Blood Vessel Structure and Layers

Blood Vessel Structure

• Arteries branch into smaller arteries called arterioles.
• Arterioles further divide into capillaries.
• Capillaries reunite to form venules.
• Venules reunite to form veins.
• Veins carry blood towards the heart.
• The aorta is the largest artery in the body.
• The aorta has three parts: ascending aorta, arch of aorta and descending aorta (thoracic and abdominal).

Blood Vessel Layers

• The tunica intima is the innermost layer of blood vessels.
• It consists of simple squamous epithelium, connective tissue basal lamina and elastic fibers.
• The tunica media is the middle and largest layer of the blood vessel walls.
• It is responsible for support and vessel diameter changes to regulate blood flow and blood pressure.
• It consists mostly of smooth muscle arranged in a circular or spiral fashion.
• The tunica externa is the outermost layer of blood vessels.
• It consists mostly of collagenous connective tissue and elastic fibers.
• The outermost layer binds blood vessels to surrounding tissue structures.
• The lumen of an artery is the hollow center through which blood flows.

Arterioles

• Arterioles are the smallest vessels with three distinguishable layers: intima, media and adventitia.
• When arterioles branch into capillaries, the tunica media and adventitia disappear, leaving only the tunica intima.

Factors Controlling Arteriole Smooth Muscle Contraction

• Epinephrine (sympathetic stimulation)
• Angiotensin
• Atrionatremic factor (ANF)
• Kinins
• Histamine

Capillaries

• Capillaries are the smallest and most numerous blood vessels.
• They contain only one layer of simple epithelium (tunica intima) with a basement membrane.
• Their thin walls allow for the exchange of nutrients and waste between blood and cells.
• The diameter of a capillary is so small that one red blood cell must pass through at a time.

Capillary Distribution

• The distribution of capillaries varies with metabolic demand.
• Skeletal muscle, liver, and kidney have the most extensive capillary distribution.

Factors Controlling Movement into Capillaries

• Hydrostatic pressure
• Osmotic pressure
• Precapillary sphincters

Precapillary Sphincters

• Located at the origin of capillaries, they control blood flow into capillaries.
• Sphincters are bands of smooth muscle cells that, when contracted, reduce blood flow through capillaries.
• When the sphincter relaxes, blood flow is increased.

Arteriovenous Shunts (AV Shunts)

• Control movement from the arterial side to the venous side of the circulatory system.
• They are also called metarterioles.
• Contraction of smooth muscle cells allows bypassing of tissue beds served by capillaries and enables the blood to be directed elsewhere.

AV Shunt Responsiveness

• Thermal stimuli
• Mechanical stimuli
• Chemical stimuli