3. Circulation

Circulation overview (describe flow of blood through relevant structures)

Circulation overview (describe flow of blood through relevant structures)

Artery > arteriole > capillary > venule > vein

What are the layers of a blood vessel?

1. Tunica intima

2. Tunica media

3. Tunica externa

Tunica intima

• Made up of endothelium

• Minimizes friction

Tunica media

• Muscular layer

• Smooth muscle

• Vasoconstriction: decrease in diameter

• Vasodilation: increase in diameter

Tunica externa

• Loosely woven collagen fibers

• Contain vasa vasorum (delivers oxygen and nutrients to arterial and venous walls and removes waste)

Types of arteries

1. Elastic arteries

2. Muscular arteries

3. Arterioles

Elastic arteries

• More central

• Conducting arteries

• Presence of elastin allows for the stretching of artery

  • Need to be able to withstand force because closer to the heart

• Continuous flow of blood

Muscular arteries

• More distal

• More active in vasoconstriction

Arterioles

Determines blood flow into capillaries

Capillaries (function, structural component)

• Responsible for exchange from blood to tissue and vice versa

• Intercellular clefts (gaps in junctions) to allow for diffusion

Capillary bed

• 10-20 capillaries supplied from one arteriole

• Can be biased or flooded depending on local conditions

Types of capillaries

1. Continuous capillary

2. Fenestrated capillary

3. Sinusoid capillary

Continuous capillary

• Least permeable (hard for things to get out)

• Most common

• Abundant in skin, muscles, lungs, and CNS

Fenestrated capillary

• Have large fenestrations (pores) that increase permeability

• Present in areas of active filtration (e.g. kidney) or absorption (e.g. small intestine), and areas of endocrine hormone secretion

Sinusoid capillaries

• Most permeable

• Least common

• Present in liver, bone marrow, spleen, and adrenal medulla

• Large intercellular clefts and fenestrations, and few tight junctions

• Allow large molecules and even cells to pass

Arterioles dilated

Blood flows through capillaries

Arterioles constricted

Not as much blood flows into capillary bed

Intrinsic/local regulation of blood flow

• Metabolic control

• Byproducts from usage (ex. exercise) cause vasodilation

  • Low O2, increased H+ (lactic acid), nitric oxide

• Byproducts of inflammation cause vasodilation

Veins (structure, function)

• Relatively little smooth muscle

• Act as blood reservoir

  • Contain up to 65% of blood in body

Pressure in veins (+ adaptation)

• Pressure in vein lower than in artery

  • Adaptation of valve (because not as much pressure to ensure correct blood flow like in arteries)

    • Prevent back flow

    • Resemble semilunar valve in heart

Venous blood flow: muscular pump

• Another mechanism to prevent back flow in veins (in addition to valves)

• When contracting skeletal muscles press against vein, they force open valves proximal to area of contraction

  • Blood is propelled toward heart

• Backflowing blood closes valves distal to area of contraction

Anastomosis/collateral circulation

The natural connection between two vessels

• Especially present in areas (like brain) where it is important for blood to reach

• Ensures that if one vessel were to get blocked/clot, blood could still get to that area of the body

Anterior Cerebral Artery (ACA)

Supplies:

• Medial and superior parts of frontal lobe

• Anterior parietal lobe

Middle Cerebral Artery (MCA)

Supplies:

• Lateral areas of frontal, temporal, and parietal lobes

Most common artery involved in stroke

Posterior Cerebral Artery (PCA)

Supplies:

• Occipital lobe

• Inferior part of temporal lobe

• Various deep structures including thalamus and posterior limb of internal capsule

Bonus exam question: compare and contrast ACA, MCA, and PCA strokes + which is most common

• ACA

  • Personality changes (frontal lobe)

  • Contralateral hemiplegia and hemisensory loss

• MCA

  • Most common

  • Contralateral vision changes

  • One-sided paralysis

  • Hemisensory loss

  • Language impairment (typically with left-sided stroke)

• PCA

  • Thalamic syndrome possible (losing temperature and pain regulation)

  • Vision and eyes impacted

• Generally, stroke has contralateral effects on body (stroke in left hemisphere likely to effect right side of body, and vice versa)

  • Reason for unilateral weakness following stroke