Blood Vessels

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Anatomy

Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart.

• Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles.
• Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues.
• A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called perfusion.
  • The exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (interstitial fluid).
  • For capillaries to function, their walls must be leaky, allowing substances to pass through.
  • Capillaries lead back to small vessels known as venules.
• Venules are small veins that converge into larger veins.
• A vein is a blood vessel that conducts blood toward the heart
  • Compared to arteries, veins are thin-walled vessels with large and irregular lumens.
  • Larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity.
  • Other ways in which the body assists the transport of venous blood back to the heart involve contractions of skeletal muscles in the extremities, as well as pressure variations caused by breathing motion in the chest.

Major Arteries and Veins

• You will find a pair of femoral arteries and a pair of femoral veins, with one vessel on each side of the body.
• In contrast, some vessels closer to the midline of the body, such as the aorta, are unique and not paired.
• The left subclavian artery becomes the axillary artery as it passes into the axillary region, and then becomes the brachial artery as it enters the upper arm.

 Physiology

• Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit.
• Systemic arteries provide blood rich in oxygen to the body’s tissues.
• The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells.
• In contrast, in the pulmonary circuit, arteries carry blood low in oxygen exclusively to the lungs for gas exchange.
• Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation.

Blood Pressure

• Five variables influence blood flow and blood pressure:
  1. Cardiac output
  2. Vessel Compliance
  3. Volume of the blood
  4. Viscosity of the blood
  5. Blood vessel length and diameter

Pulse

• Each time the heart ejects blood forcefully into the circulation, the arteries
  must expand and then recoil to accommodate the surge of blood moving
  through them.
• This expansion and recoiling of the arterial wall is called the
  pulse and allows us to measure heart rate.
• Both the rate and the strength of the pulse are important clinically.
  • A high or irregular pulse rate can be caused by physical activity or other temporary factors, but it may also indicate a heart condition.
  • The pulse strength indicates the strength of ventricular contraction and cardiac output. If the pulse is strong, then systolic pressure is high.
  • If it is weak, systolic pressure has fallen, and medical intervention may be warranted.

1. Betts, J.G., Young, K.A., Wise, J.A., Johson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J.E., Womble, M. & DeSaix, P. (2013). Anatomy and Physiology. OpenStax. http://cnx.org/content/col11496/latest/[↩]