Chapter 19: Blood Vessels – Structure, Function, and Physiology

Blood Vessels: Structure and Function

General Structure and Function of the Vascular System

The vascular system forms a closed circuit beginning and ending at the heart, responsible for transporting blood throughout the body. Blood vessels are dynamic, capable of pulsation, constriction, relaxation, and proliferation.

• Types of Blood Vessels: 1. Arteries – carry blood away from the heart. 2. Capillaries – facilitate exchange between blood and tissues. 3. Veins – return blood to the heart.

• Blood Flow Pathway: Blood moves from large arteries to smaller arteries, arterioles, capillary beds, venules, and finally veins.

• Capillaries: Only capillaries directly serve cellular needs by exchanging materials with tissue cells.

• Vessel Length: The adult body contains approximately 60,000 miles of blood vessels.

Structure of Vessel Walls

Except for capillaries, all blood vessels have three layers (tunics) surrounding a central lumen.

• Tunica Intima: Innermost layer, composed of endothelium, provides a slick surface to minimize friction.

• Tunica Media: Middle layer, mainly smooth muscle and elastic fibers, regulated by sympathetic vasomotor fibers. Responsible for vasoconstriction and vasodilation, critical for blood flow and pressure regulation.

• Tunica Adventitia (Externa): Outermost layer, made of collagen fibers, protects and anchors the vessel.

• Capillaries: Consist only of a thin tunica intima.

Arterial System: Structure and Function

Arteries generally carry oxygenated blood (except pulmonary arteries). They are classified by size and function:

• Elastic Arteries: Largest, near the heart (e.g., aorta), contain abundant elastin, withstand pressure fluctuations, and create the pulse.

• Muscular Arteries: Distribute blood to organs, have thick tunica media with more smooth muscle, active in vasoconstriction.

• Arterioles: Smallest arteries, regulate blood flow into capillary beds via diameter changes.

Capillary Structure and Function

Capillaries are the smallest vessels, ideal for material exchange due to their thin walls and small diameter.

• Continuous Capillaries: Uninterrupted endothelial lining, abundant in skin and muscles.

• Fenestrated Capillaries: Endothelial cells have pores (fenestrations), found in areas of active absorption (intestines, endocrine organs, kidneys).

• Sinusoids: Highly modified, leaky capillaries with large lumens, found in liver, bone marrow, lymphoid tissues.

• Capillary Beds: Networks of capillaries, consist of vascular shunts (metarteriole-thoroughfare channel) and true capillaries. Precapillary sphincters regulate blood flow.

Venous System: Structure and Function

Veins return blood to the heart, increasing in diameter and wall thickness from venules to large veins.

• Venules: Formed by capillary union, highly porous, allow movement of WBCs and fluid.

• Veins: Have thinner walls and larger lumens than arteries, often collapsed in tissue samples. Contain valves to prevent backflow, especially in limbs.

• Blood Reservoir: Veins hold up to 65% of the body's blood volume.

Vascular Anastomoses

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

• Arterial Anastomoses: Common in abdominal organs and joints, ensure blood supply even if one channel is blocked.

• Venous Anastomoses: More frequent than arterial, occlusion rarely causes tissue death.

Physiology of Circulation

Blood Flow, Pressure, and Resistance

Blood flow is the volume of blood moving through vessels, organs, or the entire circulation per unit time. Blood pressure is the force exerted by blood on vessel walls, and peripheral resistance is the opposition to flow due to friction.

• Blood Flow: Equivalent to cardiac output for the entire system.

• Blood Pressure: Highest in arteries near the heart, measured in mm Hg.

• Peripheral Resistance: Determined by blood viscosity, vessel length, and vessel diameter.

• Vessel Diameter: Most important factor; resistance varies inversely with the fourth power of radius.

Key Equations:

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

• Cardiac Output: $\text{Cardiac Output (ml/min)} = \text{Stroke Volume (ml/beat)} \times \text{Heart Rate (beats/min)}$

• Resistance and Radius: $\text{Resistance} \propto \frac{1}{r^4}$

Maintenance and Control of Systemic Blood Pressure

Blood pressure is maintained by cardiac output, peripheral resistance, and blood volume. Neural, chemical, and renal mechanisms regulate blood pressure.

• Neural Controls: Alter blood distribution and maintain systemic pressure via sympathetic vasoconstriction.

• Baroreceptors: Detect pressure changes, trigger vasodilation or vasoconstriction.

• Chemoreceptors: Respond to low oxygen or pH, induce vasoconstriction.

• Hormonal Controls: ADH increases water retention and vasoconstriction; adrenal medulla releases norepinephrine and epinephrine.

