APK Chapter 20

three tunics that form the wall of an artery or vein

tunica externa:

• made of areolar dense CT proper, collagen, and elastic fibers

• may contain the vasa vasorum

• provides structural support and anchors vessels to surrounding tissues

• is thickest in the veins

tunica media:

• made of smooth muscle arranged in circular layers

• has elastic fibers

• controls vasoconstriction and vasodilation

• regulates pressure and distributes blood flow in arteries (thicker in arteries)

tunica interna:

• made of simple squamous epithelial tissue

• provides smooth, frictionless surface for blood flow

• in arteries there may be an internal elastic lamina

structural and functional features of arteries

structural:

• elastic arteries: large lumen, lots of elastin

• muscular arteries: thick tunica media

• arterioles: lead into capillaries

functional:

• always carry blood away from the heart

• usually carry oxygen rich blood to the systemic system

structural and functional features of veins

structural:

• post-capillary venules

• venules

• veins

functional:

• always return blood to the heart

• carry oxygen poor blood through the IVC and SVC

• act as a storehouse for blood (65% of blood is in the venous circulation at any given moment)

• is a low pressure system

structural and functional features of capillaries

structural:

• just large enough to allow single file passage of RBCs

• have an endothelium and basement membrane

• continuous capillary: least permeable but most common (skin and muscles)

• fenestrated capillary: has pores to increase permeability. occurs in areas of active absorption or filtration (small intestine or kidneys)

• sinusoid capillary: most permeable and occurs in special locations (bone marrow, spleen, and liver)

functional:

• allows molecules to pass in and out through 4 routes

• diffusion through endothelial cell membranes

• intercellular clefts

• pinocytotic vesicles

• fenestrations

vasa vasorum

small blood vessels that supply the walls of arteries and veins. found in areas where diffusion from the lumen is insufficient

smooth muscle’s role in vasodilation, vasoconstriction, and regulation of blood flow into a capillary bed

it contracts in the tunica media (vasoconstriction) in order to increase systematic blood pressure and reduce blood flow into downstream capillary beds

it relaxes in the tunica media (vasodilation) in order lower systemic blood pressure and increase blood flow to target tissues

regulation of blood flow happens as it coordinates with metabolic signals, hormones, and the ANS input to ensure tissues receive the right amount of blood at any given moment

vessels of the capillary bed

terminal arteriole, post-capillary venules, true capillaries, and the vascular shunt (metarterioles and the thoroughfare channel)

structural basis of capillary permeability

the capillary walls allow selective exchanges of gas, nutrients, wastes, and fluids between blood and tissues.

endothelial gaps and pores: junctions that allow small solutes to pass. tightness determines permeability

basement membrane structures: thin layer of CT that supports the endothelium. acts as a filter and allows small molecules to pass through. in some capillaries, this layer is incomplete or absent

transport mechanisms: larger molecules are transported across using pinocytotic vesicles

veins that have valves vs. those that don’t

valves:

• saphenous veins

• basilic veins

• cephalic veins

• femoral veins

• popliteal veins

• brachial veins

• subclavian veins

no valves:

• SVC

• IVC

• brachiocephalic veins

• internal & external jugular veins

• facial veins

• dural venous sinuses

• hepatic veins

• renal veins

• portal vein

• pulmonary veins

ways that veins work against gravity

valves: located predominantly in the limbs. have a similar structure as the semilunar valves in the heart. help veins to fight gravity by preventing backflow of blood

skeletal muscle pump: skeletal muscles contract to push blood up a vein. this works in tandem with the valves to help veins fight gravity

vascular anastomoses

a connection between 2+ blood vessels that allow for alternate pathways for blood flow

arterial anastomoses: 2+ arteries joining to supply the same tissue. common in areas where blood flow is critical. located around joints

venous anastomoses: is the most common type and is where interconnected veins drain in the same area. located throughout the body, especially the limbs

arteriovenous anastomoses: direct connection between and artery and a vein, bypassing the capillary bed. located in the skin and fingertips

major vessels of the pulmonary circuit

arteries:

• pulmonary trunk

• right pulmonary artery

• left pulmonary artery

• arterioles

veins:

• venules

• small and large pulmonary veins

• right and left pulmonary veins

capillaries:

• specialized in the lungs for efficient gas exchange

• surround each alveolus

• thin endothelial layer with basement membrane

major arteries of the systemic circuit

• ascending aorta

• aortic arch

  • brachiocephalic trunk

  • left common carotid artery

  • left subclavian artery

• descending aorta

• common carotid artery

• internal and external carotid artery

• subclavian artery

• axillary artery

• brachial artery

• radial artery

• ulnar artery

• intercostal arteries

• celiac trunk

• SMA

• IMA

• internal and external iliac arteries

• femoral artery

• popliteal artery

• anterior and posterior tibial arteries

blood flow in the hepatic portal system

1. arterial blood flows from the abdominal aorta to the SMA

2. from the SMA, the blood enters the first capillary bed in the stomach and intestine

3. here nutrients and toxins are absorbed into the blood stream

4. the blood flows from the first capillary bed, into the hepatic portal vein, and into the second capillary bed (liver sinusoids) in the liver

5. here in the liver, hepatocytes filter out the nutrients and toxins in the blood

6. the blood then goes through the hepatic vein and into the IVC where it is now venous blood

major veins of the systemic circuit

• SVC

• IVC

• internal and external jugular veins

• radial vein

• ulnar vein

• brachial vein

• axillary vein

• subclavian vein

• renal veins

• hepatic veins

• femoral vein

• external iliac vein

• common iliac vein

• saphenous vein

• SMV

• splenic vein

major capillaries of the systemic circuit

• continuous

• fenestrated

• sinusoid