Chapter 8: The Heart and Blood Vessels – Structure and Function of the Cardiovascular System

Introduction to the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system, also known as the circulatory system, is essential for maintaining homeostasis in the human body. It consists of the heart, which acts as a pump, and a network of blood vessels that distribute blood throughout the body. The nervous system provides oversight, controlling both the rate of heart contractions and the distribution of blood to tissues and organs.

• Heart: Muscular organ that pumps blood.

• Blood vessels: Network for blood distribution, including arteries, veins, and capillaries.

• Nervous system: Regulates heart rate and blood vessel distribution.

Blood Vessels: Structure and Function

Types of Blood Vessels

Blood vessels are classified into three main types, each with distinct structural and functional characteristics:

• 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; walls are one cell layer thick and porous.

• Veins: Return blood to the heart; thin-walled and serve as blood reservoirs.

Structure of Arteries

Arteries have a specialized structure to withstand high pressure:

• Innermost layer: Endothelium of squamous epithelial cells.

• Middle layer: Smooth muscle with elastic fibers.

• Outer layer: Connective tissue.

Aneurysm

An aneurysm is a defect in the arterial wall resulting in ballooning. It may bulge inward or outward, often developing slowly and remaining symptomless until rupture, which can be fatal if it occurs in major arteries like the aorta.

Arterioles and Precapillary Sphincters

Arterioles are the smallest arteries, and precapillary sphincters regulate blood flow into capillaries:

• Vasodilation: Relaxation of vascular smooth muscle increases blood flow to capillaries.

• Vasoconstriction: Contraction of vascular smooth muscle decreases blood flow to capillaries.

Capillaries: Exchange of Substances

Capillaries are the site of selective exchange between blood and tissues:

• Structure: Smallest blood vessels, one cell layer thick, porous.

• Function: Exchange of oxygen, nutrients, and waste products with interstitial fluid.

• Fluid movement is driven by blood pressure and the presence of plasma proteins.

Lymphatic System

Role in Blood Volume Maintenance

The lymphatic system helps maintain blood volume by returning excess interstitial fluid to the circulatory system and transporting substances too large for capillaries. It also plays a major role in immune defense.

• Structure: Blind-ended capillaries and lymphatic vessels.

• Lymph: Fluid derived from interstitial fluid.

Veins: Structure and Blood Return Mechanisms

Structure and Function of Veins

Veins have three thin-walled layers and a larger lumen than arteries, allowing high distensibility. They carry blood toward the heart and serve as blood volume reservoirs.

Mechanisms Assisting Blood Return

• Contraction of skeletal muscles.

• One-way valves ensure unidirectional flow.

• Pressure changes from breathing push blood toward the heart.

The Heart: Structure and Function

General Features

The heart is a muscular pump composed of living cells. It can vary its output from 5 to 25 liters of blood per minute, with a resting rate of about 75 beats per minute, and can accelerate with exertion. The heart can beat autonomously but is modulated by the nervous system.

Layers of the Heart Wall

• Pericardium: Fibrous sac protecting and anchoring the heart.

• Epicardium: Outermost layer of epithelial and connective tissue.

• Myocardium: Thick layer of cardiac muscle; contracts during heartbeat; electrical signals flow directly from cell to cell.

• Endocardium: Innermost layer of endothelial tissue; continuous with blood vessel lining.

Chambers and Valves of the Heart

• Four chambers: Two atria (upper), two ventricles (lower).

• Septum: Muscular partition separating right and left sides.

• Four valves: Prevent backflow.

  • Atrioventricular (AV) valves: Tricuspid (right), Bicuspid/Mitral (left).

  • Semilunar valves: Pulmonary and Aortic.

Blood Flow Through the Cardiovascular System

Pulmonary and Systemic Circuits

The heart pumps blood through two circuits:

• Pulmonary circuit: Heart → lungs → heart; blood picks up O2, releases CO2.

• Systemic circuit: Heart → body → heart; delivers O2, picks up CO2 and waste.

