Circulatory System Overview

Blood vessels are the essential transport system for blood throughout the body. They consist of three distinct layers: Tunica intima, Tunica media, and Tunica adventitia.

The tunica intima is the inner layer of a blood vessel, composed of endothelium, a basement membrane, and a thin layer of connective tissue.

The tunica media is the middle layer of a blood vessel, primarily composed of smooth muscle cells and elastic fibers. It helps regulate blood pressure by contracting and dilating.

The tunica adventitia is the outer layer of a blood vessel, composed of connective tissue, including collagen fibers. It anchors the blood vessel to surrounding tissues and contains vasa vasora.

Large arteries, also called elastic arteries, are responsible for transporting large volumes of blood away from the heart. They are characterized by their thick, elastic walls that allow them to expand during ventricular systole and recoil during diastole. Examples: Aorta, Pulmonary artery, Common carotid artery.

The internal elastic lamina is a thin, fenestrated layer of elastic fibers that separates the tunica intima from the tunica media in most blood vessels. It is a prominent and distinct feature in muscular arteries.

The external elastic lamina is a similar layer of elastic fibers, but it separates the tunica media from the tunica adventitia in some blood vessels, including arteries. However, it is less prominent than the internal elastic lamina.

Muscular arteries, also called distributing arteries, are responsible for distributing blood to specific organs and body regions. They have a thicker tunica media with more smooth muscle, giving them greater ability to regulate blood flow.

Arterioles are tiny branches of arteries that connect to capillaries. These small vessels have a thick layer of smooth muscle and are responsible for regulating blood flow into capillaries.

Meta-arterioles are specialized arterioles that connect directly to capillaries. They have a band of smooth muscle called the pre-capillary sphincter, which controls blood flow into individual capillaries.

Capillaries are the smallest blood vessels and the sites of primary exchange between the blood and surrounding tissues. They have a single layer of endothelial cells with a thin basement membrane and are highly permeable.

Continuous capillaries have tight junctions between endothelial cells, forming a continuous barrier that prevents leakage of plasma proteins and large molecules. They play a vital role in the blood-brain barrier.

Fenestrated capillaries have pores or fenestrae within their endothelial cells, allowing for rapid exchange of small molecules and fluids. They are found in tissues where high-volume exchange is essential, such as the kidneys and small intestine.

Sinusoids are wider and more irregular capillaries with incomplete basal lamina, making them highly permeable. They are found in certain organs like the liver, spleen, and bone marrow where large molecules and cells need to pass.

Venules are small, thin-walled vessels that collect blood from capillaries and drain it into veins.

Medium-sized veins are larger than venules but smaller than large veins. They have thinner walls with less smooth muscle than arteries, but they often contain valves that prevent backflow of blood.

Large veins are large, thin-walled vessels that carry deoxygenated blood back to the heart. They have a thin tunica media, but their tunica adventitia is well-developed and contains smooth muscle.

Lymphatic vessels are part of the lymphatic system, which is a network of vessels that collect excess fluid (lymph) from tissues and return it to the bloodstream. They have a similar structure to veins but have valves to prevent backflow of lymph.

Lymphatic capillaries are tiny blind-ended tubes that collect interstitial fluid (lymph) from tissues. They have a unique structure with anchoring filaments and endothelial cells with gaps, which allow for the entry of fluid.

Medium-sized lymphatic vessels are larger lymphatic vessels that transport lymph towards larger lymphatic ducts. They have a similar structure to veins with a well-developed tunica adventitia.

Lymphatic ducts are the largest lymphatic vessels that collect lymph from the body and drain it into veins near the heart. They have a structure similar to large veins but with a thicker tunica adventitia.

The blood-brain barrier is a specialized barrier that prevents the passage of certain substances from the bloodstream into the brain. It is formed by tight junctions between endothelial cells in brain capillaries, a thick basal lamina, and astrocyte perivascular feet.

Vasa vasorum are small blood vessels that supply the walls of larger blood vessels, particularly in the tunica adventitia. They provide oxygen and nutrients to the vessel wall itself.

The valves within veins are thin folds of the tunica intima that prevent backflow of blood, ensuring that blood flows in one direction toward the heart.

Pre-capillary sphincters are rings of smooth muscle found at the junctions of meta-arterioles and capillaries. Contraction of these sphincters can limit blood flow into specific capillary beds, regulating blood flow and distribution to tissues.

Pericytes are smooth muscle-like cells that wrap around the capillaries. They can contract and regulate blood flow through capillaries, but their function is also involved in the repair and maintenance of the capillary structure.

Myoepithelial cells are specialized cells that surround capillaries in some tissues. They can contract and regulate blood flow, but their role is also involved in the movement of fluid within the capillaries.

The difference in the structure of arteries and veins can be recognized by observing the tunica media. Arteries typically have a thicker tunica media with more smooth muscle, while veins have a thinner tunica media with less smooth muscle.

Elastic arteries have more elastic fibers in their tunica media, allowing them to expand and recoil to accommodate changes in blood pressure. Muscular arteries have more smooth muscle in their tunica media, allowing them to constrict and dilate to regulate blood flow.

Lymphatic vessels resemble veins in structure, but they have thinner walls and more numerous valves to ensure unidirectional flow of lymph. They also lack a distinct tunica media compared to veins.