Cardiovascular System: Structure, Function, and Regulation

Cardiovascular System Overview

Major Components and Functions

The cardiovascular system consists of the heart, blood vessels, and blood. Its primary function is to transport nutrients, gases, hormones, and waste products throughout the body, maintaining homeostasis.

• Heart: Pumps blood through the circulatory system.

• Blood Vessels: Arteries, veins, and capillaries that transport blood.

• Blood: Carries oxygen, nutrients, hormones, and waste products.

Heart Structure and Blood Flow

Major Structures of the Heart

• Atria: Upper chambers that receive blood.

• Ventricles: Lower chambers that pump blood out of the heart.

• Valves: Ensure unidirectional blood flow (tricuspid, bicuspid/mitral, pulmonary, aortic).

Path of Blood Flow

1. Deoxygenated blood enters the right atrium via the vena cava.

2. Passes through the tricuspid valve to the right ventricle.

3. Pumped through the pulmonary valve to the pulmonary arteries and lungs.

4. Oxygenated blood returns via pulmonary veins to the left atrium.

5. Passes through the mitral valve to the left ventricle.

6. Pumped through the aortic valve to the aorta and systemic circulation.

Cardiac Muscle Cells

Cardiocytes: Contractile vs. Autorhythmic Cells

• Contractile Cells: Generate force for heart contraction; make up most of the myocardium.

• Autorhythmic Cells: Initiate and conduct action potentials; form the conduction system (e.g., SA node, AV node).

Electrical Activity of the Heart

Spread of Action Potentials

• Action potentials originate in the SA node (pacemaker).

• Spread through atria to the AV node, then down the Bundle of His, bundle branches, and Purkinje fibers.

Electrocardiogram (ECG) Deflections

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

Ionic Movements During Action Potentials

• Autorhythmic Cells: Spontaneous depolarization due to slow Na+ influx, Ca2+ influx, and K+ efflux.

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

Cardiac Excitation-Contraction Coupling

1. Action potential travels along sarcolemma and T-tubules.

2. Ca2+ enters cell from extracellular space and triggers further Ca2+ release from sarcoplasmic reticulum.

3. Ca2+ binds to troponin, enabling actin-myosin cross-bridge formation and contraction.

Cardiac Cycle and Output

Phases of the Cardiac Cycle

• Ventricular Filling: Blood flows from atria to ventricles.

• Isovolumetric Contraction: Ventricles contract with no volume change.

• Ventricular Ejection: Blood is pumped into arteries.

• Isovolumetric Relaxation: Ventricles relax with no volume change.

During these phases, ventricular, aortic, and atrial pressures change, as do ventricular volumes. Heart sounds are produced by valve closures.

Cardiac Output (CO)

• Definition: Volume of blood pumped by each ventricle per minute.

• Formula: $CO = HR \times SV$

• HR (Heart Rate): Beats per minute.

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

Regulation of Cardiac Output

• Intrinsic Control: Regulation by the heart itself (e.g., Frank-Starling mechanism).

• Extrinsic Control: Regulation by external factors (e.g., autonomic nervous system, hormones).

Blood Flow and Vascular Resistance

Physics of Blood Flow

• Blood flows from high to low pressure.

• Pressure Gradient ($\Delta P$): Drives blood flow.

• Resistance (R): Opposition to flow, mainly determined by vessel radius.

• Ohm's Law for Circulation: $F = \frac{\Delta P}{R}$

Vasculature Anatomy and Function

• Arteries: Thick-walled, high-pressure vessels.

• Arterioles: Major resistance vessels; regulate blood flow to organs.

• Capillaries: Thin-walled; site of exchange.

• Veins: Low-pressure, high-capacitance vessels; return blood to heart.

Arterioles and Resistance

• Arterioles adjust their radius to regulate resistance and blood flow.

• Intrinsic Control: Local factors (e.g., metabolic demand) adjust arteriole diameter.

• Extrinsic Control: Sympathetic nervous system and hormones influence arteriole tone and mean arterial pressure (MAP).

Arterial Baroreceptor Reflex

• Baroreceptors in carotid sinus and aortic arch detect changes in blood pressure.

• Reflex adjusts heart rate, contractility, and vessel diameter to maintain MAP.

Capillary Exchange and Bulk Flow

Material Exchange at Capillaries

• Exchange occurs via diffusion, transcytosis, and bulk flow.

• Capillaries allow movement of gases, nutrients, and waste between blood and interstitial fluid.

Forces Causing Bulk Flow

• Hydrostatic Pressure: Pushes fluid out of capillaries.

• Oncotic (Colloid Osmotic) Pressure: Pulls fluid into capillaries.

• Net filtration pressure determines direction of fluid movement.

Venous System and Blood Mobilization

Structural Differences of Veins

• Thinner walls, larger lumen, less smooth muscle than arteries.

• Contain valves to prevent backflow.

Venous Return and Cardiac Output

• Venous blood is mobilized by skeletal muscle pump, respiratory pump, and sympathetic stimulation.

• Increased venous return increases cardiac output.

Redistribution of Cardiac Output During Exercise

• Blood flow is redirected to skeletal muscles to meet increased metabolic demand.

• Vasoconstriction in non-essential organs; vasodilation in active muscles.

Blood Composition and Function

Major Components of Blood

Component	Function
Red Blood Cells (Erythrocytes)	Transport oxygen and carbon dioxide
White Blood Cells (Leukocytes)	Immune defense
Platelets (Thrombocytes)	Blood clotting
Plasma	Transport of nutrients, hormones, waste; contains proteins

Red Blood Cells: Function and Production

• Function: Carry oxygen via hemoglobin; transport some CO2.

• Production (Erythropoiesis): Occurs in red bone marrow; stimulated by erythropoietin (EPO) from kidneys.

Additional info: Erythropoiesis is regulated by oxygen levels in the blood; low oxygen stimulates increased RBC production.