ANATOMY #3: CH. 21 - Blood Vessels

What is the basic structure of a blood vessel from innermost to outermost?

• Tunica interna

• Tunica media

• Tunica externa

Tunica Interna

• Innermost layer of the blood vessel

  • Endothelium

  • Basement Membrane

  • Internal Elastic Lamina

Endothelium

• Simple squamous

• Continuous with endocardium

• In direct contact with blood moving through vessels

Basement Membrane

• Anchors endothelium to CT

• Regulates movement of substances

Internal Elastic Lamina

• Elastic fibers

• Boundary of tunica interna

• Especially in arteries, help stretch to accommodate pressure

• Why you can’t feel pulse

• Not seen in veins/capillaries

Tunica Media

• Middle Layer

• NOT present in capillaries

  • Smooth Muscle and CT

  • Sympathetic fibers of ANS

  • Disposes of metabolic waste

  • External Elastic Lamina

Smooth Muscle and CT

Regulates lumen diameter

Sympathetic fibers of ANS

• Innervate tunica media

• More = constriction

• Les = relaxation

How does the tunica media assist in disposing of metabolic waste?

Vasodilates

External Elastic Lamina

Boundary of tunica media

Tunica Externa

• Outermost layer

• NOT present in capillaries

• CT, collagen and elastic fibers

• Can be very thick or very thin

• Contains nerves

  • Vasa vasorum

Vasa Vasorum

• Very large vessels contain this in tunica externa

• Layer is so thick, needs its own blood supply

• Own network of blood vessels

Arteries

• Have thick tunica media (high compliance)

  • Elastic

  • Muscular

  • Arterioles

  • Metarteriole

What is used to view arteries?

Light microscopy, at 200x

Elastic Arteries

• Largest

• Aorta, pulmonary trunk, first few branches off of aorta

• Contain well-defined elastic laminae, lots of elastic fibers, looks yellowish

• Highest pressure in cardiovascular system

• Helps propel blood away from heart when ventricles are in diastole

• Walls stretch, elastic fibers store energy (pressure reservoir)

• Elastic recoil moves blood when ventricles relax

Muscular Arteries

• Medium sized arteries

• Distributing arteries throughout body, branch repeatedly

• Lots of smooth muscle in walls

Arterioles

• Microscopic

• Abundant

• ~400 million in body, 15-30 um in diameter

• Thin tunica media

• Resistance vessels, regulate resistance to regulate flow to capillaries

  • Increased resistance = decreased flow

  • Decreased diameter = increased resistance, increased friction, increased pressure

• Branch into capillary beds

Metarteriole

• End of arteriole that leads into a capillary, “terminal end”

• Precapillary sphincter: Most distal metarteriole smooth muscle cell

• Monitors flow into capillary

• Spasms open and closes several times per minute

• Not every tissue of the body needs constant flow of blood

Capillaries

• Smallest, 5-10 um diameter

• For reference, average RBC is 8 um in diameter

• SINGLE FILE RBC

• ~20 billion capillaries

• Connect arterial outflow to venous return

• Exchange vessels, exchange substances between blood and interstitial fluid

  • Lack tunica media and tunica externa

  • Makes sure to have thinnest possible membrane, get effective exchange of material

• Branched, increasing surface area = increased rate of exchange

• Blood pressure falls in capillaries, hugely branched interconnected networks

• The more surface area we have, the more places we have for gas exchange

Three types of capillaries

• Continuous

• Fenestrated

• Sinusoid

Continuous capillaries

• Endothelial cells form a continuous tube

• Intercellular clefts, small gaps between adjacent cells

• Most common/abundant type of capillary

Fenestrated capillaries

• Many small pores in plasma membranes

• Pores 70-100 nm diameter

• Found in kidneys, small intestine, endocrine glands

• Makes it easier for hormones to enter bloodstream and get transported to desired location

Sinusoid capillaries

• Unusually large fenestrations and massive intercellular clefts

• Incomplete or absent basement membrane

• Most permeable capillary type

• Fenestrations, even RBCs can fit through

• Found in bone marrow, liver and spleen

• Bone marrow: Where RBC get made

• Liver/Spleen: Where old RBC can exit

• Liver: Processing nutrients from digestive system

General order of blood flow through vessels

• Artery

• Arteriole

• Capillary

• Venule

• Vein

Order with a portal system

• Artery

• Arteriole

• Capillary

• Portal vessel

• Second capillary bed

• Venule

• Vein

Purpose of portal veins

• If you need more distributing, make parallel lines

• Hardly ever are multiple capillaries in a row

3 portal systems

Name indicates location of 2nd capillary bed

• Hepatic Portal System (Liver)

• Renal Portal System (Kidney)

• Hypophyseal Portal System (Pituitary Gland)

Hepatic Portal System

1) By intestines

2) Portal vein brings to next bed in liver

Renal Portal System

1) In kidneys

2) Portal vein brings to next bed in kidney (returns nutrients back)

Hypophyseal Portal System

1) In brain

2) Portal vein takes to next bed in pituitary/hypophyses

Venules

• Drain capillary beds

• Have very thin walls

  • Postcapillary venules

  • Muscular venules

Postcapillary venules

• Smallest, porous

• Merge together to form larger venules

Muscular venules

• Larger

• Have thicker walls

• Can expand and serve as a blood reservoir

Veins

• Thin walls

• Diameter 0.5 mm - 3 cm

• Distensible walls with large lumens

• Very low pressure

• May contain valves to prevent backflow

• Contraction of skeletal muscle and breathing help increase venous return

  • Ex. Importance of moving on flight, wearing compression socks to decrease chances of blood clots

• Action of breathing/inhaling presses down on abdomen, making abdomen a high pressure area, pushing gases/fluids to lower pressure area

  • Anastomotic Veins

Anastomotic Veins

• Connections between double sets of veins

• Many in limbs

• Allows for alternative paths to return to heart, “Collateral circulation”

At any given moment, where is the majority of blood located?

In the veins, approx. 67%, “Blood reservoirs”

Hepatic Portal Circulation

• Blood from digestive tract, taking nutrient rich blood, delivering to liver before anywhere else

• Delivers blood from GI and spleen capillaries to liver sinusoids

• Liver stores and modifies nutrients, detoxes

  • Hepatic portal vein

Hepatic Portal Vein

Leads to liver from GI and splenic

Where does the hepatic portal vein get blood from?

• Superior mesenteric

• Inferior mesenteric

• Splenic 

• Left gastric 

• Right gastric

• Cystic

Pulmonary Circulation

Deoxygenated blood in arteries, oxygenated blood in veins

• Thrombus

• Deep Vein Thrombus

Thrombus

Blood clot that develops within a vessel

Deep Vein Thrombosis (DVT)

• A blood clot that forms in a deep vein

• Often in the leg

• In half of cases, DVT causes swelling, redness and cramping in the affected leg

  • If dislodges, can travel to pulmonary arteries and produce pulmonary embolism!!!