Cardiovascular System Flashcards
Tunica Intima Layers
- Endothelium
- Basal Lamina
- Subendothelial CT
- Internal elastic lamina
- Composed of elastin
- Fenestrated
- Seperates intima from media in arteries
- Most prominant in muscular arteries

Tunica Media
- Usually the thickest layer in arteries
- Contains circularly arranged smooth muscle
- Contraction narrows lumen
- Contains elastic fibers, reticular fibers, & proteoglycans
- All produced by smooth muscle cells
- In capillaries and post-capillary venules, layer is replaced by pericytes
-
External elastic lamina
- Seperates media from adventitia
- Most prominent in muscular arteries
- Fenestrated

Tunica Adventitia
- Longitudinally oriented collagen and elastic fibers
- Primarily type I collagen
- Blends into surrounding CT
- Contains autonomic nerves that enter tunia media and innervate smooth muscle
- In thick vessels, contains small blood vessels called vasa vasorum
- Nourish adventitia & media of vessels too thick for diffusion
- More frequent in veins than arteries

Vasculature Innervation
- Smooth muscle in tunica media of most organs innervated by sympathetic fibers that release norepinephrine
- Causes sympathetic vasoconstriction response
- More prevalent in arteries than veins
- Most common in skin and kidneys
-
Skeletal muscle blood vessels innervated by sympathetic fibers that release acetylcholine
- Causes sympathetic vasodilator response
-
Parasympathetic fibers innervate very few organs
- Salivary glands
- Erectile tissue of penis
- Release acetylcholine, nitric oxide, and other co-transmitters
- Causes parasympathetic vasodilator response
Endothelial Cell
Functions
- Permeability barrier
- Secrete type IV collagen and laminin for basement membrane
- Synthesize clotting molecules
- von Willebrand factor
- Secrete vasoactive factors that control blood flow
- Endothelin - vasoconstriction
- Nitric oxide - vasodilation
- Produce growth factors
- Fibroblasts growth factor (FGF
- Platelet-derived growth factor (PDGF)
- Produce cell adhesion molecules that mediate acute inflammatory response
- Selectins
- Integrins
Weibel-Palade Bodies
Rod-shaped membrane-bound organelles within endothelial cells used for storage.
E.g. von Willebrand factor, P-selectins.
Derived from Golgi.

Pericytes
- Undifferentiated mesenchymal cells which surrounds capillaries and post-capillary venules.
- Differentiates into endothelial cells, smooth muscle, or fibroblasts.
- May be contractile.
- Branched cytoplasmic processes wrap around vessels.

Arteries
- High pressure vessels transporting blood away from heart
- Tunica media is dominant layer
- Narrow lumen with thicker wall
- Two basic types of arteries:
-
Elastic arteries
- Aorta & pulmonary trunk
- Helps convert pulsatile blood flow to steadier flow via elastic recoil
-
Muscular arteries
- Distributing arteries
- Includes most of the named arteries of the body
- Distribute blood to specific organs as needed
-
Elastic arteries
Arterioles
- Regulate blood pressure
- Regulate blood distribution to various organs & capillary beds
- Less than 5 layers of smooth muscle
-
Metarterioles
- smallest arterioles
- discontinuous layer of smooth muscle
Capillaries
- Location of metabolic exchange
- Can be smaller than diameter of RBC
- Consist of a single layer of endothelium and a basal lamina
- Discontinuous layer of pericytes may be present
Venules
- Smallest vessels of venous system
- Two types of venules:
-
Post-capillary venules
- Receive blood from capillaries
- Major site of WBC egress into extravascular tissues
-
Muscular venules
- Contains at least one complete layer of smooth muscle
-
Post-capillary venules
Elastic Artery Structure
Aorta, pulmonary artery, etc.
-
Tunica intima
- Endothelium
- Subendothelial connective tissue
- Internal elastic lamina
-
Tunica media
- Layers of smooth muscle cells alternating with and attached to 40-60 fenestrated elastic laminae
- Vessels expand with systole and recoil with diastole
- Elasticity decreases with age
- Reticular fibers and ground substance
- No distinct external elastic lamina
- Layers of smooth muscle cells alternating with and attached to 40-60 fenestrated elastic laminae
-
Tunica Adventitia
- Loose network of collagen and elastic fibers
- Fibroblasts and macrophages predominant
- Contain vasa vasorum
- poorly defined outer boundary

