A&P: CH.19 [Blood Vessels]

• blood vessels form a closed delivery system that begins and ends at the heart
• heart contracts -> arteries -> capillaries -> veins
• arteries carry AWAY blood from heart
• veins carry deoxygenated blood BACK to heart
• systemic circulation

  • arteries carry oxygenated blood and veins carry oxygen poor blood
• pulmonary circulation:

  • arteries carry deoxygenated blood to lungs, veins carry oxygenated blood to heart
• only capillaries have direct connection with tissue cells and directly serve cellular needs
• tunica intima: intimate contact with blood in lumen

  • contains simple squamous epithelium that lines lumen
  • slick surface to minimize friction
• tunica media: circularly arranged smooth muscle cells and sheets of elastin

  • regulated by sympathetic vasomotor nerve fibers of autonomic nervous system to constrict or dilate
  • critical in regulating blood pressure and circulation; bulkiest layer
• tunica externa: "tunica adventitia" loosely woven collage fibers that protect and reinforce vessel while anchoring to surrounding structures

  • infiltrated with nerve fibers, lymphatic vessels, elastic fibers (larger veins)
  • vasa vasorum: nourish external tissues of blood vessel wall
• muscular arteries

  • deliver blood to specific body organs "distributing arteries"
  • account for most of named arteries
  • thickest tunica media of all vessels
  • contains relatively more smooth muscle and less elastic tissue than elastic arteries
  • more active in vasoconstriction and less distensible
• arterioles: smallest artery

  • larger ones have all three tunics
  • tunic media chiefly smooth muscle
  • smaller ones lead into capillary beds
  • blood flow into cap beds determines by arteriolar diameter
  • constriction: bypass tissues
  • dilation: increase flow into capillaries
• capillaries are the smallest of blood vessels
• pericytes line outer surface of capillaries and are smooth muscle-like cells that stabilize capillary wall and control permeability
• given thin walls and direct access to nearly every cell in body, capillaries are ideal for material exchange between blood and interstitial fluid
• continuous capillaries

  • abundant in skin and muscle, most common
  • endothelial cells joined by tight junctions, providing uninterrupted lining
  • intercellular clefts: allow limited passage of fluids and solutes
  • numerous pinocytotic vesicles that move fluid across capillary wall
  • in brain, tight junction are continuous, blood brain barrier
• fenestrated capillaries

  • have oval pores for higher permeability to fluids and small solutes in compassion to continuous capillaries
  • found in areas of high absorption or filtration
  • small intestine: these capillaries receive nutrients from digested food
  • endocrine: allow hormones faster entry to blood
  • kidneys: always open for rapid filtration of blood plasma
• sinusoid capillaries

  • most permeable and found in liver, bone marrow, spleen and adrenal medulla
  • allow larger molecules and blood cells to pass between blood and surrounding tissues
  • blood flows slowly through sinusoid channels, allowing time for modification
• capillary beds: interweaving networks of capillaries that connect arterioles to veins (microcirculation)

  • precapillary sphincter regulates blood flow into capillary
  • relaxed (open): blood flows through true capillaries and exchanges with tissue cells
  • contracted (close): blood flows through shunts and bypasses tissue cells
  • chemical conditions and arteriolar vasomotor nerve fibers regulate amount of blood into capillary bed
• venules: united by capillaries to form them

  • postcapillary venules (smallest) have high pericyte population and extremely porous to allow white blood cells and fluid to move easily
• veins: venules join to form veins

  • walls are always thinner, lumens larger than corresponding arteries (allow for greater volume accommodation)
  • little smooth muscle or elastin in tunica media
  • tunica externa thickest layer
  • largest vena cava
  • bp is low so modifications for consistent flow
  • large lumens: little resistance
  • venous valves: prevent backward flow (abundant in lower limbs) (absent in thoracic and abdominal cav)
• venous sinuses are highly specialized, flattened veins with extremely thin walls composed of only endothelium
• varicose veins are enlarged veins that are twisted and bulging due to blood pooling
• vascular anastomoses: specially formed interconnections of blood vessels
• collateral channels: anastomoses that provide alternate pathways for blood flow
• arterial anastomoses occur around joints, abdominal organs, heart and brain
• arteries that supply retina, kidneys and spleen do not have anastomoses or have poor collateral circulation
• blood flow is the volume of blood flowing through a vessel, an organ, or entire circulation in a give period; equivalent to cardiac output
• blood pressure: force per unit area exerted on a vessel wall
• pressure gradient: differences in blood pressure within vascular system- provides driving force that keeps blood moving from high to low in the body
• three sources of resistance

  1. blood viscosity
  2. total blood vessel length
  3. blood vessel diameter
• pumping action of heart generates blood flow. pressure results when flow is opposed by resistance
• arterial blood pressure reflects

  1. how much elastic arteries close to the heart can stretch
  2. volume of blood forced into them at any time
• systolic pressure: the pressure when the ventricle is contracting
• diastolic pressure: aortic pressure drops to its lowest level
• pulse pressure: the difference between systolic and diastolic pressure; felt during systole as ventricular contraction forces blood into arteries to expand them
• mean arterial pressure: the average pressure that propels blood to tissues
• low capillary pressure is desirable because

  1. capillaries are fragile, high pressure would break them
  2. most are extremely permeable so even with low pressure, delivery wouldn't be a problem
• highest to lowest bp

  aorta -> arteries -> arterioles -> capillaries -> venules -> veins -> vena cavae
• arterial bp pulsates with each contraction of left ventricle