Study Notes on Cardiovascular System: Blood Vessels and Circulation

Cardiovascular System: Blood Vessels and Circulation

Chapter Overview

  • 20.1 Structure and Function of Blood Vessels
  • 20.2 Blood Flow, Blood Pressure, and Resistance
  • 20.3 Capillary Exchange
  • 20.4 Homeostatic Regulation of the Vascular System
  • 20.5 Circulatory Pathways
  • Assigned Reading

Blood Vessel Structure & Function

  • Closed system of vessels:
    • Arteries
    • Arterioles
    • Capillaries
    • Venules
    • Veins
  • The human body has approximately 60,000 miles of blood vessels.

Cardiovascular Circulation

  • Pulmonary Circuit:
    • Transports blood from the right side of the heart to the lungs and back to the heart.
  • Systemic Circuit:
    • Transports blood from the left side of the heart to the head and body, returning to the right side of the heart.
  • Arrows on figures indicate the direction of blood flow, with colors indicating oxygen concentration levels.

Structure of Blood Vessels

  • Three layers (tunics) of blood vessel walls:
    1. Tunica Interna/Intima
    2. Tunica Media
    3. Tunica Externa
  • Modifications in these layers account for the structural and functional differences among five types of blood vessels.
Comparison of Tunics in Arteries and Veins
  • General Appearance:
    • Arteries: Thick walls with small lumens; rounded appearance.
    • Veins: Thin walls with large lumens; typically appear flattened.
  • Tunica Intima:
    • Arteries: Wavy endothelium due to smooth muscle constriction; internal elastic membrane present in larger vessels.
    • Veins: Smooth endothelium; internal elastic membrane absent.
  • Tunica Media:
    • Arteries: Thickest layer, predominately smooth muscle cells and elastic fibers, varies with distance from the heart.
    • Veins: Typically thinner than tunica media of arteries; collagenous and smooth fibers predominate.
  • Tunica Externa:
    • Arteries: Thicker, with collagenous and elastic fibers; external elastic membrane present.
    • Veins: Thicker layer in veins with predominately collagenous fibers; valves present to prevent backflow.

Blood Vessels

Arteries & Arterioles
  • Elastic Arteries (Conducting arteries):
    • Large diameter (>10 mm), more elastic fibers, less smooth muscle.
  • Muscular Arteries (Distributing arteries):
    • Medium diameter (0.1-10 mm), more smooth muscle, fewer elastic fibers.
  • Function:
    • Arteries carry blood away from the heart to the tissues; small arteries/arterioles are resistance vessels.
Capillaries
  • Features:
    • Microscopic, exchange vessels with various structures based on permeability.
    • Types of Capillaries:
    • Least permeable: Continuous with small intracellular clefts; found in muscle, skin, lung, heart, and CNS.
    • Increased permeability: Continuous with small fenestrations; found in small intestine, kidneys.
    • Most permeable: Discontinuous with larger fenestrations; found in bone marrow, spleen, liver, and some endocrine glands.
  • Function: Facilitate exchange of materials.
Blood Flow Through Capillaries
  • Capillary Bed Formation:
    • Arterioles give rise to metarterioles.
    • Precapillary sphincters: Regulate blood flow to capillaries.
    • Thoroughfare Channels: Connect metarterioles to venules.
  • Arteriovenous Anastomosis: Direct connection from arteriole to venule; allows for prioritization of blood flow during various states such as exercise or eating.
  • Note: Only 1/4 of the body’s capillary beds are open at any given time.

Comparison of Veins and Venules

  • Many veins contain valves to prevent backflow, while venules do not.
  • Venules drain blood from capillaries into veins; folds of tunica intima assist with this process.

Comparison of Arteries and Veins

  • Direction of Blood Flow:
    • Arteries: Conduct blood away from the heart.
    • Veins: Conduct blood toward the heart.
  • General Appearance:
    • Arteries: Rounded; appear thick.
    • Veins: Irregular, often collapsed; appear thin.
  • Pressure Levels:
    • Arteries: High pressure.
    • Veins: Low pressure.
  • Relative Oxygen Concentration:
    • Higher in systemic arteries; lower in systemic veins, lower in pulmonary arteries, and higher in pulmonary veins.
  • Valves: Not present in arteries, but present in veins, particularly in limbs and veins inferior to the heart.

