The Circulatory System: Blood Vessels and Circulation

Chapter 20 - The Circulatory System: Blood Vessels and Circulation

20.1 General Anatomy of the Blood Vessels

  • Expected Learning Outcomes:

    • Understand the anatomy and functional roles of different types of blood vessels.

    • Define the terms artery, capillary, and vein.

    • Identify and describe the three tunics associated with most blood vessels.

    • Compare and contrast the thickness, composition, and lumen diameter among arteries, capillaries, and veins.

    • Describe the structure and function of specific types of blood vessels:

    • Elastic (conducting) arteries

    • Muscular (distributing) arteries

    • Arterioles

    • Capillaries

    • Venules

    • Veins

    • Define vasoconstriction and vasodilation.

    • List types of capillaries and describe their locations and anatomical structures related to functions.

    • Explain the functional significance of the venous reservoir.

    • Define anastomosis and its significance.

    • Define portal system.

  • Principal categories of blood vessels:

    • Arteries: Carry blood away from the heart.

    • Veins: Carry blood back to the heart.

    • Capillaries: Connect the smallest arteries to the smallest veins, forming a circuit.

20.1a The Vessel Wall

  • Structure of blood vessel walls:

    • Consist of three layers (tunics):

    • Tunica interna (tunica intima):

      • Lines the blood vessel and is exposed to blood.

      • Composed of endothelium (simple squamous epithelium).

      • Acts as a selectively permeable barrier and secretes chemicals for dilation/constriction.

      • Normally repels blood cells and platelets to prevent clotting.

      • Produces cell-adhesion molecules during inflammation to attract leukocytes.

    • Tunica media:

      • Middle layer made of smooth muscle, collagen, and elastic tissue.

      • Strengthens the vessels and controls blood vessel diameter.

    • Tunica externa (tunica adventitia):

      • Outermost layer consisting of loose connective tissue.

      • Anchors the vessel, allows passage for small nerves and lymphatic vessels.

      • Contains vasa vasorum, small vessels that supply blood to the outer half of larger vessels.

20.1b Arteries

  • Classification of arteries by size:

    • Conducting (elastic) arteries:

    • Largest arteries, e.g., aorta, common carotid.

    • Features internal and external elastic laminae.

    • Expand during systole and recoil during diastole to maintain pressure.

    • Distributing (muscular) arteries:

    • Deliver blood to specific organs (e.g., brachial, femoral).

    • Three-fourths of their wall is smooth muscle, with thick elastic laminae.

    • Resistance (small) arteries and arterioles:

    • Thick tunica media relative to lumen size; can control blood flow to organs.

  • Aneurysm:

    • Weak point in artery or heart wall resulting in a thin-walled, pulsating sac.

    • Dissecting aneurysm occurs when blood accumulates between tunic layers.

    • Commonly affects the abdominal aorta, renal arteries, and cerebral arteries.

    • May cause pain or rupture, leading to hemorrhage. Causes include atherosclerosis and hypertension.

  • Arterial sense organs:

    • Located in major arteries; monitor blood pressure and chemistry.

    • Carotid sinuses: baroreceptors that transmit signals related to blood pressure.

    • Carotid bodies: chemoreceptors that monitor blood chemistry, affecting respiration.

    • Aortic bodies: similar to carotid bodies but innervated by the vagus nerve.

20.1c Capillaries

  • Function of capillaries:

    • Exchange vessels for gases, nutrients, wastes, and hormones between blood and tissue.

    • Part of microvasculature, with nearly every body cell close to a capillary.

  • Types of capillaries:

    • Continuous capillaries: Found in most tissues; small solutes can pass; proteins and large molecules cannot.

    • Fenestrated capillaries: Located in organs requiring rapid absorption; contain filtration pores allowing movement of small molecules.

    • Sinusoids: Found in liver, bone marrow; allow passage of proteins and new blood cells due to large gaps in the endothelial layer.

  • Capillary beds:

    • Networks made up of 10-100 capillaries, normally supplied by a single arteriole;

    • Precapillary sphincters regulate blood flow; constriction can bypass capillary beds.

20.1d Veins

  • Characteristics of veins:

    • Known as capacitance vessels; larger capacity than arteries for blood storage.

    • At rest, ~64% of blood volume is found in veins vs. 13% in systemic arteries.

    • Veins possess steady blood flow and are subjected to low blood pressure (averaging 10 mm Hg).

  • Types of veins (from smallest to largest):

    • Postcapillary venules: Smallest veins, highly porous for fluid exchange.

    • Muscular venules: Larger than postcapillary, with smooth muscle layers.

    • Medium veins: Have venous valves to prevent backflow; associated with muscular pumps.

    • Large veins: Diameter >10 mm; thick tunica externa and moderate smooth muscle.

    • Venous sinuses: Modified veins with thin walls, unable to constrict; found in select body locations.

  • Varicose veins:

    • Caused by blood pooling in veins, leading to their stretching and valve failure.

20.1e Circulatory Routes

  • Typical blood route:

    • Heart → arteries → arterioles → capillaries → venules → veins.

    • Blood typically journeys through a single capillary network.

  • Alternative routes:

    • Portal system: Blood flows through two consecutive capillary networks before returning to heart; examples include the kidneys.