Steps in Pulmonary Circuit

1. Deoxygenated blood enters right atrium via vena cava.

2. Passes through right AV valve to right ventricle.

3. Through pulmonary semilunar valve to pulmonary trunk and arteries.

4. Blood is oxygenated in pulmonary capillaries.

5. Oxygenated blood returns via pulmonary veins to left atrium.

Steps in Systemic Circuit

1. Oxygenated blood flows from left atrium through left AV valve to left ventricle.

2. Through aortic semilunar valve to aorta.

3. Through arteries and arterioles to tissues.

4. Capillaries deliver nutrients and oxygen, pick up wastes.

5. Blood returns via venules and veins to vena cava and right atrium.

Blood Flow Pattern

• Blood passes through the heart twice per body circuit: once deoxygenated (right side), once oxygenated (left side).

• Deoxygenated and oxygenated blood do not mix.

Coronary Circulation

Coronary Arteries and Veins

• Coronary arteries: Supply myocardium; small diameter, susceptible to blockage (atherosclerosis).

• Coronary veins: Collect blood from myocardial capillaries, 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): Both atria and ventricles relax, semilunar valves close.

Heart Sounds

• Lub: Closing of AV valves during ventricular systole.

• Dub: Closing of semilunar valves during ventricular diastole.

• Heart murmurs: Indicate disturbed blood flow, possibly defective valves.

Cardiac Conduction System

Coordination of Heart Contraction

• Sinoatrial (SA) node: Cardiac pacemaker in right atrium; initiates heartbeat.

• Atrioventricular (AV) node: Relays impulse between atria and ventricles.

• AV bundle and Purkinje fibers: Carry impulse through septum and ventricles.

Electrocardiogram (ECG/EKG)

Recording Heart's Electrical Activity

• P wave: Impulse across atria.

• QRS complex: Spread of impulse through ventricles.

• T wave: End of electrical activity in ventricles.

• EKG detects arrhythmias and ventricular fibrillation.

Blood Pressure

Definition and Measurement

• Systolic pressure: Highest pressure during ventricular systole.

• Diastolic pressure: Lowest pressure during ventricular diastole.

• Measured in mm Hg using a sphygmomanometer.

Normal Blood Pressure Values

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

Regulation of the Cardiovascular System

Homeostatic Regulation

• Maintaining constant arterial pressure is key.

• Regulated by cardiac output and arteriole diameter.

Baroreceptors

• Pressure receptors in aorta and carotid arteries monitor blood pressure.

• Negative feedback loop adjusts heart rate and vascular resistance.

Hormonal Regulation

• Epinephrine and norepinephrine: Increase heart rate and contraction force.

• ADH, aldosterone, renin-angiotensin, ANH: Influence blood pressure.

Local Control Mechanisms

• Precapillary sphincters adjust local blood flow based on tissue metabolic activity.

Cardiovascular Disorders

Major Disorders

• Hypertension: High blood pressure; risk factor for cardiovascular disease.

• Aneurysm: Ballooning of arterial wall; risk of rupture.

• Atherosclerosis: Cholesterol buildup in arteries; leads to heart attack, stroke.

• Angina: Chest pain from reduced blood flow to heart muscle.

• Heart attack: Permanent damage to heart tissue from blocked blood flow.

• Heart failure: Reduced efficiency of heart pumping.

• Embolism: Blockage of blood vessel by floating material.

• Stroke: Brain damage from interrupted blood supply.

Risk Factors for Hypertension

Risk Factor	Description
Obesity	Excess body weight increases strain on heart
High-fat diet	Raises cholesterol and triglycerides
Smoking	Damages blood vessels
Diabetes	Increases risk of vascular damage
Family history	Genetic predisposition

Reducing Cardiovascular Disease Risk

• Do not smoke.

• Monitor and control cholesterol and blood pressure.

• Exercise regularly.

• Maintain healthy weight.

• Control diabetes.

• Avoid chronic stress.

Key Equations

Blood Pressure Equation

• Blood pressure is determined by cardiac output and peripheral resistance:

$\text{Blood Pressure} = \text{Cardiac Output} \times \text{Peripheral Resistance}$

Cardiac Output Equation

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

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

Example

Example: If a person has a heart rate of 75 beats/min and a stroke volume of 70 mL/beat, their cardiac output is:

$75 \times 70 = 5250$ mL/min = 5.25 L/min

Additional info: These notes expand on the original slides by providing definitions, context, and equations relevant to Anatomy & Physiology students.