The Blood Vessels and Circulatory System

The Blood Vessels

Objectives

  • By the end of this lesson, you should be able to…
    • Describe the three layers that typically form the wall of a blood vessel, and state the function of each.
    • Define vasoconstriction and vasodilation.
    • Compare and contrast the structure and function of the three types of arteries.
    • Describe the structure and function of a capillary bed.
    • Describe the structure and function of veins and explain how veins differ from arteries.
    • Define blood flow, blood pressure, and resistance, and explain the relationships between these factors.
    • Describe how blood pressure differs in the arteries, capillaries, and veins.
    • List and explain the factors that influence blood pressure and describe how blood pressure is regulated.
    • Define hypertension. Describe its manifestations and consequences.
    • Explain how blood flow is regulated in the body in general and in specific organs.
    • Outline factors involved in capillary dynamics, and explain the significance of each.
    • Define circulatory shock. List several possible causes.
    • Trace the pathway of blood through the pulmonary circuit and state the importance of this special circulation.
    • Describe the general functions of the systemic circuit.
    • Name and give the location of the major arteries and veins in systemic circulation.
    • Describe the structure and special function of the hepatic portal system.

Helpful Terminology

  • angi/o: vessel
  • thromb/o: clot
  • arteri/o: artery
  • varic/o: dilated
  • ather/o: plaque, fatty substance
  • vas/o: vessel
  • -ectasis: expansion or dilation
  • vascul/o: vessel
  • -emia: blood
  • ven/o: vein
  • hem/o; hemat/o: blood
  • phleb/o: vein

Introduction -- A Circle of Blood

  • Historical Background:
    • Earlier belief by Galen (Greek physician) that blood flowed in a tide-like manner returning through the same vessels carrying impurities.
    • William Harvey (1628) proposed a model of continuous blood circulation, stating blood flows from the heart through arteries and returns via veins, forming a circular flow.
    • Harvey’s theories were initially scorned but ultimately validated by the discovery of capillaries and experimentation revival in the early second millennium.
    • Due to these contributions, Harvey is often called the "Father of modern physiology."

General Plan of Circulation

  • Closed System: Blood is confined to circulatory vessels; it never directly bathes tissues.
  • Dual Circulatory System:
    • Pulmonary Circuit: Right side of the heart pumps blood into the lungs to receive oxygen and return it to the left side.
    • Systemic Circuit: Left side of the heart pumps oxygenated blood throughout the body, returning deoxygenated blood to the right side.
  • Lymphatic System: Assists in returning tissue fluid to circulation but is not a part of the cardiovascular system.

Pattern of Flow

  • Vasculature Description:
    • Blood vessels include arteries, arterioles, capillaries, venules, and veins.
    • Arteries carry blood away from the heart; large arteries branch into smaller arterioles, which lead to capillaries for material exchange.
    • Blood returns via venules into veins leading back to the heart.
Blood Vessel Classification
  • Arteries: Carry blood away from the heart, usually oxygen-rich (except pulmonary artery).
  • Arterioles: Smallest arteries feeding into capillaries.
  • Capillaries: Smallest vessels where exchange occurs between blood and tissues.
  • Venules: Small veins carrying blood toward the heart, merging into larger veins.
  • Veins: Carry deoxygenated blood towards the heart, except pulmonary veins.
General Blood Vessel Structure
  • Three Tunics:
    • Tunica Interna (Tunica Intima): Innermost layer, reduces friction, lined with endothelial cells.
    • Tunica Media: Middle layer, composed of smooth muscles, regulates blood flow and pressure.
    • Tunica Externa (Tunica Adventitia): Outer layer made of connective tissue, includes vasa vasorum (small vessels).

Arteries

  • Types of Arteries:
    • Elastic (Conducting) Arteries: Largest, nearby heart, include the aorta.
    • Muscular (Distributing) Arteries: Medium-sized, distribute blood to body sections, prominent tunica media.
    • Arterioles: Smallest arteries, critical in controlling blood pressure and flow to capillaries.
Capillaries
  • Function: Functional units of the vascular system for nutrient and waste exchange.
  • Structure: Composed of a single layer of epithelial cells, extremely thin walled to allow high permeability.
  • Types: Continuous, fenestrated, and sinusoids based on permeability levels.
Capillary Beds
  • Arrangement: Networks connecting arterioles to venules.
  • Flow Regulation: Controlled by precapillary sphincters and influenced by local conditions (vasomotion).

Venules and Veins

  • Structure: Similar layers to arteries but thinner, larger lumens for blood reservoirs.
  • Valves: Prevent backflow; most numerous in limb veins.
  • Mechanisms for Venous Return: One-way valves, skeletal muscle pump, and respiratory pump.

Vascular Anastomoses

  • Definition: Union of branches of two or more vessels providing alternate routes for blood flow.
  • Types: Arterial, venous, and arteriovenous anastomoses.

Hemodynamics (Physiology of Circulation)

  • Blood Flow: Volume per unit time, influenced by pressure and resistance.
  • Blood Pressure: Force exerted on vessel walls, measured in mmHg. Factors include cardiac output, resistance, elasticity, and blood volume.
  • Resistance Sources: Diameter, length, and viscosity of vessels.
  • Pressure-Flow Relationship: Flow equals pressure difference divided by resistance.
Blood Pressure Measurements
  • Components: Systolic and diastolic pressure measured with a sphygmomanometer.
  • Korotkoff Sounds: Sounds heard during blood pressure measurement.

