Blood Vessels

• There are 60,000 miles of blood vessels in the human body, enough to travel around the earth's equator twice
• Hormones that lower blood pressure


  • ADH
  • Antidiuretic hormone
  • Aldosterone
  • Angiotensin
  • Epinephrine
  • Norepinephrine
• Major types of blood vessels

  • Arteries
  • Veins
  • Capillaries

  
• Vessel tunics


  • Tunica intima (tunica interna)
  • Tunica media
  • Tunica externa (tunica adventitia)
• Arteries


  • Thicker tunica media when away from the heart
  • Carry oxygenated blood away from the heart
  • Veins carry deoxygenated blood towards the heart
• Arteries and veins


  • Composed of three tunics: tunica interna, tunica media, and tunica externa
• Types of arteries


  • Conducting arteries (many elastic fibers, e.g. aorta)
  • Distributing arteries (many muscular fibers, deliver blood to body)
  • Resistance arteries (smaller distributing arteries, arterioles)
  • Arterioles (contain mostly smooth muscle, regulate blood delivery and pressure)

  
• Arteries


  • Elastic arteries have the highest pressure
  • Muscular arteries lose elasticity and gain more muscle as they distribute blood
• Types of capillaries


  • Continuous
  • Fenestrated
  • Sinusoidal
• Continuous capillaries


  • Epithelial cells held together with tight junctions, allow fluid passage
  • Form blood-brain barrier
• Fenestrated capillaries


  • Epithelium riddled with filtration pores (fenestrations), increase permeability
  • Found where active absorption or filtration occurs (e.g. intestines, endocrine glands, kidneys)
• Sinusoidal capillaries


  • Highly modified, leaky capillaries with large diameters
  • Allow large molecules and cells to pass into tissues
  • Found in liver, bone marrow, lymphoid tissue, some endocrine organs
• Capillary beds


  1. Vascular shunts (metarteriole-thoroughfare channel)

  2. True capillaries (branching off metarteriole, regulated by precapillary sphincters)
• Vascular shunts open


  Blood flows through true capillaries
• Vascular shunts closed


  Blood flow restricted, forces blood through vascular shunt
• Types of circulatory routes


  • Capillaries connecting arteries to veins
  • Portal systems (two capillary networks connected)
  • Anastomoses (merging blood vessels)

  
• Anastomoses


  • Provide alternate pathways (collateral channels) for blood to reach a body region
  • Thoroughfare channels are examples of arteriovenous anastomoses

  
• Laminar flow


  Smooth blood flow, faster in center and slower along vessel wall
• Turbulent flow


  Disrupted flow due to constriction or vessel curve, audible
• Blood flow from arterioles to veins (venules)

  1. Arterioles

  2. Capillaries

  3. Venules

  4. Veins

  
• Portal system


  Connects two capillary networks together (e.g. hepatic portal system)
• Anastomosis


  Merging of blood vessels (arteries to veins or arteries to arteries)

  
• Arterial anastomoses


  • Provide alternate pathways (collateral channels) for blood to reach a given body region

  
• Thoroughfare channels


  Examples of arteriovenous anastomoses

  
• Shunt


  Circulatory route
• Laminar flow


  Smooth blood flow; flow is faster in center and slower along vessel wall (encounters friction); seen in smaller vessels (arterioles)
• Turbulent flow


  Flow is disrupted due to constriction or vessel curve; "audible"; seen in larger vessels
• Control of blood flow to tissues

  1. Chemoreceptors detect low oxygen levels to release nitric oxide which dilates vessels

  2. Vasomotor tone: Medulla Oblongata sends SNS impulses to increase vasoconstriction

  3. Hormones (epi/norepi, renin, angiotensin II, ADH) increase vasoconstriction

  4. Angiogenesis: growth of new blood vessels over time (e.g. cancer, coronary vessel blockage)
• Resistance


  Opposition to flow; results in decreased blood pressure
• Sources of resistance


  • Blood viscosity
  • Total blood vessel length
  • Blood vessel diameter

  
• Blood Flow = Radius^4

  
• Blood pressure


  Force per unit area exerted on the wall of a blood vessel by its contained blood; measured in mm Hg on Brachial Artery

  
• Systolic pressure


  1st Korotkoff Sound
• Diastolic pressure


  Disappearance of Korotkoff Sounds
• Normal BP: 120/75 mm Hg; Hypotension: Systolic BP < 90/60 mmHg; Hypertension: BP ≥ 140/90 mmHg
• Primary vs. Secondary Hypertension
• Systemic pressure is highest in the aorta and declines throughout the length of the pathway, with the steepest change in blood pressure occurring in the arterioles
• Factors aiding venous return

  1. Pressure gradient

  2. Gravity

  3. Respiratory "pump"

  4. Skeletal muscular "pump"

  5. Cardiac suction

  6. Valves prevent backflow
• Mean arterial pressure (MAP)


  Cardiac output (CO) x Peripheral resistance (PR) = (Heart rate (HR) x Stroke volume (SV)) x PR

  
• Short-term regulation of blood pressure

  1. Baroreceptors inhibit SNS and stimulate PNS to decrease HR and SV, decreasing BP

  2. Chemoreceptors stimulate SNS and inhibit PNS to increase HR, SV, and vasoconstriction, increasing BP

  3. Epi/Norepi increase HR, SV, and vasoconstriction, increasing BP