The Cardiovascular System: Structure, Function, and Disorders

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The Cardiovascular System

Introduction to the Cardiovascular System

The cardiovascular system, also known as the circulatory system, plays a critical role in maintaining homeostasis by transporting blood throughout the body. It consists of the heart (the pump), blood vessels (the distribution network), and is regulated by the nervous system.

Heart: Pumps blood through the vessels.
Blood vessels: Network for blood distribution, including arteries, capillaries, and veins.
Nervous system: Controls heart rate and blood distribution.

Blood Vessels: Structure and Function

Types of Blood Vessels

Arteries: Carry blood away from the heart under high pressure; thick-walled.
Capillaries: Microscopic vessels where exchange of solutes and water with body cells occurs; thin-walled and porous.
Veins: Return blood to the heart; thin-walled with large lumens and high distensibility.

Structure of Arteries

Three layers:
- Endothelium: Innermost layer of squamous epithelial cells.
- Smooth muscle with elastic fibers: Middle layer for flexibility and strength.
- Connective tissue: Outer layer for support.
Function: Carry blood away from the heart under pressure.

Arterial Disorders

Aneurysm: Defect in arterial wall causing ballooning; may be symptomless until rupture, which can be fatal.

Arterioles and Precapillary Sphincters

Arterioles: Smallest arteries, regulate blood flow into capillaries.
Precapillary sphincters: Control blood flow into individual capillaries.
Vasodilation: Relaxation increases blood flow.
Vasoconstriction: Contraction decreases blood flow.

Capillaries: Exchange with Tissues

Smallest, most numerous blood vessels; one cell layer thick.
Form capillary beds for extensive exchange surface.
Selective exchange of substances (O2, nutrients, wastes) with interstitial fluid.
Fluid movement:
- At arterial end: Fluid filtered out by blood pressure.
- At venous end: Most fluid reabsorbed by diffusion (driven by plasma proteins).

Lymphatic System

Maintains blood volume by returning excess interstitial fluid to circulation.
Transports large substances (lipids, microorganisms).
Major role in immune defense.
Structure: Blind-ended capillaries, lymphatic vessels, lymph (fluid derived from interstitial fluid).

Veins: Return Blood to the Heart

Three layers, but thinner walls and larger lumen than arteries.
High distensibility; serve as blood reservoirs.
Blood return assisted by:
- Contraction of skeletal muscles (muscle pump).
- One-way valves.
- Pressure changes during breathing.

The Heart: Structure and Function

Heart Anatomy

Composed mostly of cardiac muscle tissue.
Pericardium: Fibrous sac surrounding the heart for protection and anchoring.
Wall layers:
- Epicardium: Outer layer.
- Myocardium: Thick muscle layer; contracts to pump blood.
- Endocardium: Inner endothelial layer, continuous with blood vessels.

Heart Chambers and Valves

Four chambers:
- Two atria (upper chambers).
- Two ventricles (lower chambers).
Septum: Muscular partition separating right and left sides.
Four valves prevent backflow:
- Two atrioventricular (AV) valves: Tricuspid (right), Bicuspid/Mitral (left).
- Two semilunar valves: Pulmonary, Aortic.

Pattern of Blood Flow

Two circuits:
- Pulmonary circuit: Heart → Lungs → Heart (gas exchange).
- Systemic circuit: Heart → Body → Heart (nutrient/waste exchange).
Deoxygenated and oxygenated blood do not mix.

Pulmonary Circuit Steps

Deoxygenated blood enters right atrium via vena cava.
Passes through right AV valve to right ventricle.
Pumped through pulmonary semilunar valve into pulmonary arteries to lungs.
Blood is oxygenated in pulmonary capillaries.
Returns via pulmonary veins to left atrium.

Systemic Circuit Steps

Oxygenated blood flows from left atrium through left AV valve to left ventricle.
Pumped through aortic semilunar valve into aorta.
Distributed via arteries and arterioles to tissues.
Exchange of nutrients and wastes in capillaries.
Returns via veins to right atrium.

Coronary Circulation

Coronary arteries: Supply myocardium; susceptible to atherosclerosis.
Coronary veins: Collect blood from myocardium and return it to right atrium.

The Cardiac Cycle

Phases of the Cardiac Cycle

Atrial systole (0.1 s): Atria contract, AV valves open, ventricles fill.
Ventricular systole (0.3 s): Ventricles contract, AV valves close, semilunar valves open, blood ejected.
Diastole (0.4 s): All chambers relax, semilunar valves close.

Heart Sounds

"Lub": Closing of AV valves during ventricular systole.
"Dub": Closing of semilunar valves during ventricular diastole.
Heart murmurs: Abnormal sounds due to disturbed blood flow, often from defective valves.

Cardiac Conduction System

Sinoatrial (SA) node: Pacemaker, initiates heartbeat.
Atrioventricular (AV) node: Relays impulse between atria and ventricles.
AV bundle and Purkinje fibers: Conduct impulse through septum and ventricles.

