3. Circulation

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

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* 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