Anatomy & Physiology: Structure and Function of Blood Vessels

Anatomy & Physiology: Structure and Function of Blood Vessels

General Information

• Textbook for Reference: Anatomy & Physiology: An Integrative Approach, Third Edition by Dr. Douglas, Bio 2060.

• Copyright: © 2019 McGraw-Hill Education. All rights reserved.

Chapter 20 Overview

1. Structure and Function of Blood Vessels

   • Three types of blood vessels: arteries, capillaries, and veins.

   • Learning objectives for Chapter 20 include:

     1. Describe the three tunics common to most vessels.

     2. Explain distinguishing features of the tunics in arteries, capillaries, and veins.

     3. Distinguish among elastic arteries, muscular arteries, and arterioles.

     4. Describe general anatomic structure and function of capillaries.

     5. Compare the structure, function, and location of continuous capillaries, fenestrated capillaries, and sinusoids.

     6. Trace blood movement through a capillary bed.

     7. Describe the structure and function of veins.

     8. Explain how veins serve as blood reservoirs.

     9. Compare and contrast simple and alternative pathways of blood vessels.

20.1 Structure and Function of Blood Vessels

Types of Blood Vessels

• Arteries: Convey blood from the heart to capillaries.

• Capillaries: Microscopic porous blood vessels for substance exchange between blood and tissues.

• Veins: Transport blood from capillaries back to the heart.

General Structure of Vessels

• Blood vessel walls consist of three layers known as tunics:

  • Tunica Intima: Innermost layer; consists of a simple squamous epithelium (endothelium) and a subendothelial layer of areolar connective tissue.

  • Tunica Media: Middle layer; composed of circularly arranged smooth muscle cells mixed with elastic fibers. This layer is responsible for vasoconstriction (narrowing of lumen) and vasodilation (widening of lumen).

  • Tunica Externa: Outermost layer; consists of areolar connective tissue with elastic and collagen fibers that anchor the vessel to surrounding structures, may contain vasa vasorum for nutrient supply to large vessels.

Comparison of Vessels

• Arteries vs. Veins:

  • Arteries have a thicker tunica media and a narrower lumen, more elastic fibers, and a greater ability to recoil.

  • Veins feature thicker tunica externa and a larger lumen but fewer elastic and collagen fibers. Vein walls may collapse without blood.

• Capillaries:

  • Composed only of tunica intima; allow rapid gas and nutrient exchange due to their thin walls.

Types of Arteries

• Elastic Arteries:

  • Largest arteries, diameters of 2.5-1 cm (e.g., aorta).

  • High elastic fiber content for stretch and recoil which maintains blood flow during diastole.

• Muscular Arteries:

  • Medium-sized arteries, diameters ranging from 1 cm to 0.3 mm.

  • Distribute blood to specific body regions; contain more smooth muscle for vasoconstriction and vasodilation.

• Arterioles:

  • Smallest arteries (0.3 mm to 10 micrometers).

  • Control blood flow via vasomotor tone; regulated by the brain's vasomotor center;

  • Their structure allows for regulation of systemic blood pressure.

Clinical Views

• Atherosclerosis:

  • Progressive disease characterized by the formation of atheromas (plaque) in arteries leading to narrowing and potential blockages.

  • Risk factors include high cholesterol (hypercholesterolemia), smoking, and hypertension.

  • Treatment options include angioplasty and coronary bypass surgery.

• Aneurysm:

  • Weakening of arterial walls leading to ballooning; prevalent in the aorta and arteries at the brain's base, increasing the risk of rupture and potential fatal bleeding.

Capillaries

• Characteristics: Small vessels connecting arterioles to venules; average diameter of 8-10 micrometers and length of 1 mm.

• Types of Capillaries:

  • Continuous Capillaries: Continuous endothelial lining with tight junctions, allows molecules like glucose to pass; found in muscle, skin, lungs, CNS.

  • Fenestrated Capillaries: Endothelial cells have pores for greater permeability; found in areas of fluid absorption, such as the intestines and kidneys.

  • Sinusoids: Larger gaps for more robust exchange (larger molecules, cells like red blood cells), located in bone marrow, spleen, and certain glands.

• Capillary Beds: Groups of capillaries with metarterioles, precapillary sphincters controlling blood flow. Only a fraction of capillary beds are open at a given time due to vasomotion.

20.2 Total Cross-Sectional Area and Blood Flow Velocity

Key Concepts

• Blood flow velocity is inversely related to total cross-sectional area—velocity is slowest in capillaries, facilitating gas and nutrient exchange.

• Cross-sectional area of one vessel contrasts with the total cross-sectional area across a network of vessels, with capillary networks having the largest combined cross-sectional area.