Electrocardiogram (ECG/EKG)

Records electrical activity of the heart.
Key waves:
- P wave: Atrial depolarization.
- QRS complex: Ventricular depolarization.
- T wave: Ventricular repolarization.
Used to detect arrhythmias and other cardiac abnormalities.

Blood Pressure and Its Regulation

Blood Pressure

Force exerted by blood on vessel walls.
Systolic pressure: Highest pressure during ventricular contraction.
Diastolic pressure: Lowest pressure during ventricular relaxation.
Highest in arteries, lowest in veins.

Measuring Blood Pressure

Measured with a sphygmomanometer (inflatable cuff and gauge).
Units: mm Hg (millimeters of mercury).
Normal: Systolic < 120 mm Hg, Diastolic < 80 mm Hg.

Table: Systolic and Diastolic Blood Pressure Categories

Category	Systolic (mm Hg)	Diastolic (mm Hg)
Normal	<120	<80
Elevated	120–129	<80
Hypertension Stage 1	130–139	80–89
Hypertension Stage 2	≥140	≥90
Hypertensive Crisis	>180	>120

Hypertension (High Blood Pressure)

Sustained elevation of blood pressure; "silent killer" due to few symptoms.
Risk factor for cardiovascular disease; causes vessel hardening and heart strain.

Table: Risk Factors for Hypertension

Risk Factor	Description
Genetics	Family history increases risk
Obesity	Excess body weight raises blood pressure
High sodium intake	Increases fluid retention and pressure
Physical inactivity	Reduces cardiovascular efficiency
Excessive alcohol	Can raise blood pressure
Chronic stress	May contribute to sustained high pressure
Smoking	Damages blood vessels

Hypotension (Low Blood Pressure)

May cause dizziness or fainting, especially after sudden position changes.
Can result from blood or fluid loss.

Regulation of the Cardiovascular System

Homeostasis centers on maintaining constant arterial pressure.
Regulated by cardiac output and arteriole diameter.
Local blood flows adjusted to meet tissue needs.

Baroreceptor Reflex

Baroreceptors: Pressure sensors in aorta and carotid arteries.
Negative feedback loop maintains arterial pressure.
Increased pressure stretches vessels, activating baroreceptors, which signal the brain to lower heart rate and dilate vessels, reducing pressure.

Hormonal Regulation

Epinephrine and norepinephrine: Increase heart rate and contraction force.
Antidiuretic hormone (ADH), aldosterone, renin-angiotensin system, atrial natriuretic hormone (ANH): Influence blood pressure by affecting blood volume and vessel tone.

Local Control of Blood Flow

Precapillary sphincters adjust flow to local tissue needs.
Active tissues receive more blood via vasodilation.
During low blood pressure, vasoconstriction shunts blood to vital organs (brain, heart).

Exercise and Cardiac Output

Blood flow to active muscles increases during exercise.
Cardiac output rises to maintain blood pressure (up to 20–35 L/min in athletes).

Cardiovascular Disorders

Overview

Leading cause of death in the U.S. (>655,000 deaths/year).
Major disorders: Hypertension, aneurysms, atherosclerosis, angina, heart attack, heart failure, embolism, stroke.

Atherosclerosis

Build-up of cholesterol deposits in arteries.
Leads to heart attacks, strokes, aneurysms, peripheral vascular disease.
Risk factors: High cholesterol/triglycerides, obesity, sedentary lifestyle, high-fat diet, smoking, diabetes, hypertension, family history.

Angina

Chest pain due to reduced blood flow in coronary arteries.
Symptoms: Pain, tightness, shortness of breath.
Diagnosis: Angiography.
Treatment: Medication, balloon angioplasty, coronary artery bypass graft (CABG).

Heart Attack (Myocardial Infarction)

Sudden blockage of blood flow to heart muscle, causing tissue death.
Symptoms: Intense chest pain, radiating pain, nausea.
Diagnosis: ECG, blood enzymes.
Treatment: Arrhythmia control, clot-dissolving drugs, CABG.

Heart Failure

Heart becomes less efficient; fluid backs up in tissues (congestive heart failure).
Causes: Age, prior heart attacks, valve disease, lung disease, hypertension.
Treatment: Diuretics, medications to improve heart function.

Embolism

Blockage of a blood vessel by material (clot, fat, air, etc.) traveling in the bloodstream.
Types:
- Pulmonary embolism: Affects lungs.
- Cerebral embolism: May cause stroke.
- Cardiac embolism: May cause heart attack.

Stroke

Brain damage from interrupted blood supply (embolism or vessel rupture).
Symptoms depend on affected brain area; immediate care is crucial.
Treatment: Clot-dissolving drugs (embolism), surgery (rupture).

Heart Replacement

Heart transplants: Average survival ~15 years; limited by donor availability.
Alternatives: Artificial hearts, xenotransplants (animal hearts).

Reducing Cardiovascular Risk

Do not smoke (doubles risk).
Control cholesterol levels.
Exercise regularly.
Monitor and treat high blood pressure.
Maintain healthy weight.
Control diabetes.
Avoid chronic stress.

Key Equations

Blood Pressure:
Cardiac Output (CO):

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