ally-production.s3.amazonaws.com

CHAPTER 21: THE CARDIOVASCULAR SYSTEM: BLOOD VESSELS AND HEMODYNAMICS

I. INTRODUCTION

  • Hemodynamics: The means by which blood flow is altered and distributed, and blood pressure is regulated.
  • Overview of the histology of blood vessels and the anatomy of the primary routes of the arterial and venous systems.

II. STRUCTURE AND FUNCTION OF BLOOD VESSELS

A. Basic Structure of Blood Vessels
  1. Three Layers in a Generalized Blood Vessel:
    • Tunica Interna (Intima)
    • Tunica Media
    • Tunica Externa
  2. Tunica Interna (Intima):
    • Closest layer to the lumen; in direct contact with blood; continuous with endocardium.
    • Consists of:
      • Inner endothelium (simple squamous epithelium)
      • Basement membrane
  3. Tunica Media:
    • Composed of smooth muscle and elastic fibers that regulate lumen diameter.
  4. Tunica Externa:
    • Contains elastic and collagen fibers that form a protective outer layer.
  5. Angiogenesis: The formation of new blood vessels; stimulated by certain cancers; inhibition of angiogenesis could be potential treatment.
  6. Major structural and functional characteristics are outlined in Table 21.1.
B. Arteries
  1. Function: Carry blood away from the heart's ventricles to the tissues.
    • Structure includes:
      • Tunica interna
      • Tunica media (for elasticity and contractility)
      • Tunica externa
  2. Functional Properties:
    • Elasticity:
      • Achieved through elastic tissue in tunica interna and media; allows arteries to accommodate blood under high pressures from ventricular contraction.
    • Contractility:
      • Due to smooth muscle in tunica media; enables change in lumen size and limits bleeding from wounds.
    • Types of Arteries:
      • Elastic Arteries:
      • Large arteries with a high elastin content and less smooth muscle.
      • Also called conducting arteries; function as a pressure reservoir.
      • Muscular Arteries:
      • Have more smooth muscle; distribute blood to various body parts.
C. Anastomoses
  1. Definition: Union of branches from two or more arteries supplying the same region.
  2. Purpose: Provide alternate routes for blood supply to tissues or organs.
  3. Collateral Circulation: Alternate flow of blood through anastomoses.
  4. End Arteries: Do not anastomose; occlusion can lead to necrosis.
  5. Types of Anastomoses:
    • Can consist of two veins or a vein and an artery.
D. Arterioles
  1. Definition: Very small arteries that regulate blood flow to capillaries.
  2. Function: Through vasoconstriction and vasodilation, arterioles control blood flow into capillaries and affect arterial blood pressure.
E. Capillaries
  1. Definition: Microscopic vessels connecting arterioles and venules.
  2. Microcirculation: Refers to blood flow through capillaries.
  3. Location: Found near almost every body cell; distribution varies with tissue metabolic activity.
  4. Primary Function: Exchange of nutrients and wastes between blood and tissue cells via interstitial fluid.
  5. Capillary Wall Composition: Composed of a single layer of endothelial cells and a basement membrane.
  6. Capillary Networks: Extensive branching increases surface area for material exchange.
    • Regulatory Mechanism: Flow is controlled by smooth muscles in arterioles and by precapillary sphincters (rings of smooth muscle).
  7. Types of Capillaries:
    • Continuous Capillaries:
    • Fenestrated Capillaries:
    • Sinusoids:
F. Venules
  1. Definition: Small vessels formed from several capillary unions; they merge to form veins.
  2. Function: Drain blood from capillaries into veins.
G. Veins
  1. Structure: Similar to arteries but with thinner tunica interna and media, and a thicker tunica externa; contain valves to prevent backflow.
  2. Vascular (Venous) Sinuses: Thin-walled veins without smooth muscle; examples include the superior sagittal sinus and coronary sinus.
  3. Clinical Connection: Weak valves can lead to varicose veins.
H. Blood Distribution
  1. Blood Reservoirs: At rest, a significant portion of blood is stored in systemic veins and venules.
    • Allows for compensation through venous vasoconstriction during hemorrhage.
  2. Principal Reservoirs: Veins of abdominal organs (liver and spleen) and skin.

III. CAPILLARY EXCHANGE

A. Mechanisms of Exchange
  1. Substances enter and leave capillaries through:
    • Diffusion
    • Transcytosis
    • Bulk flow (filtration and absorption)
B. Diffusion
  1. Most important method of exchange; substances like O2, CO2, glucose, amino acids, and hormones diffuse down concentration gradients.
  2. Most plasma proteins and red blood cells do not pass through the capillaries of continuous and fenestrated types; exceptions exist in the brain due to the blood-brain barrier.
C. Transcytosis
  1. Involves enclosing substances within vesicles that enter endothelial cells by endocytosis and exit by exocytosis.
D. Bulk Flow: Filtration and Reabsorption
  1. Bulk flow is essential for regulating blood and interstitial fluid volumes.
  2. Movement of water and dissolved substances depends on hydrostatic and osmotic pressures.
  3. Starling’s Law of the Capillaries: Balance of fluid flow in and out at the arterial and venous ends.
  4. Edema: Abnormal fluid accumulation may occur due to several factors including high hydrostatic pressure, low plasma protein concentration, increased capillary permeability, or lymphatic blockage.

