Vessels & Circulation

Vessels & Circulation

Chapter Overview

  • Goals for Lecture:

    • Name and describe each type of vessel found in the body including examples, locations, sizes, histological differences, and contents.

    • Discuss how blood pressure differences influence vessel morphology.

    • Explain the process of embryonic blood vessel development.

    • Trace the path of blood from the heart through both circuits, noting major vessels and their destinations.

Blood Vessels

  • Naming Convention:

    • Vessels are named based on the direction they deliver blood in relation to the heart, not based on oxygenated versus deoxygenated status.

  • Types of Vessels:

    • Arteries: Carry blood away from the heart.

    • Arterioles: Deliver blood to capillaries.

    • Capillaries: Sites of substance exchange through their walls.

    • Venules: Drain blood from capillaries.

    • Veins: Return blood to the heart.

Blood Vessel Histology

  • Layers of Blood Vessels:

    • Intima:

    • Innermost layer consisting of simple squamous epithelium atop an internal elastic membrane.

    • Media:

    • Middle layer made up of smooth muscle and collagen; some vessels have an external elastic membrane.

    • Adventitia:

    • External layer of dense irregular connective tissue (CT); blends with adjacent tissues for stability.

  • Variability by Vessel Type:

    • Arteries: Typically have thicker walls relative to lumen due to higher blood pressure.

    • Veins: Thinner walls relative to lumen allow for greater blood volume accommodation.

    • Thick-walled vessels obtain nutrients from vasa vasorum (vessels of vessels).

Types of Arteries

  • Elastic Arteries:

    • Closest to the heart (examples: pulmonary arteries, aorta, carotids, subclavians, common iliacs).

    • Features:

    • Diameter: ~1.5 cm

    • Thick walls with prominent elastic membranes and less smooth muscle.

    • Muscular Arteries:

    • Further from the heart (examples: femoral, brachials, mesenterics).

    • Features:

    • Diameter: ~0.4 cm

    • Thicker media with smooth muscle to withstand moderate pressures.

    • Arterioles:

    • Function as vessels within tissues under low pressure.

    • Features:

    • Diameter: ~0.0003 cm (microscopic), thin adventitia, 1-2 smooth muscle cell layers in media.

Capillaries

  • Characteristics of Capillaries:

    • Microscopic vessels (may be the width of an erythrocyte).

    • Very low pressure, consisting solely of intima for nutrient exchange.

  • Types of Capillaries:

    • Continuous Capillaries:

    • Found in dermis, CNS, PNS, muscles, lungs.

    • Features: Many tight junctions and desmosomes; allows passive diffusion and active transport only.

    • Fenestrated Capillaries:

    • Found in kidneys, intestines, endocrine glands, choroid plexuses.

    • Features: Porous (fenestra = window); allows leakage of small proteins and solutes.

    • Sinusoid Capillaries:

    • Found in red marrow, spleen, liver.

    • Features: Flattened with large gaps between intima cells (sinus = cavity); allows movement of all plasma components.

Capillary Beds

  • Functional Characteristics:

    • Capillaries form interconnected networks, or beds, between arterioles and venules with regulated blood flow.

    • Precapillary Sphincters: Smooth muscle rings that regulate blood flow into capillaries.

    • Thoroughfare Channels: An arteriole allowing blood flow even when the capillary bed is closed off.

    • Arteriovenous Anastomosis:

    • Exists around beds; allows blood flow when postural changes hinder blood flow.

Types of Veins

  • Venules:

    • Microscopic with very low pressure (diameter ~0.0002 cm), thin adventitia, incomplete media, few muscle cells.

  • Medium Veins:

    • (Examples: radial, ulnar, femoral).

    • Diameter: ~0.4 cm with thin walls, thicker adventitia, presence of valves.

  • Large Veins:

    • (Examples: superior/inferior vena cava, renal, mesenteric, portal).

    • Diameter: ~2 cm with thin walls but thick adventitia.

Venous Circulation

  • Characteristics:

    • Veins have stretchy walls and large lumens, allowing them to function as blood reservoirs.

    • Approximately 64% of the body's blood resides in the venous system at any time.

  • Response to Blood Loss:

    • Smooth muscles contract to expedite blood movement from reservoirs to arteries.

  • Valves in Veins:

    • Flap-like folds of the media projecting inward preventing backflow, especially in extremities.

  • Venous Blood Movement:

    • Skeletal Muscle Pump: Contraction of muscles squeezes veins, pushing blood through.

    • Thoracoabdominal Pump: Expansion of the thoracic cavity helps draw blood through the veins.

Clinical Note: Atherosclerosis

  • Definition: Thickening and hardening of arteries due to plaque build-up (fat, cholesterol, calcium) along vessel walls.

  • Causes and Complications:

    • Develops with age, wear, or lifestyle choices.

    • Can lead to reduced blood flow to organs, resulting in pain, organ atrophy, and peripheral necrosis.

  • Major Diseases Associated:

    • Coronary Artery Disease: May cause angina, myocardium atrophy (ischemia), and heart attack (infarction).

    • Stroke: Resulting from halted blood flow to the brain.

    • Peripheral Artery Disease: Causes reduced blood flow to legs.

  • Potential Treatment: Balloon angioplasty.

Major Vessel Development

  • Arterial Development:

    • Superior heart tubes form into six sets of pharyngeal arch arteries.

    • Arches transform into different structures affecting artery formation.

    • Artery Formation:

    • First, second, and fifth sets disappear.

    • Third set forms carotid arteries for the head.

    • Fourth set forms brachiocephalic trunk (right) and aorta (left) connecting to the left ventricle.

    • Sixth set forms pulmonary arteries connecting to the right ventricle.

  • Venous Development:

    • Inferior heart tubes become the sinus venosus which contributes major veins.

    • Important Structures:

    • Sinus venosus develops into part of the right atrium.

    • Umbilical veins bring nutrient-rich blood from mother; left umbilical vein branches into hepatic portal vein.

    • Anterior cardinal veins develop into veins above the heart (superior vena cava, jugulars, etc.).

    • Posterior cardinal veins separate to become iliacs.

    • The inferior vena cava develops from liver vessels.

    • Note: The pulmonary veins develop from lung tissue, connecting to the left atrium.

Circuits

  • Pulmonary Circuit:

    • Short system with about 9% of total blood volume.

    • The right ventricle has a thinner wall to reduce excessive force.

    • Arteries carry deoxygenated blood; veins carry oxygenated.

    • Pathway:

    • Deoxygenated blood flows from the right ventricle into the pulmonary trunk, pulmonary arteries, and to the lungs.

    • Blood is oxygenated in alveolar capillaries.

    • Oxygenated blood returns to the left atrium via pulmonary veins.

  • Systemic Circuit:

    • Extensive system containing roughly 84% of blood volume.

    • The left ventricle features thicker walls to provide adequate force for blood delivery.

    • Arteries carry oxygenated blood; veins carry deoxygenated blood.

    • Pathway:

    • Oxygenated blood exits the left ventricle through the aorta to systemic arteries.

    • Blood travels through muscular arteries, arterioles, delivering nutrients and oxygen to tissues.

    • Deoxygenated blood returns to the right atrium via venules, medium veins, and large veins.