Alterations in Blood Pressure

  • Hypotension: Systolic < 90 mmHg, symptoms include dizziness.
  • Hypertension: Systolic ≥ 140/90 mmHg, can lead to serious health issues. Types include primary and secondary hypertension.

Autoregulation

  • Tissue’s Ability: Local blood flow adjustment based on metabolic needs.
  • Mechanisms: Metabolic (decreased oxygen, increased CO2) and myogenic (muscle response to stretch).

Blood Flow in Special Areas

  • Skeletal Muscles: Blood flow increases during activity.
  • Brain: Constant blood flow regulated by autoregulatory systems sensitive to local gases.
  • Lungs: Blood flow adjusted to oxygen levels, low oxygen induces vasoconstriction.
  • Heart: Blood flow increases during exercise, influenced by local accumulation of CO2.

Forms of Circulatory Shock

  • Hypovolemic Shock: Most common form related to significant blood loss.
  • Vascular Shock: Caused by vasodilation, leading to low pressure.
  • Cardiogenic Shock: Heart’s inability to sustain adequate circulation, often due to myocardial damage.

Blood Circuits

  • Systemic Circuit: Supplies the body, arteries and veins involved in nutrient and waste exchange.
  • Pulmonary Circuit: Transports deoxygenated blood to lungs for gas exchange and returns oxygenated blood to the heart.

Unique Areas of Circulation

  • Hepatic Portal System: Carries nutrient-rich blood from GI tract to liver for processing.
  • Cerebral Circulation: Circle of Willis for brain blood supply.
  • Fetal Circulation: Modified structures allow nutrient exchange with maternal blood, includes foramen ovale and ductus arteriosus.

Blood Vessel Pathologies

  • Arteriosclerosis & Atherosclerosis: Hardening and narrowing of arteries, potential cause of heart-related issues.
  • Varicose Veins: Results from valve incompetence leading to vein dilation.
  • Deep Vein Thrombosis (DVT): Formation of clots in veins, potentially leading to serious complications.

Benefits of Exercise

  • Cardiovascular Improvement: Regular aerobic exercise enhances circulation, lowers blood pressure, and improves cardiac output.

Aging and the Cardiovascular System

  • Age-Related Changes: Increased vessel stiffness, reduced cardiac output, and declines in heart muscle efficiency.

Lab Practice

  • Artery and Vein Identification: Recognizing major arteries and veins based on names indicative of their location or function is crucial in anatomy studies.
Branches of the Aorta
  • Ascending Aorta: Supplies the heart.
  • Aortic Arch: Brachiocephalic, left common carotid, and left subclavian arteries.
  • Descending Thoracic Aorta: Supplies thoracic structures.
  • Abdominal Aorta: Supplies abdominal organs and branches into the iliac arteries.
Major Systemic Veins
  • Venous Drainage: Detailed understanding of the venous pathway returning blood to the heart, noting that most veins mirror arterial pathways except for unique vein arrangements.

References

  • Marieb, E. N. (2010). Human Anatomy and Physiology (v. 2). Boston: Pearson Learning Solutions.
  • OpenStax College, Anatomy and Physiology. OpenStax College, 19 June 2013.
  • William Harvey [Internet]. The Famous People website; 2015.[cited 2015 Mar 1]. Available from: //www.thefamouspeople.com/profiles/william-harvey-492.php.
Objectives - Describe blood vessel layers and their functions. - Define vasoconstriction and vasodilation. - Compare types of arteries. - Detail capillary beds and veins. - Explain blood flow, pressure, and resistance. - Differentiate blood pressure in arteries, capillaries, and veins. - Discuss factors affecting blood pressure. - Define hypertension and its effects. - Describe blood flow regulation in organs. - Outline capillary dynamics. - Define circulatory shock and its causes. - Trace pulmonary blood circulation. - Describe systemic circuit functions. - Identify major arteries and veins in systemic circulation. - Discuss the hepatic portal system structure. ### General Plan of Circulation - Blood is confined to vessels; it circulates continuously in a closed system. - Pulmonary Circuit: Right heart to lungs; Systemic Circuit: Left heart to body. - Lymphatic system aids in fluid return. ### Blood Vessel Classification - Arteries: Carry oxygen-rich blood from the heart. - Arterioles: Small arteries leading to capillaries. - Capillaries: Site for nutrient and waste exchange. - Venules and Veins: Carry deoxygenated blood to the heart. ### General Structure - Three layers in blood vessels: - Tunica Interna: Reduces friction. - Tunica Media: Smooth muscle for flow regulation. - Tunica Externa: Connective tissue support. ### Hemodynamics - Blood Flow: Influenced by pressure and resistance. - Blood Pressure: Affected by cardiac output, resistance, elasticity, and volume. ### Autoregulation - Local blood flow adjustment based on tissue needs. ### Unique Areas of Circulation - Hepatic Portal System: Carries nutrient-rich blood from the GI tract to the liver. ### Blood Vessel Pathologies - Arteriosclerosis: Hardening of arteries. - Varicose Veins: Valve incompetence leads to dilation. - DVT: Clots in veins, potential complications.