Mathematical Relationships

• Mean velocity of blood flow can be expressed as inversely proportionate to total cross-sectional area: $v = \frac{Q}{A}$ Where:

  • $v$ is velocity

  • $Q$ is blood flow (volume/time)

  • $A$ is total cross-sectional area

20.3 Capillary Exchange

Mechanisms of Exchange

• Diffusion: Major process involving substances moving based on concentration gradients (e.g., O2, CO2).

• Vesicular Transport: Substances transmitted via vesicles in endothelial cells (e.g., certain hormones).

• Bulk Flow: Governed by pressure gradients, consisting of filtration (fluid exiting capillaries) and reabsorption (fluid re-entering capillaries).

• Net Filtration Pressure (NFP): Calculated as: $NFP = (HPb - HPif) - (COPb - COPif)$

  • $HPb$: Blood hydrostatic pressure

  • $HPif$: Interstitial fluid hydrostatic pressure

  • $COPb$: Blood colloid osmotic pressure

  • $COPif$: Interstitial fluid colloid osmotic pressure

Role of the Lymphatic System

• Lymphatic vessels recover excess interstitial fluid not reabsorbed at the venous end of capillaries, accounting for approximately 15% of fluid movement.

20.4 Local Blood Flow

Regulation of Blood Flow

• Local blood flow varies according to tissue metabolic activity.

• Autoregulation: Mechanism by which tissues adjust blood flow based on immediate needs. Changes occur through release of vasodilators and vasoconstrictors in response to varying oxygen and nutrient levels.

20.5 Blood Pressure, Resistance, and Total Blood Flow

Concepts of Blood Pressure

• Blood Pressure: The force exerted by circulating blood on vessel walls.

• Measured during heart cycles:

  • Systolic Pressure: Maximum pressure during heart contraction (e.g., 120 mm Hg).

  • Diastolic Pressure: Minimum pressure during relaxation (e.g., 80 mm Hg).

  • Pulse Pressure: Difference between systolic and diastolic (e.g., 40 mm Hg).

  • Mean Arterial Pressure (MAP): Calculated as:
    $MAP = DBP + \frac{1}{3} (SBP - DBP)$

  • Capillary Pressure: Fluctuates from 40 mm Hg (arterial end) to <20 mm Hg (venous end) to prevent vessel damage and allow for nutrient exchange.

• Resistance: The friction blood encounters, influenced by viscosity, vessel length, and radius (smaller radius increases resistance). Flow can be expressed as:
  $F \propto \frac{\Delta P}{R}$
  Where $\Delta P$ is the pressure gradient and $R$ is resistance.

20.6 Regulation of Blood Pressure and Blood Flow

Neural and Hormonal Mechanisms

• Neural Regulation: Involves autonomic reflexes from the medulla oblongata regulating cardiac output and vessel diameter (vasomotor center).

• Hormonal Regulation: The renin-angiotensin-aldosterone system significantly alters blood volume and pressure through various mechanisms, including vasoconstriction and fluid retention.

Important Hormones

• Angiotensin II: Raises blood pressure through vasoconstriction and increasing blood volume via thirst stimulation.

• Aldosterone: Promotes sodium and water reabsorption in the kidneys, regulating blood volume.

• ADH (Antidiuretic Hormone): Enhances water retention, further increasing blood pressure.

• Atrial Natriuretic Peptide (ANP): Decreases blood pressure via vasodilation and increased urine output.

20.7 Blood Flow Distribution During Exercise

Redistribution During Physical Activity

• Blood flow to muscles, heart, and skin increases significantly during exercise. Blood flow to non-essential areas like the abdomen and kidneys decreases.

20.8 Pulmonary Circulation

Pathway of Blood Flow in Pulmonary Circulation

• Right ventricle pumps deoxygenated blood through the pulmonary trunk to lungs, where gas exchange occurs in pulmonary capillaries.

• Lower pressures in pulmonary vessels facilitate efficient gas exchange.

20.9 Systemic Circulation

Arteries and Veins Overview

• Major systemic arteries: branch from the aorta, supplying oxygenated blood to the body organs.

• Major venous return vessels: superior and inferior vena cavae return deoxygenated blood to the right atrium.

20.10 Blood Vessel Overview of Head, Trunk, and Limbs

Major Structures of Blood Supply

• Detailed understanding of arteries and veins supplying various body regions (head, neck, thorax, abdomen, upper and lower limbs, highlighting significant branches and their associated functions).

20.11 Comparison of Fetal and Postnatal Circulation

Fetal Circulation Pathway

• Unique pathways such as umbilical vein, ductus venosus, foramen ovale, and ductus arteriosus facilitate efficient fetal circulation, bypassing non-functional lungs.

• Post-birth changes lead to the closure of these structures and a transition to independent pulmonary circulation.