4 - Structure and Function of Blood Vessels

What are arteries?

Large blood vessels that carry blood away from the heart

What are arterioles?

Microscopic blood vessels that distribute blood within tissues

What are capillaries?

Smallest type of blood vessel, site of exchange of substances (gas exchange) between the blood and tissues

What are venules?

Microscopic blood vessels that drain capillary beds and turn into veins

What are veins?

Smaller blood vessels (compared to arteries) that carry blood back towards the heart

What are the three tunics of a typical blood vessel?

Tunica intima - endothelium/inside layer

Tunica media - smooth muscle+connective tissue / middle layer

Tunica externa - anchors bv/ outside layer

What is the tunica intima of the blood vessel?

-Inside layer-

Lining of the interior of the blood vessel, in contact with the blood. Made of a single layer of endothelium with the outermost layer being comprised of elastic lamina (elastic tissue)

What is the tunica media of the blood vessel?

-Middle layer-

Consists of smooth muscle and connective tissue, which contracts to promote vasoconstriction or relax to promote vasodilation. Thick layer and most prominently found in arteries

In small arteries, it can form a "makeshift tourniquet) aka vasospasm in response to damage to the vessel

What is the tunica externa (aka tunica adventitia) of the blood vessel?

-Outside layer-

Anchors blood vessel to surrounding tissue, may also contain vaso vasorum (bvs that supply blood to bvs)

Are arteries thicker or thinner than veins? How and why?

Arteries are more thicker as they contain a thicker muscular layer (tunica media) and a thicker elastic lamina on all layers compared to veins.

This is due to arteries being pressure vessels, and thicker layers serve to regulate blood flow and pressure

Are veins thicker or thinner than arteries? How and why?

Veins are thinner than arteries as they contain a thinner tunica intima and tunica media, to the point that the tunica externa is the thickest layer. Due to this thinness, veins typically have less elastic tissue, but lots more collagen to give strength to the thin walls

This is due to veins acting as blood volume reservoirs (distensible) due to their large lumen size.

What are the two major types of arteries?

1. Elastic arteries - stretch to propel blood

2. Muscular arteries - contract for vasoconstriction/relax for vasodialation

What are elastic arteries?

AKA conducting arteries

Contain an abundance of elastic fibers, found in pulmonary trunk+aorta+major branches

Allow bvs to stretch under pressure during ventricular systole, acting as a pressure reservoir, and releasing that pressure during ventricular diastole to bring extra force to push blood into capillaries

What are muscular arteries?

AKA distributing arteries

Contains more smooth muscle in the tunic media of the bv, and less elastic tissue than elastic arteries. Branch extensively to supply tissues

Allows greater bv vasoconstriction and vasodilation. Also maintains vascular tone, which is required resistance to blood to keep pressure for blood to flow

Describe the structure and function of arterioles

Contains a fenestrated (holes) internal elastic membrane and two layers of smooth muscle cells in tunica intima and a precapillary sphincter (smooth muscle which can contract to prevent blood entering capillaries)

Responsible for regulating blood pressure in the body (due to there being A LOT of arterioles)

Describe the structure and function of capillaries

Lack tunica media and tunica externa (only contains tunica intima), therefore being VERY THIN (for exchange)

Responsible for exchange of nutrients, ions, oxygen,waste etc. between blood and tissues

Describe the structure and function of venules

Drains blood from capillaries

Begins as postcapillary venules (still allow exchange with tissues) and turn into muscular venules (responsible for volume reservoir in veins)

What are the role of precapillary sphincters?

Smooth muscles at the end of arterioles (metarterioles)

Can contract to reduce blood flow into the capillaries, or relax to increase blood flow into the capillaries

What are the role of throughfare channels?

Direct channel from the arteriole to the venule, bypassing the capillary beds

Allows blood to be diverted away from tissues with low metabolic needs or when blood is needed in an emergency (hemorrhage/blood loss)

What are the three different types of capillaries?

Continuous - continuous lining, tight regulation of fluids

Fenestrated - small pores, diffusion of small molecules

Sinusoid - large pores + intercellular clefts, diffusion of large molecules

What are continuous capillaries? Where can they be found in the body?