Muscular Artery Structure
-
Tunica Intima
- Endothelium
- Thinner subendothelial layer
- Prominent internal elastic lamina
- Appears scalloped if smooth muscle of media contracted
-
Tunica Media
- Up to 40 layers of circularly arranged smooth muscle cells
- Few elastic fibers
- External elastric lamina often fragmented into several thin layers that extend into adventitia
-
Tunica Adventitia
- Collagen and adipose
- Larger vessels have vasa vasorum
- Blends in with surrounding CT

Arteriole Structure
- Endothelium
- +/- internal elastic lamina
- Tunica media only a few layers thick (call it less than 5)
- External elastic lamina typically absent
- Contains pre-capillary sphincters

Metarterioles
- Smallest arterioles
- Discontinuous layers of smooth muscle
- Form preferential channels through capillary beds
- Arise from ordinary capillaries and drain into venules
- Contains precapillary sphincters
Arteriosclerosis
Any thickening and hardening of arterial walls.
Atherosclerosis most common form:
- Hardening of arterial wall caused by fatty plaque (atheroma)
- Foam cells accumulate
- Smooth muscle cells proliferate in intima and produce ECM components contributing to fibrosis
- Weakens vessel & reduces luminal diameter
- Platelet aggregation & thrombus formation can cause sudden blockage
Classes of Capillaries
Continuous
- found in most tissues
- tight junctions between endothelial cells
- no fenestrations
- pinocytotic vesicles transport substances across endothelium
Fenestrated
- Found in GI tract, kidney, endocrine organs
- Fenestrations or pores occur in thin areas of cytoplasm
- Have diaphragms in most organs
- Glomerular capillaries lack diaphragms
- More permeable than continuous
Sinusoidal
- Found in liver, bone marrow, spleen, adrenal cortex
- Leakiest capillary
- Wider diameter
- Irregular shape
- Large gaps between endothelial cells
- Basal lamina discontinuous or absent
Venous Circulation
- Low pressure vessels returning blood to heart
- Larger lumens with thinner walls
- Valves prevent backflow
- Formed by a fold of the tunica intima
- Most common in large veins below level of heart
Postcapillary Venules (PCVs)
Structure
- Consists mainly of endothelium, basal lamina, and discontinuous pericytes
- Reabsorb filtrate which leaked from capillaries
- Major site for vasoactive mediators
- Ex. Histamine and serotonin
- Main site of leukocyte diapedesis
- Via selectins, integrins, and ligands
Diapedesis
- Lymphocytes leave PCV’s of lymph nodes
- Requires L-selectins on lymphocyte to bind ligand on endothelial surface
- In inflammation, all types of leukocytes leave PCVs at site
- Requires P-selectins and E-selectins on endothelial surface to bind ligands on leukocytes
- Expression of selectins induced by products of inflammatory process
- Binding causes cell to slow and roll
- Integrins then strengthen binding and cause cell to stop
- Leukocyte inserts pseudopod into endothelial intercellular junction
- Widened by histamine and leukotrienes from mast cells and basophils
Muscular Venules
- Receives blood from postcapillary venules
- Differ from PCV in that they have true tunica media with 1-2 complete layers of smooth muscle
- Have thin tunica adventitia
Medium & Large Vein
Structure
In artery-vein companion pairs, vein usually larger diameter and appears collapsed/irregular in shape.
-
Tunica Intima
- Endothelium
- Thin subendothelial CT layer
- Occasional internal elastric lamina
- valves present in veins with >2mm diameter
- paired folds of intima
-
Tunica Media
- Much thinner smooth muscle layer
-
Tunica Adventitia
- Typically thickest layer of vessel wall
- Collagen and elastic fibers
- Fibroblasts
- Large veins like IVC may also contain bundles of longitudinally arranged smooth muscle

Lymphatic Vessels
Characteristics
- Low pressure
- More permeable than capillaries
- Picks up excess interstitial fluid and returns it to circulation
- Found in most tissues except nervous system and bone marrow
- Major metastatic pathway
Lymphatic Capillaries
- Begin as open ended endothelial tubes located near capillary beds
- Single layer of thin endothelial cells
- Basal lamina discontinuous or missing
- Often have valves
-
Achoring filaments connect basal lamina to perivascular CT
- Prevents collapse
- Irregularly shaped
- May contain many lymphocytes