    • Anastomosis: Connection between two vessels (artery-direct vein bypass, or one vein empties into another).

20.2 Blood Pressure, Resistance, and Flow

  • Expected Learning Outcomes:

    • Understanding blood pressure interactions with cardiac output, resistance, and hemodynamics.

    • Define and explain blood flow, blood pressure, and peripheral resistance.

    • Describe arterioles' role in regulating blood flow and pressure.

    • Discuss factors affecting peripheral resistance: local, hormonal, and neural influences.

    • Explain mean arterial pressure (MAP) and total peripheral resistance (TPR).

  • Blood Flow:

    • The volume of blood flowing through a vessel over time (mL/min).

    • Perfusion: Flow related to a specific mass of tissue (mL/100g/min).

  • Factors determining blood pressure:

    • Blood pressure (BP): The force exerted by blood on vessel walls; measured typically in the brachial artery.

    • Systolic pressure: Peak BP during ventricular contraction (e.g. 120 mm Hg).

    • Diastolic pressure: Minimum BP during ventricular relaxation (e.g. 75 mm Hg).

    • Normal young adult value: 120/75 mm Hg.

  • Mean arterial pressure (MAP):

    • MAP = Diastolic pressure + (1/3 * Pulse Pressure).

  • **Calcifying Effect of Pressure on Flow:

    • The flow through arteries is pulsatile due to variability in BP during the cardiac cycle.

  • Hypertension and Hypotension:

    • Hypertension: Chronic BP > 130/80, can lead to heart failure.

    • Hypotension: Chronic low BP without a specific numerical criterion; may cause dizziness or syncope.

20.2b Peripheral Resistance

  • Peripheral resistance (PR): Resistance encountered in vessels away from the heart.

    • Factors affecting PR: blood viscosity, vessel length, and radius.

  • Vessel Diameter Control: PR is most influenced by vessel radius due to its fourth power relation to flow.

  • Vasomotion: Refers to the processes of vasoconstriction and vasodilation, which modulate blood flow and pressure.

20.2c Regulation of Blood Pressure and Flow

  • Local control: Autoregulation of tissue blood supply.

    • Metabolic Response: Wastes in poorly perfused tissue encourage vasodilation.

    • Myogenic Response: Vasoconstriction occurs with increased pressure, vasodilation with reduced pressure.

  • Neural Control: Mediated by the central and autonomic nervous systems.

    • Baroreflex: Response to changes in BP; controls rapid adjustments for posture changes.

    • Chemoreflex: Adjusts respiration based on blood chemistry & vasomotion.

  • Hormonal Control: Hormones affecting BP include:

    • Angiotensin II: Raises BP via vasoconstriction.

    • Aldosterone: Promotes renal water retention.

    • Natriuretic peptides: Promote excretion and vasodilation.

    • ADH: Promotes water retention and vasoconstriction in high concentrations.

    • Epinephrine/Norepinephrine: Causes vasoconstriction to raise BP.

20.3 Capillaries and Fluid Exchange

  • Capillary exchange: The process of fluid and solute movement between blood and tissues.

    • Substances exchanged include:

    • Water, oxygen, nutrients, hormones, carbon dioxide, and wastes.

    • Exchange routes include endothelial cell cytoplasm, intercellular clefts, and filtration pores.

    • Mechanisms include: Diffusion, transcytosis, filtration, and reabsorption.

  • Filtration & Reabsorption Dynamics:

    • Fluid filters out at arterial ends due to hydrostatic pressure; reabsorbed at venous ends due to oncotic pressure.

    • Net Filtration Pressure (NFP) drives fluid out (starts at 13 mm Hg) and net reabsorption (7 mm Hg inward).

    • Overall, ~85% of filtered fluid is reabsorbed; the rest is absorbed by the lymphatic system.

  • Edema: Abnormal fluid accumulation in tissues; caused by increased filtration or reduced reabsorption.

20.4 Venous Return and Circulatory Shock

  • Mechanisms of Venous Return:

    • Pressure gradient: Blood pressure from venules to heart; drifts 7 to 13 mm Hg.

    • Gravity: Aids blood return from the head/neck.

    • Skeletal muscle pump: Muscle contraction helps push blood towards the heart.

    • Thoracic pump: Inhalation creates pressure changes for blood movement.

    • Cardiac suction: Ventricular contraction that draws blood into atria.

  • Circulatory Shock: Insufficient cardiac output to meet metabolic needs, categorized as:

    • Cardiogenic shock: Heart's pumping insufficiency.

    • Low venous return (LVR) shock: Insufficient blood returning to the heart, e.g., hypovolemic shock from trauma, dehydration, or obstruction.

  • Shock Responses:

    • Compensated shock: Homeostatic mechanism restores function.

    • Decompensated shock: Compensation fails, leading to worsened conditions.

Summary

  • This study guide conveys the anatomy, physiology, and pathology relevant to the circulatory system, detailing the structure and dynamics of blood vessels, the principles of blood pressure and flow regulation, capillary exchange, and visceral shock. This comprehensive overview is crucial for understanding the fundamentals of human circulatory dynamics and serves as an indispensable resource.