Question

Anatomy Tutorial: Histology of Blood Vessels

Answer 1

\({\bf{Blood~Vessel~Layers:}}\) three layers (going from outermost to innermost, adventitia, media, intima)



- adventitia: made of CT (collagen + elastic), surrounds the entire blood vessel
- media: smooth muscle and loose CT; involved in vasoconstriction/vasodilation. bound to the intima and adventita via collagen
- intima (also called tunica intima): endothelial lining and underlying elastic CT. in large arteries this layer will be thicker.

vasa vasorum: supplies muscle cells, fibroblasts, and fibrocytes to large vessels

\({\bf{Arteries~vs~Veins:}}\) unlike valves, arteries have...
- thicker walls (more CT and muscle cells in the media)
- round cross-section (better able to hold this shape rather than collapsing)
- less lumen
- folds in the epithelium ("pleated")
- do not have valves

Answer 2

\({\bf{Arteries:}}\) three types, from largest diameter to smallest: elastic, muscular, and arterioles



Elastic arteries:
- transport blood away from heart
- thick intima w/ a lot of elastic fibers and little muscle
- not controlled by SNS/local stimulation

Muscular arteries: transport blood to muscles/organs
- higher proportion of smooth muscle in media
- diameter controlled by sympathetic NS and local stimulation
- connected to elastic arteries

Arterioles:
- thin adventita
- sympathetic/endocrine/local control of diameter
- control blood flow btwn arteries + capillaries

Answer 3

\({\bf{Capillaries:}}\)
- thin
- slow blood flow
- substances can diffuse in between endothelial cells or through them
- basic structure: 1-3 endothelial cells + basal lamina
- subtypes: continous (endothelial lining is complete), fenestrated (endothelial lining has pores for the passage of small peptides/proteins) and sinusoids (which also have pores but are flattened/irregularly shaped, with gaps between endothelial cells and thin basal lamina)



exchange of substances occurs via: diffusion across endothelial cells (lipid-solubles, gases, water)
diffusion around endothelial cells: (water, small solutes)
diffusion thruogh fenestrated capillaries/sinusoids (water, solutes)
vessicles (water, bound/unbound solutes)

Answer 4

\({\bf{Veins:}}\) return blood to heart
- venules: smallest, collect blood from capillaries, mostly CT
- medium-sized veins: near muscular arteries, similar 3-layer organization as arteries
- large-sized veins: thick adventita, and isolated muscle cells. no valves.
- venous valves: in medium-sized veins, help prevent backflow of blood. arteries have enough blood pressure/flow to keep blood moving forward but veins do not

- veins are capacitance vessels + blood reservoirs (can handle changes in blood volume) --> can vasoconstrict to reduce venule blood volume and increase arterial blood volume

general blood distribution: arteries/heart/capillaries get 30-35% of total blood volume while venous system gets the rest

Answer 5

Adapted from Human Anatomy, Martini, et. al. 9th edition