IV. HEMODYNAMICS: FACTORS AFFECTING BLOOD FLOW

A. Blood Flow Distribution
  • Dependent on factors driving blood flow and those resisting it.
B. Blood Pressure (BP)
  1. Definition: Pressure exerted on blood vessel walls, clinically refers to arterial pressure.
  2. Equation: ext{Cardiac Output (CO)} = rac{ ext{Mean Aortic Blood Pressure (MABP)}}{ ext{Total Resistance (R)}}
  3. Factors affecting BP:
    • Cardiac output
    • Blood volume
    • Viscosity
    • Resistance
    • Elasticity of arteries
  4. Blood pressure falls from the aorta to 0 mm Hg at the right atrium.
C. Vascular Resistance
  1. Definition: Opposition to blood flow due to friction between blood and vessel walls.
  2. Influencing factors:
    • Diameter of blood vessel
    • Blood viscosity
    • Total length of blood vessel
  3. Systemic Vascular Resistance: Total resistance from systemic blood vessels; primarily found in arterioles, capillaries, and venules due to their diameter.
D. Venous Return
  1. Blood return to the heart due to pressure gradient between venules and right atrium.
  2. Maintained by skeletal muscle contractions, valves in veins, and changes in pressure associated with breathing.
E. Velocity of Blood Flow
  1. Definition: Volume passing through tissues per unit time.
  2. Inverse relationship with the cross-sectional area of blood vessels; blood flows slowest where the area is greatest.
  3. Blood flow decreases from aorta to capillaries and increases returning to the heart.
F. Summary of Factors Affecting Blood Pressure
  • Refer to Figure 21.10 for a graphical summary.
G. Syncope
  1. Definition: Sudden, temporary loss of consciousness; often due to cerebral ischemia.

V. CONTROL OF BLOOD PRESSURE AND BLOOD FLOW

A. Role of the Cardiovascular Center (CV)
  1. Located in the medulla oblongata; regulates heart rate, contractility, and blood vessel diameter.
  2. Receives input from:
    • Higher brain regions
    • Sensory receptors (baroreceptors, chemoreceptors, proprioceptors)
  3. Output through sympathetic and parasympathetic fibers:
    • Sympathetic: Increases HR and contractility; maintains vasomotor tone.
    • Parasympathetic: Decreases HR through vagus nerve impulses.
B. Neural Regulation of Blood Pressure
  1. Baroreceptors: Pressure-sensitive neurons that monitor vessel wall stretching.
    • Cardiac Sinus Reflex: Maintains normal brain blood pressure via carotid sinus baroreceptors.
    • Aortic Reflex: Regulates systemic BP via aortic baroreceptors.
    • Reflex actions in response to BP falls include increased HR, contractility, and vasoconstriction.
  2. Carotid Sinus Massage: A technique used to slow heart rate in paroxysmal supraventricular tachycardia.
  3. Chemoreceptors: Monitor blood levels of O2, CO2, and hydrogen ions, enhancing respiratory and blood flow responses.
C. Hormonal Regulation of Blood Pressure
  1. Various hormones regulate BP and blood flow:
    • Angiotensin II
    • Epinephrine
    • Norepinephrine
    • Antidiuretic hormone (ADH)
    • Aldosterone
    • Atrial natriuretic peptide
  2. Refer to Table 21.2 for a summary of the hormonal relationships with blood pressure regulation.
D. Autoregulation of Blood Flow
  1. Definition: Tissue's ability to adjust its own blood flow based on metabolic needs.
  2. Major stimulus is oxygen levels, though not direct.
  3. Autoregulation is influenced by physical and chemical factors.

VI. CHECKING CIRCULATION

A. Pulse
  1. Definition: Alternate expansion and recoil of arterial walls with each heartbeat; strongest where arteries are closest to the heart.
  2. Normal Resting Pulse Rate: 70-80 beats/min.
    • Tachycardia: Resting heart rate > 100 beats/min.
    • Bradycardia: Resting heart rate < 50 beats/min.
B. Measuring Blood Pressure
  1. Blood Pressure: Pressure exerted by blood on arterial walls during ventricular systole and diastole; measured using sphygmomanometer.
  2. Systolic Pressure: Force during ventricular contraction.
  3. **Diast