Vascular Structure and Function

Vascular Structure and Function

• Arteries transport blood away from the heart
• Ventricles pump blood into arteries, which branch into smaller arteries, then arterioles, and eventually into capillaries
• Arteries "branch", "diverge", and "fork" because they transport blood away from the heart
• All arteries transport oxygenated blood, with the exception of the pulmonary arteries
• The lumen is the hollow, central blood-containing space in arteries
• Arteries feature three layers, listed as follows:
  • Tunica intima: The innermost layer made of simple squamous epi, and is the same as the endocardium
  • Tunica media: The middle layer made of smooth muscle, closing arteries and regulating blood distribution
    • Vasoconstriction: Lumen squeezing to reduce blood flow
    • Vasodilation: Lumen widening to increase blood flow
  • Tunica adventitia: The outer layer made of collagen fibers and is very elastic
• Arteries act as a conduit between the heart and capillaries, acting as a pressure reservoir, controlling the flow
• The arteries dampen pressure and flow oscillations, resulting in more even flow in the capillaries, and controlling blood distribution with vasoconstriction and vasodilation
• Arteriosclerosis is the hardening of the arteries from smoking, stress, age, and inactivity
  • Increased blood pressure and reduced flow to capillaries can result from arteriosclerosis

Capillaries

• Capillaries are the body's smallest blood vessels
• Red blood cells travel through capillaries in single file
• Small arterioles transport blood to capillaries, while small venules receive blood from the capillaries
• Capillaries consist of simple squamous epi
• The functional unit of the circulatory system, capillaries facilitate nutrient and gas exchange (diffusion)

Veins

• Veins transport blood toward the heart
• Veins "merge", "join", and "converge" for transport purposes
• Small venules merge into small veins, which in turn merge into larger veins
• All veins carry unoxygenated blood, with the exception of the pulmonary veins
• Veins hold 65% of the body's blood supply at any given time
• Like arteries, veins have three layers; however, the tunica media is much thinner
  • Veins are "limp" due to the thinner tunica media
  • Veins also have a much wider lumen
• Veins also have valves to prevent the backflow of blood
• Skeletal muscle contractions (muscular pump) and breathing (respiratory pump) cause blood movement in veins
• Varicose veins result from enlarged and distended veins due to failed valves

Blood Pressure and Pulse

• Blood pressure measurement uses a sphygmomanometer in mm Hg that include the following two measures:
  • Systolic = arterial blood pressure due to ventricular contraction (normal adult range: 110-140 mm Hg)
  • Diastolic = blood pressure from arterial elastic recoil (normal adult range: 75-80 mm Hg)
• Pulse is the difference between systolic and diastolic pressure as blood flows through arteries
• The radial and carotid pulses are the most accessible for palpation (taking a pulse) at various sites on the body
• Normal pulse rate: 60-80 bpm