What are the three components of cardiovascular circuits?
- Arteries- transport blood away from the heart
- Veins- return blood to the heart
- Capillaries- smallest vessels where exchange of gases, nutrients, hormones, etc occurs
What are the two cardiovascular circuits?
- Pulmonary circuit (Arteries low O2)
- Systemic circuit (Arteries high O2)
Which germ layer are blood vessels derived from?
How are blood vessels organized?
- Outer wall consists of up to three layers - tunica intima, tunica media, and tunica adventitia
- Central lumen contains blood
- Tunics vary in tissue composition and structure depending on the vessel type
Describe the structure of the tunica intima.
- endothelium=innermost lining=simple squamous to cuboidal epithelium
- pericytes on the basal side- somewhat contractile
- lamina propria
- internal elastic lamina = thin layer of elastic connective tissue
What is the lamina propria?
Subendothelial layer of connective tissue in the tunica intima
Describe the structure of the tunica media.
- varies widely depending on blood vessel type
- connective tissue varies in amount and type
- may have concentrically arranged smooth muscle
- may have concentrically arranged elastic fibers
Describe the structure of the tunica adventitia.
- Outermost connective tissue, varies in thickness and type
- May contain 'vasa vasora' - small blood vessels that supply walls of large vessels
- Largest veins may have longitudinal smooth muscle
What are the three types of arteries?
- 1. Elastic arteries - largest, highest pressure
- 2. Muscular arteries - medium sized distributing arteries
- 3. Arterioles - smallest arteries
Describe the structure of elastic arteries.
- Very thick walls
- T. Intima has prominent lamina propria
- T. Media has prominent elastic fibers that allow for expansion and recoil with each heartbeat
- T Adventita has prominent vasa vasorum
What is an aneurysm?
- Ballooning of the wall of a blood vessel
- Caused due to a weakness in T. Media/decreased elastic fibers in the elderly
- Scarring occurs and collagen replaces the elastic fibers
- Treatment: monitor rate of expansion, put a stint in
Describe the structure of muscular arteries.
- T. Media has up to 40 layers of concentrically arranged smooth muscle fibers
- T. Intima folds like an accordian when smooth muscle is contracted; has lots of elastic fibers
What is Masson's Trichrome Stain used for?
Shows collagen which is prominent in T. Adventitia- appears blue
What is Weigert's histochemical stain used for?
Shows elastin - appears blue
Describe the structure of arterioles.
- T. Intima = endothelium + basement membrane, little lamina propria
- T. Media = 1-3 layers smooth muscle fibers
- T. Adventitia = very loose CT
How is contraction of arterioles controlled?
Contraction of smooth muscle cells controlled both by release of paracrine factors and by autonomic nerves (contraction/vasoconstriction=sympathetic/epinephrine; relaxation/vasodilation=parasympathetic/acetylcholine/nitric oxide)
What is purpose does contraction of arterioles serve?
Helps regulate blood pressure and blood flow into capillary beds at the smallest pre-capillary metarterioles
What is the precapillary sphincter?
- Regulates blood flow into the capillary beds at the metarteriole therefore determines blood pressure
- Regulated by the autonomic nervous system
What are the three types of capillaries?
- 1. Continuous Capillaries - Function where exchange between blood and the surrounding tissue is highly regulated
- 2. Fenestrated Capillaries- Function where exchange between blood and surrounding tissues is selective
- 3. Sinusoidal Capillaries (aka Venous Sinusoids) - Function where there is free exchange between blood and the surrounding tissues
Which cell type associated with the exterior of capillaries?
- Cytoplasmic processes wrap around endothelium discontinuously
- Pluripotent stem cell activity; slight contractile activity
Describe the structure of continuous capillaries.
- Endothelium has tight junctions; most macromolecular transport is highly regulared; exchange across endothelium between capillary lumen and surrounding tissues is facilitated by vesicular trancytosis
- Prominent in the brain and lungs
Describe the structure of fenestrated capillaries.
- Exchange is facilitated by small holes or fenestrations in the wall of the endothelium
- May have a proteoglycan 'diaphragm' that prevents free passage of large macromolecules
- Prominent in the kidney, intenstines, and endocrine glands
Describe the structure of sinusoidal capillaries.
- Endothelium has large gaps for free passsage of macromolecules and some cells
- Often lg diameter lumen
- Prominent in liver, spleen, and bone marrow
What are the three kinds of veins?
- 1. Venules - immediately post-capillary with prominent T. Intima
- 2. Small and Medium Veins - folds of T. Intima push into the lumen; prominent T. Adventitia
- 3. Large Veins - Very prominent T. Adventitia
Contrast venules and arterioles.
- Venules: larger diameter lumen, prominent TI, sparse CT in TM, diapedesis occuring in venule wall
- Arterioles: Smaller diameter lumen, less prominent TI, concentric smooth muscle in TM
Describe the structure of small and medium veins.
- Valves - characteristic folds of T. Intima with core of collagen I and elastin- open in one direction only
- TM: Much less prominent than the corresponding arteries
- TA: Relatively thick, considerable collagen I
How does the muscular (deep) venous pump work?
- Skeletal muscles constrict vein
- Blood moves towards heart
- Connot backflow due to valve orientation
Describe the structure of large veins.
- Vena Cavae and Immediate suppliers
- TI= prominent; lamina propria fibroelastic (dense) CT
- TM = not well developed
- TA = very prominent; fibroelastic CT and longitudinal smooth muscle; vaso vasora present
What is atherosclerosis?
- Subendothelial/Lamina Proprial Plaque formation, facilitated by excess LDL
- In early stages, there is subendothelial inflammation and foam cells- results in tearing of the epithelium
- In later stages, thrombus breaks away (thromboembolus) and triggers clot formation inside the lumen of blood cells