CH 20 Anatomy & Physiology: Blood Vessels Structure and Function

20.1 Structure and Function of Blood Vessels

  • Three types of blood vessels:
    • Arteries:
    • Conduct blood from the heart to capillaries.
    • Capillaries:
    • Microscopic, porous vessels.
    • Main role is to exchange substances between blood and tissues.
    • Veins:
    • Transport blood from capillaries back to the heart.

20.1a General Structure of Vessels

  • Walls composed of three layers (tunics):
    • Tunica intima:
    • Innermost layer, composed of simple squamous epithelium (endothelium) and a subendothelial layer of loose connective tissue.
    • Tunica media:
    • Middle layer made of smooth muscle and elastic fibers.
    • Vasoconstriction narrows lumen; vasodilation widens lumen.
    • Tunica externa (adventitia):
    • Outermost layer consists of connective tissue to anchor vessels.
    • May include vasa vasorum for larger vessels.

20.1b Arteries

  • Arteries branch into smaller vessels:
    • Smaller diameters, less elastic fibers, and more smooth muscle relative to size.
  • Types of arteries:
    • Elastic arteries:
    • Largest arteries (e.g., aorta).
    • High elastic fiber content allows stretching and recoil.
    • Muscular arteries:
    • Medium-sized arteries.
    • Distribute blood to specific regions with elastic tissue for constriction and dilation.
    • Arterioles:
    • Smallest arteries, control blood flow to capillaries.

20.1d Veins

  • Venules:
    • Small veins formed from merging of capillaries.
  • Types of veins:
    • Small and medium-sized veins:
    • Companion to muscular arteries; contain valves to prevent blood pooling.
    • Large veins:
    • Travel with elastic arteries and serve major body regions.
  • Systemic veins serve as blood reservoirs:
    • At rest, 70% of blood is in systemic circulation.

20.1e Pathways of Blood Vessels

  • Simple pathway:
    • One major artery supplying an organ; branches into arterioles and capillary beds draining into venules.
  • Alternative pathways:
    • Arterial anastomosis:
    • Multiple arteries supplying the same region (e.g., superior and inferior epigastric arteries).
    • Venous anastomosis:
    • Multiple veins draining the same area.
  • Portal systems:
    • Blood flows through two capillary beds before returning to the heart (e.g., hepatic portal system).

20.2 Total Cross-Sectional Area and Blood Flow Velocity

  • Total cross-sectional area is largest among capillaries due to their number.
  • Blood flow velocity is inversely related to total cross-sectional area; slower in capillaries to facilitate exchange of substances (gases, nutrients).

20.3 Capillary Exchange

  • Functions of capillaries:
    • Exchange of gases, nutrients, and wastes occurs through diffusion and bulk flow.
  • Diffusion:
    • Substances move according to concentration gradients (e.g., oxygen moves into tissues, CO2 moves into blood).
  • Vesicular transport:
    • Endothelial cells transport substances using vesicles by pinocytosis and exocytosis.

20.4 Local Blood Flow

  • Local blood flow varies based on metabolic needs, myogenic responses, and regulatory factors.
  • Angiogenesis:
    • Formation of new vessels in response to increased tissue demand (e.g., during exercise).

20.5 Blood Pressure

  • Blood pressure is the force exerted by blood against vessel walls.
  • Arterial pressure includes systolic (when the heart beats) and diastolic (when the heart is at rest) pressure.
  • Mean arterial pressure (MAP) is calculated considering systolic and diastolic pressure and reflects perfusion.

20.6 Regulation of Blood Pressure and Blood Flow

  • Blood pressure regulated through cardiac output, resistance, and blood volume.
  • Neural and hormonal mechanisms work together for short-term and long-term regulation.
  • Baroreceptors:
    • Detect changes in blood vessel stretch and signal cardiovascular centers to adjust heart rate and vessel tone accordingly.
  • Renin-Angiotensin System:
    • Involves a cascade affecting blood pressure via vasoconstriction and fluid retention mechanisms.

20.7 Blood Flow Distribution During Exercise

  • Increased cardiac output and redistribution of blood flow during exercise:
    • Increased blood to muscles and heart, decreased supply to abdominal organs and kidneys.

20.8 Blood Flow Through the Pulmonary Circulation

  • Right ventricle pumps deoxygenated blood to the lungs via pulmonary arteries.
  • Gas exchange occurs in pulmonary capillaries, resulting in oxygenated blood returning to the left atrium via pulmonary veins.

20.9 Major Arteries and Veins of the Body

  • Details of arterial branches from the aorta include the brachiocephalic trunk, common carotid arteries, and subclavian arteries.
  • Major veins include the superior and inferior vena cavae that return blood to the right atrium.

20.10 Summary of the Blood Supply to Various Organs

  • Supply and drainage of the head, neck, thoracic organs, and abdominal organs structured through a network of arteries and veins.

20.11 Upper and Lower Limb Blood Supply

  • Upper limb arteries:
    • Subclavian to axillary to brachial, branching down to the forearm and hand.
  • Lower limb arteries:
    • External iliac to femoral artery branching into popliteal, anterior, and posterior tibial arteries.

20.12 Fetal vs. Postnatal Circulation

  • Fetal circulation:
    • Includes shunts (e.g., foramen ovale, ductus arteriosus) to bypass pulmonary circulation.
  • After birth, these shunts close, establishing independent blood flow.