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.
Blood Vessel Diameter:
o This is the variable that constantly changes PR!!
Arterial Blood Pressure:
o Force per unit area exerted on the wall of a blood vessel by the blood (= a hydrostatic pressure)
o Expressed in mm Hg and taken at the brachial artery.
o Contraction of the ventricles generates the BP.
o Highest in the aorta, decreases progressively, lowest at the right atrium, biggest drop of BP occurs at the arterioles.
o The pressure gradient provides the driving force that keeps blood moving from higher to lower pressure areas and back to the heart.
o Systolic blood pressure: the highest pressure exerted during ventricular systole (contraction) = the high number of a blood pressure reading
o Diastolic pressure: lowest level of arterial pressure during ventricular diastole = the lower number of the blood pressure reading)
o Pulse pressure = difference between systolic and diastolic pressure and amount of pressure to feel a pulse
o BP is measured by using a sphygmomanometer or via a catheter within a blood vessel
o Normal BP = 120/80 mmHg (systolic BP/diastolic BP)
o Hypertension = HTN = >140/90 mmHg
o Hypotension = low blood pressure (from blood loss, dehydration, anemia, etc…)
o Mean arterial blood pressure (MABP): defined as an approximation of an “average” blood pressure in the arteries between systole and diastole. Because diastole lasts longer than systole, MEAN pressure is not simply the value halfway between systole and diastole: MABP = diastolic pressure + (pulse pressure divided by 3).
Equation for BP Regulation:
o MABP is proportional to CO (cardiac output) X PR (peripheral resistance)
remember, CO = SV X HR
o When CO goes up, so does BP.
o When Peripheral Resistance goes up, so does MABP.
o Increased stroke volume or heart rate will increase MABP.
o When anything about peripheral resistance (PR) goes up (i.e., increased viscosity, vasoconstriction) MABP will go up.
o When anything about CO or PR goes down, MABP goes down.
Capillary Blood Pressure:
o MABP ranges from 15 to 35 mm Hg in capillaries.
o Low capillary pressure is a must.
o High BP would rupture fragile, thin-walled capillaries.
o Most capillaries are very permeable, so low pressure still pushes filtrate into interstitial spaces.
Venous Blood Pressure:
o MABP of the venous vessels changes little during the cardiac cycle.
o Venous Blood pressure represents a small pressure gradient = about 15 mm Hg (as compared to 35 mm Hg at arterioles).
o Due to low venous pressure, adequate blood return to the right atrium is compromised.
