

Pulse

• Pulse is a pressure wave felt in arteries due to ventricular contraction.

• Pulse rate equals heart rate; regularity and strength indicate circulation efficiency.

Blood Pressure

Definition and Measurement

• Force exerted by blood against vessel walls.

• Measured as systolic/diastolic in mmHg (e.g., 120/80 mmHg).

Factors Affecting Blood Pressure

• Cardiac output

• Peripheral resistance (vasoconstriction increases BP, vasodilation decreases BP)

Maintaining Blood Pressure

• Baroreceptors monitor BP and signal the cardiac center in the medulla oblongata.

• High BP triggers reflexes to lower HR and contractility; low BP triggers reflexes to raise HR and contractility.

Hypertension

• High blood pressure: Systolic >140 mmHg, Diastolic >90 mmHg.

• "Silent Killer"—often asymptomatic.

• Causes: Genetics, vasoconstrictor hormones, lifestyle factors.

Anaphylaxis

• Systemic allergic reaction causing widespread vasodilation and shock.

• Results in low blood pressure and cardiac output; can be fatal without intervention (e.g., EpiPen).

Cardiac Conduction System

• Specialized cells generate and distribute action potentials for heart contraction.

• Components: SA node, AV node, AV bundle, bundle branches, Purkinje fibers.

SA Node

• Located in right atrium; initiates heartbeat (~75/min).

• Pacemaker of the heart.

AV Node

• Located near right ventricle; relays signal to ventricles.

• Creates slight delay for proper filling.

Electrocardiogram (ECG)

• Comprehensive recording of heart's electrical activity.

• Three main waves:

  • P wave: Atrial depolarization

  • QRS complex: Ventricular depolarization

  • T wave: Ventricular repolarization

Arrhythmias

• Abnormal heart rhythms due to faulty electrical impulses.

• Tachycardia: Fast HR (>100 bpm)

• Bradycardia: Slow HR (<60 bpm)

• ECG can help diagnose arrhythmias before symptoms arise.

Atrial Fibrillation

• Chaotic atrial activity; no defined P waves.

• Increases risk of stroke or heart attack.

Ventricular Fibrillation

• Rapid, irregular contractions; no effective pumping.

• Requires immediate defibrillation.

Asystole

• Absence of electrical and mechanical activity; no pulse or breathing.

Myocardial Infarction (Heart Attack)

• Portion of heart muscle dies due to lack of oxygen.

• Partial blockage causes angina; complete blockage causes MI.

Treatments

• Dissolving blood clots (t-PA, aspirin)

• Stents to open blocked vessels

• Coronary bypass surgery

• Heart transplant

Blood Vessel Type	Wall Structure	Function	Pressure
Artery	Thick muscle, elastic tissue	Carry blood away from heart	High
Arteriole	Thin muscle, less elastic	Regulate blood flow to capillaries	Moderate
Capillary	Single cell layer (endothelium)	Exchange of gases, nutrients, wastes	Low
Venule	Thin wall, some muscle	Collect blood from capillaries	Low
Vein	Thin muscle, valves present	Return blood to heart	Lowest

Additional info: This guide expands on the original notes with definitions, examples, and academic context for key terms and processes, ensuring completeness and clarity for college-level Anatomy & Physiology students.