- Elastic arteries: These are the largest arteries (e.g., the aorta) and contain high amounts of elastic fibers. They act as a "pressure reservoir," stretching when the heart contracts and recoiling when it relaxes to maintain continuous blood flow.
- Muscular arteries: Medium-sized vessels that contain more smooth muscle. Often called "distributing arteries," they use vasoconstriction and vasodilation to direct blood to specific organs.
To ensure safety and redundancy, many arteries form anastomoses, which are unions between branches of two or more arteries supplying the same region. This creates collateral circulation, providing an alternate route for blood if a vessel becomes blocked. However, some vessels are end arteries, meaning they do not anastomose; if these are obstructed, the tissue they supply may suffer necrosis (death).
Arterioles: The Resistance Vessels
Arterioles are microscopic branches of small arteries that regulate blood flow into capillary beds. They are known as resistance vessels because they provide significant opposition to blood flow through friction. The terminal end of an arteriole is called a metarteriole, which leads into a capillary bed. At the entrance to these beds, precapillary sphincters (rings of smooth muscle) control exactly how much blood enters the capillaries by regulating resistance.
Capillaries: The Site of Exchange
Capillaries are the smallest vessels, and they serve as the primary site for microcirculation—the flow of blood from an arteriole through capillaries and back to a venule. Because their walls consist of only a single layer of endothelial cells and a basement membrane, they allow for the efficient exchange of substances.
There are three structural types of capillaries:
- Continuous capillaries: The most common type, featuring uninterrupted endothelial tubes with small gaps called intercellular clefts.
- Fenestrated capillaries: Contain small pores (fenestrations) that allow for faster exchange, found in organs like the kidneys and small intestine.
- Sinusoids: The widest capillaries with large openings that allow even large proteins and blood cells to pass through; these are found in the liver and spleen.
Study Tip: Capillary Flow Patterns
In most tissues, blood does not fill every capillary. It may flow through a thoroughfare channel, which is a direct route from the metarteriole to the venule, bypassing the capillary bed entirely when the precapillary sphincters are contracted.
The Venous System: The Blood Reservoir
As blood moves away from the capillaries, it enters venules, which are formed by the union of capillaries. Larger venules eventually unite to form veins, which carry blood back to the heart.
Veins differ significantly from arteries because they operate under much lower pressure. Consequently, veins have thinner walls and much larger lumens to accommodate volume. Because they hold about 64% of the body's total blood volume, they are referred to as blood reservoirs. To assist blood in returning to the heart against gravity, veins utilize valves to prevent backflow, as well as the "skeletal muscle pump" and "respiratory pump".
Specialized venous structures include:
- Vascular sinuses: Veins with very thin walls and no smooth muscle, such as the dural venous sinus.
- Superficial vs. Deep veins: Superficial veins reside in the subcutaneous layer, while deep veins are located between skeletal muscles.
Hemodynamics and Blood Pressure Regulation
The movement of blood is a balance of pressures and resistances. Blood pressure is the force exerted by blood against vessel walls. This pressure is influenced by vascular resistance, which depends on three factors: the size of the lumen, blood viscosity (thickness), and the total length of the blood vessels.
The body regulates these variables through several mechanisms:
- Cardiovascular Center: Located in the medulla oblongata, it uses baroreceptors (pressure sensors) and chemoreceptors (chemical sensors) to adjust heart rate and vessel diameter via the autonomic nervous system.
- Hormonal Regulation: The Renin-Angiotensin-Aldosterone (RAA) system and Antidiuretic Hormone (ADH) can increase blood pressure by causing vasoconstriction or increasing blood volume.
- Autoregulation: Tissues can locally adjust their own blood flow in response to chemical changes (like low O2 or high CO2) or temperature changes.
Clinical Correlation: Shock
When the cardiovascular system fails to deliver adequate oxygen and nutrients to tissues, a state called shock occurs. This can be categorized into four types:
- Hypovolemic: Due to loss of blood or fluid.
- Cardiogenic: Due to heart failure.
- Vascular: Due to widespread vasodilation (e.g., anaphylactic or septic shock).
- Obstructive: Due to a physical blockage (e.g., pulmonary embolism).
Shock progresses from a compensated stage, where negative feedback loops attempt to restore homeostasis, to a decompensated stage, where positive feedback loops cause the system to spiral toward failure. If medical intervention is not successful, it reaches the irreversible stage, where the heart can no longer produce the ATP required for contraction.
