Chapter 19: Blood Vessels Flashcards
Blood Vessels:
o Are dynamic and ever changing conduits for delivery (nutrients) and removal (wastes) to and from the body tissues that begins and ends at the heart
o Arteries: carry blood away from the heart; oxygenated except for pulmonary circulation
o Capillaries: contact tissue cells and directly serve each individual cell
o Veins: carry oxygen poor blood toward the heart
Structure of Blood Vessel Walls:
o Arteries and veins:
o Tunica intima = innermost and next to the lumen and contains the endothelium (simple squamous) that lines the inner walls of the BVs. Allows for smooth, frictionless movement of blood
o Tunica media = smooth muscle for vasoconstriction and vasodialation (affects overall blood flowà BP via sympathetic nervous system)
o Tunica externa = loose woven collagen fibers to support and anchor the BV. Vasa vasorum = small blood vessels that nourish the tunica externa (also nerves and lympathatics infiltrate).
Conducting (Elastic) Arteries:
o Large thick-walled arteries near the heart with elastin in all three tunics.
o Examples are the aorta and its major branches (vessels are 2.5 cm to 1 cm in diameter).
o Large lumens offer low-resistance and hence called Conducting Arteries.
o Elastic arteries act as pressure reservoir. Expand and recoil as blood is ejected from the heart.
Muscular (Distributing) Arteries:
o Muscular Arteries distal to elastic arteries: delivers blood to the body organs
o Have thick tunica media with more smooth muscle and less elastin (1cm to 0.3 mm in diameter).
o Since more smooth muscle is present these arteries are they are active in vasoconstriction/vasodialation.
Arterioles:
o Arterioles are the smallest arteries. Have a Thick tunic media = large amount of smooth muscle with very little elastin. Resistance vessels.
o Arterioles lead to capillary beds.
o Arterioles control flow into capillary beds via vasodilation and vasoconstriction which adjusts blood flow to capillary beds.
Capillaries:
o Capillaries are the smallest blood vessels and microscopic in size. o The walls consist of thin tunica intima = one cell thick + a basement membrane. o Pericytes (spider shaped smooth muscle cells) help stabilize capillary walls and help control permeability. o Size allows only a single RBC to pass at a time with direct contact with every body cell. o In all tissues except for cartilage, epithelia, cornea and lens of eye. o Functions = exchange of gases, nutrients, wastes, hormones, etc. between blood and interstitial fluids.
The 3 Types of Capillaries:
o Continuous Capillaries
o Fenestrated Capillaries
o Sinusoidal Capillaries (sinusoids)
Continuous Capillaries:
o CO2 / O2 and nutrient exchange occurs in the skin and muscles (most common but the least permeable).
o Tight junctions connect endothelial cells of capillaries.
o Although tight junctions, there are intercellular clefts (small openings) between endothelial cells that allow the passage of fluids and small solutes.
o Continuous capillaries of the brain.
o Tight junctions are complete, forming the blood-brain barrier.
o Pinocytic vesicles allow nutrient exchange through endothelial cells.
Fenestrated Capillaries:
o Some of the endothelial cells contain pores (or fenestrations)
o These capillaries also have intercellular clefts and pinocytic vesicles
o More permeable than continuous capillaries to fluids and solutes
o Function in absorption or filtrate formation:
o Small intestines (absorption of nutrients).
o Endocrine glands (dumping hormones into blood).
o Kidneys (blood plasma filtration).
Sinusoids (Sinusoidal Capillaries):
o Most permeable.
o Fewer tight junctions between epithelial cells, larger intercellular clefts and very leaky, large lumens with incomplete basement membranes.
o Sinusoids are fenestrated (pores).
o Allow large molecules blood cells to pass between the blood and surrounding tissues.
o Found in the liver, bone marrow (RBC’s and leukocytes manufactured), liver, and spleen.