• Renal Controls: Direct (water retention) and indirect (renin-angiotensin system) regulation.

Variations from Normal Blood Pressure

Blood pressure can deviate from normal, leading to hypotension or hypertension.

• Hypotension: Systolic pressure below 100 mm Hg; often benign, but chronic cases may indicate poor nutrition.

• Hypertension: Sustained arterial pressure of 140/90 mm Hg or higher; major risk factor for heart failure, stroke, and vascular disease. Most cases are primary (essential) hypertension, with no identifiable cause.

• Secondary Hypertension: Due to identifiable disorders (e.g., kidney disease, endocrine disorders).

Physiology of Blood Flow

Blood velocity changes throughout the systemic circulation, being fastest in large arteries and slowest in capillaries. Velocity is inversely related to the total cross-sectional area.

• Capillaries: Slow flow allows efficient exchange of materials.

Autoregulation of Localized Blood Flow

Autoregulation is the automatic adjustment of blood flow to meet tissue needs, controlled by local factors.

• Mechanisms: Declining oxygen or nutrient levels trigger vasodilation; myogenic responses adjust vessel diameter based on stretch.

• Long-Term Autoregulation: Involves growth and enlargement of vessels in response to sustained needs (e.g., high altitude, coronary occlusion).

Blood Flow in Special Areas

Different organs have unique blood flow requirements and regulatory mechanisms.

• Skeletal Muscles: Blood flow increases with activity; vasoconstriction diverts blood from other areas during exercise.

• Brain: Maintains constant flow; sensitive to pH and CO2 levels; protected by myogenic mechanisms.

• Lungs: Low resistance and pressure; low oxygen causes vasoconstriction, opposite to other tissues.

• Heart: Blood flow influenced by aortic pressure and ventricular activity; increases during exercise to meet high oxygen demand.

Blood Flow Through Capillaries

Capillary blood flow is slow and intermittent, regulated by precapillary sphincters. Fluid movement is governed by hydrostatic and osmotic pressures.

• Hydrostatic Pressure: Forces fluid out of capillaries at the arterial end.

• Osmotic Pressure: Draws fluid into capillaries at the venous end.

• Net Fluid Movement: More fluid leaves at the arterial end than returns at the venous end; excess is picked up by lymphatic vessels.

Forms of Circulatory Shock

Circulatory shock occurs when blood vessels are inadequately filled, leading to poor circulation and potential organ damage.

• Hypovolemic Shock: Due to large-scale blood loss; managed by rapid fluid replacement.

• Vascular Shock: Caused by extreme vasodilation and loss of vasomotor tone; blood volume is normal.

• Cardiogenic Shock: Pump failure due to myocardial damage.

Blood Flow Through Circuits

Blood circulates through pulmonary, systemic, and hepatic portal circuits.

• Pulmonary Circuit: Pulmonary trunk → right/left pulmonary arteries → lobar arteries → pulmonary capillaries → pulmonary veins.

• Systemic Circuit: Aorta (ascending, arch, thoracic, abdominal) branches to all body regions; veins return blood to the heart via superior/inferior vena cava.

• Hepatic Portal System: Nutrient-rich blood from digestive organs → mesenteric veins → hepatic portal vein → liver (sinusoids) → hepatic veins → inferior vena cava.

Unique Aspects of Fetal Circulation

Fetal circulation includes several shunts to bypass non-functional organs.

• Foramen Ovale: Connects atria, bypasses pulmonary circuit.

• Ductus Arteriosus: Connects pulmonary trunk and aorta.

• Ductus Venosus: Bypasses liver.

• Umbilical Vein and Arteries: Exchange blood between fetus and placenta.

Effects of Exercise on the Cardiovascular System

Regular exercise enhances cardiovascular health.

• Heart Adaptation: Increases in size and efficiency.

• Vessel Health: Clears fatty deposits, reduces risk of atherosclerosis and coronary heart disease.

Summary Table: Vessel Types and Their Features

Vessel Type	Main Features	Function
Arteries	Thick walls, high pressure, elastic and muscular types	Carry blood away from heart
Capillaries	Thin walls (tunica intima only), small diameter	Exchange of materials with tissues
Veins	Thin walls, large lumens, valves present	Return blood to heart, blood reservoir

Summary Table: Types of Circulatory Shock

Type	Cause	Main Features
Hypovolemic	Blood loss	Weak pulse, rapid heart rate, vasoconstriction
Vascular	Extreme vasodilation	Low resistance, falling blood pressure
Cardiogenic	Pump failure	Inadequate circulation, myocardial damage

Additional info: Some details about fetal circulation and exercise effects were inferred from standard A&P knowledge to ensure completeness.