Overview of Blood Vessels

Blood Vessels Overview

• Blood vessel walls are composed of smooth muscle, connective tissue, and an endothelial lining.
• The concentration and arrangement of tissues in the vascular wall depend on metabolic and mechanical factors (e.g., blood pressure).

Layers of Blood Vessels

• Tunica Intima: The inner layer, comprised of endothelium and a thin layer of connective tissue. Larger vessels may also contain elastin.
• Tunica Media: The middle layer, containing either smooth muscle or elastic tissue (or both), arranged in a spiralled manner, providing structural support and allowing for changes in vessel diameter.
• Tunica Adventitia: The outer layer, composed of fibrous connective tissue (Type I collagen).

Endothelium

• The endothelial lining of the cardiovascular system comprises simple squamous epithelium, allowing for simple and active diffusion between blood and interstitial fluid.
• Endothelial cells control how relaxed or constricted blood vessels are by releasing various substances like endothelin 1, ACE, nitric oxide (NO), and prostacyclin. These substances cause either constriction or relaxation in the surrounding muscle.
• Endothelial cells are non-thrombogenic (preventing blood clots).
• They are involved in inflammation and immune responses.
• They secrete growth factors.
• Lymphatic endothelial cells are in contact with lymph, directly interacting with the lymphatic system's fluid.

Types of Capillaries

• Continuous Capillaries: Tight junctions between endothelial cells; basement membrane is continuous. (e.g., muscle tissue, lung and brain).
• Fenestrated Capillaries: Small pores (fenestrations) in the endothelial cells; basement membrane is continuous. (e.g. kidneys, endocrine glands and intestines). Permeability is higher.
• Sinusoidal Capillaries: Discontinuous endothelium and basement membrane with large gaps. (e.g., liver, spleen, bone marrow). The most permeable type.

Tunica Media - Smooth Muscle

• The tunica media is the smooth muscle layer of all blood vessels except capillaries.
• The smooth muscle is arranged spirally or helically to provide support and adapt to changes in vessel diameter.
• Contraction of this layer leads to vasoconstriction; relaxation leads to vasodilation.
• Some vessels also contain elastic fibers for stretching and recoil.

Tunica Adventitia

• The outermost layer of blood vessels.
• This layer provides protection and support.
• Larger blood vessels have their own small blood vessels (vasa vasorum) which supply them with nutrients and oxygen.
• Nervi vasorum (nerve fibers) regulate the contraction of smooth muscle in the tunica media.

Blood Vessel Anatomy (Specific examples)

• Elastic arteries have a prominent internal elastic lamina and a thick tunica media, filled with elastic fibres interwoven with smooth muscle arranged helically. The tunica adventitia is thin compared to the media.
• Muscular arteries have a thick tunica media, comprised of smooth muscle, with fewer elastic fibers. This assists in regulating blood pressure.
• Arterioles have a very thin tunica adventitia and 1-3 layers of smooth muscle. Allows for precise control of blood flow into capillary beds.
• Capillaries are the smallest blood vessels; they consist only of an endothelium and have no tunica adventitia or media. These vessels are thin enough for exchange of substances between blood and tissues.
• Venules are a transitional type of vessel between capillaries and veins, featuring endothelium and a very thin layer of smooth muscle in the tunica media, with or without valves.
• Veins have thinner tunica media and adventitia compared to arteries of the same size, with valves to prevent backflow of blood.
• Capillary beds consist of a network of capillaries connected to an arteriole and venule. Precapillary sphincters regulate blood flow into the capillaries at the capillary bed entrance.

Blood Vessel Ageing Related Changes

• The number of elastic lamellae in the tunica media increases with aging, but elasticity decreases due to changes in elastin and collagen composition.
• Such changes can lead to arteriosclerosis (stiffening of arteries).

Lymphatic Vessels

• Lymphatic capillaries have thin walls compared to blood vessels, and are more permeable to fluid and larger molecules.
• Overlapping endothelial junctions allow interstitial fluid and larger molecules to enter lymphatic capillaries.
• Lymphatic vessels transport fluid back to the bloodstream.
• The tunica media is largely smooth muscle (esp in larger lymphatic vessels) and absent in smaller vessels.
• The tunica adventitia anchors lymphatic vessels to surrounding tissues.

Capillary Types

• Continuous capillaries have a continuous, uninterrupted lining of endothelial cells with tight junctions. The basement membrane is also continuous.
• Fenestrated capillaries have pores (fenestrations) in endothelial cells, allowing higher permeability. The basement membrane is continuous.
• Sinusoidal capillaries have large gaps between endothelial cells and a discontinuous basement membrane, permitting the largest exchange of materials.

Sensors

• Peripheral chemoreceptors in the carotid and aortic bodies detect changes in blood chemical composition.
• Baroreceptors in the carotid sinus and aortic arch detect changes in arterial wall tension, which correspond to blood pressure changes.

This quiz covers the structure and function of blood vessels, detailing the different layers including tunica intima, tunica media, and tunica adventitia. Understand the role of endothelial cells and how they affect blood flow and vessel dilation. Join us to test your knowledge on vascular anatomy!