Capillary Beds:
o Capillary beds are interwoven networks of capillaries form the microcirculation between arterioles and venules.
o Consist of two types of vessels:
o Vascular shunt (or metarteriole—thoroughfare channel): Directly connects the terminal arteriole and a postcapillary venule.
o True capillaries:
o Woven 10 to 100 exchange vessels per capillary bed
o True capillaries usually branch off the metarteriole (proximal end of vascular shunt that comes off a terminal arteriole) returns via the thoroughfare channel to join with a postcapillary venule).
Blood Flow Through Capillary Beds:
o Precapillary sphincters act like valves that regulate blood flow into the true capillaries at the point where they join with the metarteriole.
o Blood goes through the terminal arteriole to go either through the true capillary bed or be shunted directly to the thoroughfare channel into the postcapillary venule.
o Regulated by local chemical conditions and vasomotor nerves (whether sphincters are relaxed or contracted).
Venules:
o Formed when the capillary beds unite.
o Very porous; allow fluids and WBCs to go back and forth easily.
o Postcapillary venules consist of endothelium and a few pericytes.
o Larger venules have one or two layers of smooth muscle cells.
Veins:
o Formed when venules join together.
o Have thinner walls, larger lumens compared with corresponding arteries.
o Blood pressure is lower than in arteries.
o Thin tunica media (little smooth muscle) and a thick tunica externa (consists of collagen fibers and elastic networks).
o Veins are called capacitance vessels (blood reservoirs); and contain up to 65% of the blood supply at any time.
o Specialization of veins that ensure return of blood to the heart:
o Large-diameter lumens offer little resistance.
o Venous Valves prevent backflow of blood. Most abundant in veins of the limbs (to help counteract effects of gravity).
o Venous sinuses: flattened veins with extremely thin walls of only endothelium (e.g., coronary sinus of the heart and dural venous sinuses of the brain).
Vascular Anastomoses:
o Interconnections of branches of blood vessels where BVs come together.
o Arterial anastomoses provide alternate blood pathways (collateral channels) to a given body region = collateralization.
o Common at joints (movement of joint blocks blood flow), in abdominal organs, brain, and heart.
o Allows for adequate blood flow if another vessel is blocked (e.g.blood clot = emboli).
o Vascular shunts of capillaries are examples of arteriovenous anastomoses.
o Venous anastomoses allow multiple routes for blood.
Blood Flow:
o Volume of blood flowing through a vessel, an organ, or the entire circulation in a given period of time.
o Measured as ml/min or L/min.
o Equivalent to cardiac output (CO) for entire vascular system, except, blood to certain organs may be shunted based on needs.
o Blood Flow varies at any given time through INDIVIDUAL ORGANS depending upon their metabolic needs (blood shunting).
o Blood flow is relatively constant when at rest.
o Pressure Differences: Blood always flows form areas of high pressure to areas of lower pressure. The greater the pressure gradient, the greater the flow.
o Resistance (opposition to blood flow): the greater the resistance, the more the blood flow is impeded.
Resistance: Definition and Three Important Sources:
o Opposition to flow in systemic system.
o Measure of the amount of friction blood encounters.
o 1) Blood viscosity.
o 2) Total blood vessel length.
o 3) Blood vessel diameter.
o Small-diameter arterioles are the major determinants of PR
o Resistance varies inversely with the 4th power of the vessel radius =
o Doubling the size of a vessel =
o 1/ 2x2x2x2 = 1/16th the previous resistance larger blood vessels near the heart do not influence peripheral resistance, but vasoconstriction/dialation of the smaller arterioles pretty much determine PR.
o Abrupt changes in diameter due to fatty plaques from atherosclerosis dramatically increase PR.
Blood Viscosity:
o The thickness or viscosity of the blood due to formed elements (RBCs) and plasma proteins.
o The greater the viscosity, the greater the PR and the greater the BP (usually a constant except for e.g. polycythemaia, anemias).
Blood Vessel Length:
o The longer the vessel, the greater the PR encountered and the greater the BP (obese people have many more miles of blood vessels) but usually a constant.