Contains endothelial cells forming a continuous lining, with the basement membrane + the bv being joined by tight junctions

Allows tight regulation of flow of fluids between the tissues and the bv

Found in CNS, lungs, muscle (tightly wrapped with connective tissue), and skin

What are fenestrated capillaries? Where can they be found in the body?

Contains endothelial cells with fenestrations (small pores)

Allows the diffusion of small molecules across the capillary wall (amino acids, glucose, hormones)

Found in kidneys, ciliary process, and endocrine glands

What are sinusoid capillaries? Where can they be found in the body?

Contains endothelial cells with large fenestrations (large pores) and intercellular clefts (gaps within basement membrane)

Allows the diffusion of large molecules across the capillary wall, esp prominent in structures where proteins, rbcs, and wbcs need to pass

Found in red bone marrow (production of rbcs), liver, and spleen

Do arteries or veins contain valves?

Veins, prevents backflow of blood, ensure that blood flows in one direction back to the heart

What causes varicose veins?

Valvular incompetence of the vales in the veins, leading to swelling

What are sinuses?

Veins that lack smooth muscles

Why does the venous circuit require pumps to aid venous return? What are the two pumps that aid venous return?

Venules have very low pressure compared to arteries, therefore, they need pumps to pump blood back to the right atrium of the heart

1. Skeletal muscle pump - skeletal muscle compression

2. Respiratory pump - diaphragm -> increased intrabdominal pressure

What is the skeletal muscle pump?

Skeletal muscle "squishes" veins, causing blood to be pumped back towards the right atrium of the heart

What is the respiratory pump?

Diaphragm contraction causes increased intrabdominal pressure, resulting in abdominal veins being compressed and blood flowing back towards the heart

Where can the majority of human blood be found at rest?

Found in veins and venules, act as blood reservoirs during times in need by venoconstriction (ex: SNS, blood loss)

What are the three mechanisms that support capillary exchange in the capillaries?

1. Diffusion

2. Transcytosis (vesicles)

3. Bulk flow (pressure by mass movement of ions)

How does diffusion promote capillary exchange?

Simple diffusion by moving from high concentrated to low concentrated area via endothelial cells (only lipid soluable), fenestrations, or intercellular clefts (fenestrated/sinusoid arteries)

How does transcytosis promote capillary exchange?

Vesicles formed from endothelial cells transport large lipid-insoluble molecules through the cytosol of the cell (endocytosis) and then excreted from the cell via vesicles (exocytosis)

How does bulk flow promote capillary exchange?

Large number of ions moving in same direction due to pressure from the arterioles (BHP) and pressure remaining in the venules (BCOP)

What type of pressure promotes filtration and reabsorption of ions from the arterioles and venules

Blood hydrostatic pressure (BHP) - drive filtration (arterioles into tissue)

Blood colloid oncotic pressure (BCOP) - drives reabsorption (tissue into venules)

What are the four principle divisions of the aorta and what do they branch off as/supply?

Arch of aorta - leads to brachiocephalic trunk, left common carotid, left subclavian

Ascending aorta - Right and left coronary arteries

Thoracic aorta - Supplies structures in the thorax

Abdominal aorta - Supplies abdominal pelvic organs and lower limbs

What are the four paired branches of the abdominal aorta and what do they supply?

Suprarenal arteries - supplies adrenal glands

Renal arteries - supplies kidneys

Gonadal arteries - supplies ovaries/testes

Lumbar arteries - supplies vertebrae and body wall

What are the three unpaired branches of the abdominal aorta and what do they supply?

Celiac trunk - supplies liver, stomach, spleen, pancreases, duodenum of small intestine (upper GI)

Superior mesenteric artery - supplies rest of small intestine, proximal 2/3 of large intestine (lower GI)

Inferior mesenteric artery - supplies distal 1/3 of large intestine (lower lower GI)

What are the three major systemic veins returning blood to the heart and which structures does blood flow from to these structures?

Superior vena cava - head, upper trunk, upper limbs

Coronary sinus - coronary circulation

Inferior vena cava - lower trunk, lower limbs