Circulatory Routes

  • Types of Circulatory Routes:
    • Simple Circulation.
    • Alternative Pathways:
    • Anastomoses: Arterial, Venous, Arteriovenous.
    • Portal Systems: Example - Circle of Willis.
Venous Return
  • Definition: The volume of blood returning to the right atrium via systemic veins every minute; facilitated by:
    • Pressure gradient between veins and right atrium.
    • Assisted by:
    • Valves.
    • Respiratory pump.
    • Skeletal muscle pump.
    • Venoconstriction and vascular tone.

Factors Affecting Blood Pressure

  • Increased Factors:
    • Blood volume.
    • Increased sympathetic impulses and hormones from adrenal medulla.
    • Increased cardiac output (CO) on MAP (Mean Arterial Pressure).
    • Increased total blood vessel length due to obesity.
  • Decreased Factors:
    • Decreased parasympathetic impulses.

Capillary Exchange

Mechanisms of Exchange
  • Substances cross capillary walls by:
    • Diffusion
    • Transcytosis/Vesicular transport
    • Bulk Flow:
    • A passive process where large quantities move across membranes together. It primarily regulates blood and interstitial fluid volumes.
  • Forces Involved in Bulk Flow:
    1. Blood hydrostatic pressure (BHP)
    2. Interstitial fluid hydrostatic pressure (IFHP)
    3. Blood colloidal osmotic pressure (BCOP)
    4. Interstitial fluid colloidal osmotic pressure (IFOP)
Filtration and Reabsorption
  • Filtration: Pressure-driven movement from capillaries into interstitial fluid, promoted by BHP and IFOP.
  • Reabsorption: Movement from interstitial fluid back into capillaries, promoted by IFHP and BCOP.
  • Net Filtration Pressure (NFP): NFP = (BHP + IFOP) − (BCOP + IFHP) indicates the direction of fluid movement.

Clinical Applications: Edema

  • Definition: Abnormal increase in interstitial fluid occurring when filtration exceeds reabsorption.
  • Causes: Increased filtration, inadequate reabsorption, and obstruction of lymphatic drainage.

Hemodynamics of Total Blood Flow

  • Influencing Variables:
    • Cardiac Output
    • Compliance
    • Blood Volume
    • Blood Viscosity
    • Blood Vessel Length and Diameter.
  • Velocity of Blood Flow:
    • Inversely related to the cross-sectional area of blood vessels; slowest in capillaries due to we largest total cross-sectional area.

Homeostatic Regulation of Blood Pressure

  • Three Mechanisms to Ensure Adequate Blood Flow and Pressure:
    1. Autoregulation
    2. Neural mechanisms
    3. Endocrine mechanisms
  • Baroreceptor Reflexes: Increased blood pressure leads to increased baroreceptor firing; decreased blood pressure leads to slower firing, both engaging homeostatic responses to restore balance.

Hormones and Blood Pressure Regulation

  • Renin-Angiotensin-Aldosterone Mechanism:
    • Increase in angiotensin II stimulates production of antidiuretic hormone and aldosterone, increasing blood volume via erythropoietin production, enhancing red blood cell quantity and blood pressure stability.
Summary of Mechanisms Regulating Blood Vessels
  • Most blood vessels lack direct parasympathetic innervation, relying instead on autoregulation and hormonal factors for maintenance of blood pressure and flow.

Circulatory Routes

  • Systemic Circulation:
    • Starting from the left ventricle, blood travels through arterial branches, including:
    • Elastic arteries (e.g., Aorta)
    • Muscular arteries
    • Arterioles
    • Capillaries for gas exchange
    • Venules merging into veins, leading back to the heart.
Pulmonary and Hepatic Circulation
  • Pulmonary Circulation: Right ventricle to pulmonary trunk, bifurcating to lungs for gas exchange and returning oxygenated blood to the left atrium via pulmonary veins.
  • Hepatic Portal System: Blood from systemic circulation and organs enters the liver both via the hepatic artery and hepatic portal vein for processing.
Fetal Circulation & Birth Adjustments
  • Exchange occurs via umbilical arteries and a single umbilical vein. Post-birth adaptations involve closure of the foramen ovale and ductus arteriosus, facilitating normal circulation patterns and signaling organ maturation.