Blood Vessel Structure and Function

Demonstrate knowledge, understanding, and application of the structure and functions of arteries, arterioles, capillaries, venules, and veins.

Typical of a mammalian circulatory system.
Total length of blood vessels in an average adult: approximately 100,000 miles, or four times around the Earth's circumference.

Elastic fibers: Composed of elastin, providing flexibility through stretching and recoiling.
Smooth muscle: Contracts or relaxes to change the size of the lumen (channel within the blood vessel).
Collagen: Provides structural support to maintain the shape and volume of the vessel.

Arteries carry blood away from the heart to the body's tissues.
Carry oxygenated blood, except for:
- Pulmonary artery: carries deoxygenated blood from the heart to the lungs.
- Umbilical artery (during pregnancy): carries deoxygenated blood from the fetus to the placenta.
Blood pressure in arteries is higher than in veins.
Artery walls contain elastic fibers, smooth muscle, and collagen.
- Elastic fibers: Withstand the force of blood pumped from the heart and stretch to accommodate the larger blood volume.
- Collagen: Maintains the limits of stretching.
- Recoil of elastic fibers: Evens out surges of blood, providing continuous flow between heart contractions.
- Pulse: A surge of blood is still felt when the heart contracts, despite the effect of elastic fibers.
- Endothelium: The lining of arteries is smooth, allowing blood to flow easily.
Arterioles link arteries and capillaries.
- More smooth muscle, less elastin than arteries.
- Constrict or dilate to control blood flow into individual organs.
- Vasoconstriction: Smooth muscle contraction constricts the vessel and prevents blood flow into a capillary bed.
- Vasodilation: Smooth muscle relaxation allows blood to flow into a capillary bed.

Aneurysm: A bulge or weakness in a blood vessel, commonly in the aorta and brain arteries.
Often undetected until it bursts, which can be fatal.
Risk factors include high blood pressure and changes in collagen to elastin ratio in the aorta wall.
Collagen:Elastin Ratio:
- Normal Aorta: $1.85:1$
- Small Aneurysm: $3.75:1$
- Large Aortic Aneurysm: $7.91:1$

Microscopic blood vessels linking arterioles and venules.
Form an extensive network through all tissues of the body.
Lumen is so small (approximately $7.5-8$ $μm$ ) that red blood cells must travel in single file.
Substances are exchanged through capillary walls between tissue cells and blood.
Gaps between endothelial cells allow substances to pass out of the capillaries into the surrounding fluid.
Exception: Capillaries in the central nervous system have very tight junctions between cells.

Large surface area for diffusion of substances into and out of the blood.
Total cross-sectional area is greater than the supplying arteriole, which reduces the rate of blood flow.
Slow movement of blood allows more time for the exchange of materials by diffusion.
Walls are a single endothelial cell thick, providing a thin layer for diffusion.

Veins carry blood away from the cells of the body towards the heart.
Carry deoxygenated blood, except for:
- Pulmonary vein: carries oxygenated blood from the lungs to the heart.
- Umbilical vein (during pregnancy): carries oxygenated blood from the placenta to the fetus.
Deoxygenated blood flows from capillaries into venules, then into larger veins.
The inferior vena cava carries deoxygenated blood from the lower parts of the body, and the superior vena cava carries deoxygenated blood from the head and upper body.
Veins do not have a pulse, as surges from the heart are lost in the capillaries.
Veins hold up to 60% of the body's blood volume.
Blood pressure in veins is very low compared to arteries.
Medium-sized veins have valves to prevent backflow of blood.
Walls contain lots of collagen and relatively little elastic fiber, with a wide lumen and smooth, thin lining (endothelium).
Venules link capillaries with veins, have thin walls with little smooth muscle, and several venules join to form a vein.

One-way valves at intervals prevent the backflow of blood.
Larger veins run between active muscles, which squeeze the veins during contraction, forcing blood towards the heart. Valves prevent backflow when muscles relax.
Breathing movements of the chest act as a pump, with pressure changes and squeezing actions moving blood towards the heart.