Blood Vessel Anatomy - BIO153 Module 2

Overview

• Resource Material
  • Lecture closely follows textbook: Saladin, Anatomy & Physiology: The Unity of Form and Function, 10th Edition.
  • Students are encouraged to refer to the textbook for further details and clarifications.

Course Outcomes

• Outcome 36: Understand and Describe Vessels of the Circulatory System
  • Objectives:
  • A. Differentiate between arteries, veins, and capillaries regarding anatomy, function, and direction of flow.
  • B. Describe the anatomy of an artery, a vein, and a capillary.
  • C. List and describe the three layers that comprise blood vessel walls.
  • D. Describe the flow of blood from arterial flow through a capillary bed to venous flow.
  • E. Identify the major arteries and veins of the body.

Introduction to Blood Vessels

• Blood vessels are crucial organs of the cardiovascular system.
• Form a closed circuit leading to and from the heart.
• Classification:
  • Blood vessels are classified based on size and histological organization.
  • Largest vessels:
  • Pulmonary Trunk: Carries blood from the right ventricle to pulmonary circulation.
  • Aorta: Carries blood from the left ventricle to systemic circulation.

Types of Blood Vessels

Arteries

• Function: All arteries carry blood away from the heart.
• Arterioles:
  • Smallest branches of arteries leading to capillary beds.
  • Receive blood from arteries and carry blood to capillaries.

Capillaries

• Smallest blood vessels with thin walls.
• Function: Sites for exchange of substances between blood and interstitial fluid.

Venules

• Smallest branches of veins.
• Receive blood from capillaries.

Veins

• Function: All veins carry blood toward the heart.

Structure of Blood Vessels

Blood Vessel Layers (Tunics)

1. Tunica Externa (Adventitia)

   • Outermost layer.
   • Anchors vessels to adjacent tissues.
   • Contains:
     • Collagen fibers.
     • Elastic fibers.
     • Smooth muscle cells (in veins).
   • Vasa Vasorum:
     • Small arteries and veins in walls of large arteries and veins.
     • Serve cells of tunica media and tunica externa.

2. Tunica Media

   • Thick middle layer.
   • Contains concentric sheets of smooth muscle in loose connective tissue.
   • Encircles the endothelium lining the lumen of the blood vessel.
   • Binds to the inner and outer layers.
   • External Elastic Membrane (in arteries):
     • Separates tunica media from tunica externa.
     • Large arteries have the thickest tunica media.

3. Tunica Intima (Interna)

   • Innermost layer.
   • Composed of:
     • Endothelial lining with a smooth surface to prevent blood clotting.
     • Connective tissue layer.
     • Internal Elastic Membrane (in arteries):
     • Contains elastic fibers in the outer margin of tunica intima.
     • Secretes substances to dilate and constrict blood vessels.

Differences between Arteries and Veins

• Walls: Arteries have thicker walls than veins.
• Blood Pressure: Higher in arteries.
• Lumen Shape: A constricted artery has a small, round lumen; veins have a large, irregular lumen.
• Endothelium: The endothelium in constricted arteries is folded; veins do not have this.
• Elasticity: Arteries are more elastic than veins.
• Valves: Veins have valves, whereas arteries do not.

Types of Arteries

• Elastic Arteries (Conducting Arteries):
  • Large vessels (e.g., pulmonary trunk and aorta).
  • Tunica media has many elastic fibers and fewer muscle cells.
  • Elasticity helps to even out pulse force.
• Muscular Arteries (Distributing Arteries):
  • Medium-sized arteries.
  • Tunica media has numerous muscle cells.
• Arterioles (Resistance Vessels):
  • Small vessels.
  • Have tunica intima (endothelial lining) but little or no tunica externa.
  • Have thin or incomplete tunica media.

Clinical Note: Aneurysm

• Defined as a bulge in an arterial wall due to a weak spot in elastic fibers.
• High pressure may cause vessel rupture.

Capillaries: Structure and Function

• Smallest vessels composed of a single layer of flattened endothelial cells.
• Form barrier between blood and interstitial fluid.
• Microscopic networks permeate all active tissues.
• Function:
  • Site of all exchange functions in the cardiovascular system.
  • Allows diffusion of materials between blood and interstitial fluid; semipermeable.

Types of Capillaries

1. Continuous Capillaries:

   • Found in all tissues except epithelia and cartilage.
   • Complete endothelial lining; permit diffusion of small solutes and lipid-soluble materials but block blood cells and plasma proteins.
   • Specialized forms exist with very restricted permeability (e.g., blood-brain barrier).

2. Fenestrated Capillaries:

   • Contain pores in the endothelial lining.
   • Allow rapid exchange of water and larger solutes; found in choroid plexus, endocrine organs, kidneys, and intestinal tract.

3. Sinusoids (Sinusoidal Capillaries):

   • Have gaps between adjacent endothelial cells.
   • Allow free exchange of water and large plasma proteins; typically found in liver, spleen, bone marrow, and endocrine organs.

Capillary Beds and Regulation of Flow

• Capillary Bed: Connects one arteriole and one venule; denser in tissues with higher metabolic demands (e.g., muscle, nerve).
• Precapillary Sphincters: Guard the entrance to capillaries; respond to the needs of the surrounding cells by opening or closing to regulate blood flow based on oxygen and nutrient availability.

Connections Between Blood Vessels

• Thoroughfare Channels: Direct capillary connections between arterioles and venules within a capillary bed.
• Arteriovenous Anastomoses: Direct connections between arterioles and venules that bypass the capillary bed.
• Collateral Arteries: Multiple arteries that contribute to one capillary bed, allowing circulation if one artery is blocked.

Angiogenesis

• Defined as the formation of new blood vessels.
• Stimulated by vascular endothelial growth factor (VEGF) under conditions such as embryonic development or hypoxia (oxygen shortage).

Structure of Veins

• Veins collect blood from capillaries and return it to the heart; they have larger diameters, thicker tunica externa, thinner tunica media, and operate under lower blood pressure.
• Venous Valves: Flaps of tunica intima prevent backward flow of blood and assist in the return of venous blood to the heart through compression by muscular movements and breathing.

Summary of Blood Flow

• Arteries:
  • Carry blood away from the heart to lungs (Right & Left Pulmonary arteries) and body (Aorta).
• Veins:
  • Carry blood back to the heart, from the body to the right atrium (Superior Vena Cava, Inferior Vena Cava, Coronary Sinus) and from the lungs to the left atrium (Pulmonary Veins).

Oxygenation of Blood Flow

• Oxygen-poor (deoxygenated) blood is carried in the:
  • Aorta and pulmonary veins.
• Oxygen-rich (oxygenated) blood is carried via:
  • Superior Vena Cava, Inferior Vena Cava, and pulmonary arteries.

Passage Through the Heart

• Right Side:
  • Deoxygenated blood passes through right atrium → tricuspid valve → right ventricle → pulmonary valve.
• Left Side:
  • Oxygenated blood passes through left atrium → bicuspid (mitral) valve → left ventricle → aortic valve.