Vasculature

Question 1

1. Describe the structure, organization, and function of the basic layers of blood vessel walls.

Answer 1

Tunica intima:

• Inner layer of vessel
• Contains a layer of endothelial cells in contact with blood + layers of elastic/collagenous tissues.

Tunica media:

• Middle layer
• comprised of multiple layers of elastic laminae, smooth muscle, and/or collagen.

Tunica adventitia:

• outer support layer
• collagenous.
• May have its own blood vessels (vasa vasorum) in larger vessels since it can’t get oxygen/nutrients from blood in capillary (wall too thick).

Question 2

2. Discuss the morphological characteristics that distinguish the different types of blood vessels.

Answer 2

Arteries:

• Thick walled.
• Large arteries have thick media and much elastic (necessary for expansion to decrease pressure from systole), but elastic layers decrease as vessels get smaller.
• Smooth muscle found in media from aorta to arterioles.
• Pulmonary arteries have lower pressure.

1) Elastic aorta and large arteries:
- Intima has thin endothelial layer –> layer of collagen/elastin-rich fibers +fibroblasts and myointimal cells (like smooth muscle)
- Media with many elastic and smooth muscle layers
- Adventitia (has vasa vasorum).

2) Muscular arteries:
- Thin intima with endothelial cells and thin layer of CT –> inner elastic lamina –> media with lots of smooth muscle –> outer elastic lamina –> adventitia (thick, lots of collagen and elastin).

3) Small muscular arteries:
- No outer elastic lamina (still have inner).
- Thin lamina (endothelia + collagenous material) –> large smooth muscle layer in media –> adventitia (about same size as media, merges with surrounding CT).

Microvasculature

1) Arterioles:
- Inner lining of endothelial cells on a thin basement membrane
- Surrounded by 1-2 layers of smooth muscle + outer collagenous tissue (blends in with surrounding CT).
- Arterioles are gatekeepers and can restrict access to capillary beds.
- Metarterioles and arteriole-venule shunts connect larger arterioles (or venules) –> direct blood flow or permit bypass of capillary beds.

2) Capillaries:
- smallest vessels. 1-2 endothelial cells around lumen.
- No muscular layer, but surrounded by pericytes (can give rise to smooth muscle and aid with vessel growth after injury; don’t really contract).
- Then surrounded by collagenous fibrils that connect capillary to CT.

• Continuous capillaries = uninterrupted lining; pinocytotic vessels can transfer fluid across.
• Fenestrated capillaries = have pores in endothelial cells (covered by a diaphragm) that allow bulk flow of plasma.

Discontinuous endothelia:
-have much wider pores that can permit whole RBCs or leukocytes through.

3) Post-capillary venules:
- larger in diameter, also have pericytes.
- Responsive to vasoregulatory substances (ex: histamine) which makes them more sensitive to controlled permeability.
- Larger venules have 1-2 layers of smooth muscle in media (muscular venules).
- This is also where leukocytes interact with each other before entering tissues (diapedesis).

Vein:

• generally thinner walls (seen as collapsed); with lower BP.
• Wall thickness increases with vein diameter.
• Small veins have no inner elastic lamina, media has some smooth muscle, and adventitia is collagenous and blends with CT.
• Medium-sized have endothelial lining + muscle layer, and adventitia is thicker.
• Large veins have thin endothelial intima –> media of interlayered smooth muscle and collagen (and some elastin) –> thick/large adventitia (with vasa vasorum in bigger veins).
• Veins use hydrostatic pressure to get blood to heart; aided by contraction of smooth muscle and compression of skeletal muscles.
• Valves help prevent backflow (loss of valves = varicose veins).
• Lymphatics flow one way and empty at junction between internal jugular and subclavian vein.
• Generally single endothelial layer, blend into the CT, and typically go through local lymph node.

Question 3

3. Explain the structure and function of the different types of capillaries.

Answer 3

See card 2

Question 4

4. Outline the unique functions of post-capillary venules.

Answer 4

Areas of permeability control (can be altered by vasoregulatory substances, like histamine or serotonin). Also leukocytes interact in PCVs before entering tissues (diapedesis?).

Question 5

5. Describe how blood flow is regulated in capillary beds.

Answer 5

Arteriolar-venule shunts  smooth muscle. Metarterioles = gatekeepers that connect to small pre-capillary sphincters; system of vessels leading from arteriole through capillary beds to post-cap veins; controlled by sphincters. Local control of BP is through metarterioles, sphincters, and shunts.

Question 6

6. Discuss the general structure and functional significance of arterio-venous shunts, portal systems, pampiniform plexus, anastomoses, and end arteries.

Answer 6

Anastomoses = connections between arteries and veins that permit collateral circulation. Helps if a vessel becomes occluded  alternative path.

End arteries = arteries in a tissue that can’t have an alternate arterial supply – bad if it gets occluded.

Portal systems = begin and end in a capillary bed (hypothalamic-anterior pituitary, hepatic).

Pampiniform plexus = countercurrent arrangement between artery and venous network. Ex: spermatic cord.

Arterio-venous shunts connect vascular passages between arterioles and post-capillary veins and are controlled by smooth muscle sphincters.