ch 16 blood vessels and circulation
What causes the drop in systolic blood pressure from 120 mm Hg in a major systemic artery to about 35 mm Hg at the arteriole end of a capillary?
A small part of the drop is due to friction. A larger part is due to the fact that the total cross sectional area at the capillary level (of all capillaries) is larger than the cross sectional area of the main arterial channels supplying those capillaries. "Ballooning" of the thin-walled peripheral vessels also plays a role.
Arteries are named on the basis of their course and distribution. What two kinds of anatomic features occur at the origin of arteries?
Arteries originate at branching points or at major landmarks (where the parent vessel passes a particular anatomic feature).
The standard treatment for circulatory shock is the administration of intravenous fluids, corti-costeroids, and antibiotics. What is the logic behind this regimen?
Fluids elevate the blood pressure (which has dropped). Antibiotics combat bacterial invaders (which may have crossed mucous membranes during the low blood pressure). Steroids restore capillary wall integrity (which is often compromised).
Explain the mechanisms by which each one of the major causes of cardiovascular shock lowers the blood pressure.
Hypovolemic shock—so much blood has been lost that there is not enough to fill the left ventricular lumen. Cardiogenic shock—due to fibrillation or heart failure, the heart does not contract with sufficient force to raise the blood pressure up to reasonable levels. Endotoxic shock—due to the presence of bacterial toxins, too many precapillary sphincters are forced to relax simultaneously resulting in too many capillary beds open at the same time. Neurogenic shock—smooth muscle tissue in too many vessels is relaxed at one time resulting in generalized vasodilation.
Explain this apparent discrepancy: the systemic circulation contains about 80% of the total blood volume, but the amount of blood leaving the systemic and pulmonary circulations per minute is the same.
Imagine a figure-8 racetrack with a very small loop and a very large loop and space out 20 race cars equidistant from each other on this track. There will be more cars (blood) in the big loop than in the small one, but the same number of cars (amount of blood) per unit of time leave the big loop as leaves the little loop.
Four organs have a dual blood supply (receive blood of both high and low oxygen levels). At the tissue level, the organs differ in how that blood is distributed. Explain.
In the liver and in the pituitary gland, the high and low-oxygen blood mixes within the tissue. In the heart, the low-oxygen blood passes through the chambers (but not through the cardiac tissue). In the lungs, the low-oxygen blood enters capillary beds where it becomes oxygenated. The small, high oxygen vessels associated with the lungs are the bronchial vessels which supply primarily the bronchial tree.
What does the term trunk imply when used in the name of an artery?
Most "trunks" divide (branch) almost immediately into two or three arteries.
Why is it that a major arterial channel can often be ligated (tied off) without seriously disrupting the blood supply to its area of distribution?
Most regions of the body have multiple vessels that supply them. If one is ligated, the others typically enlarge to supply the same total flow to the area under consideration. This concept is called collateral circulation.
Three principal organs supplied by branches of the celiac trunk are the stomach, spleen, and liver. List several other organs that these vessels supply.
Pancreas, duodenum, gallbladder.
What are the three inputs to the cerebral arterial circle?
Right and left internal carotid arteries and the basilar artery.
Which aortic branches are present in only about 50% of the population?
Testicular arteries and ovarian arteries.
Compare the overall diameter, wall thickness, volume of blood transported per unit time, and the relative flow speed (mm/sec) of blood within an artery and its satellite vein.
The artery is smaller in overall diameter than the vein. It also has a thicker wall than the vein, carries a slightly larger volume of blood per unit of time, and the blood in it is moving much faster than that in the vein.
systemic circulation.
The flow of blood from the left ventricle to the rest of the body and back to the right atrium (heart-body-heart). pressures are higher than pulmonary circulation which are needed because a larger % of the body is supplied by the systemic circulation.
pulmonary circulation
The flow of blood from the right ventricle to the lungs and then back to the left atrium (heart-lungs-heart). Its major function is to allow blood to exchange carbon dioxide for oxygen in the lungs
Why do some veins but no arteries have valves that prevent backflow of blood?
The pressure in some veins is so low—especially in limbs, that centrifugal force produced by limb movements can overcome the movement of blood proximally. Arteries have much higher pressure so no "backflow preventers" are needed.
Why is patent (open) foramen ovale usually inconsequential, but patent ductus arteriosus is usually quite serious and often requires surgical intervention?
The pressures in the two atria are nearly identical and so there is little or no blood flow (right-left or left-right) through a patent foramen ovale. On the other hand, a patent ductus arteriosus allows significant left-right flow through the ductus. This blood "should" have gone through the aorta to the body.
Why do the testicular arteries originate fairly high on the abdominal aorta instead of closer to the testes themselves?
The testes begin their development (as do the ovaries) fairly high in the abdominal cavity just below the kidneys. When the testes migrate through the lower anterior abdominal wall to enter the scrotum, their arteries (which originate near the renal arteries) migrate with them by "stretching". However, the origins of the testicular vessels do not move.
Why is the systemic blood pressure so much lower than the pulmonary blood pressure?
The total resistance to blood flow through the pulmonary circulation is much lower than the total resistance to flow through the systemic circulation.
Why do arteries and veins have tiny vessels within their walls?
These tiny vessels (vasa vasorum) are necessary to nourish the tissues within their walls which are too thick for nutrients and waste products to diffuse through from the blood within their lumina.
What purpose do arteriovenous shunts serve?
They allow variation in the amount of high oxygen blood delivered to particular loci and allow some thermoregulation.
Since the lungs in a fetus are nonfunctional in O2/CO2 exchange, why does some blood travel to them in the pulmonary arteries? (i.e., why aren't the lungs completely bypassed by the foramen ovale and ductus arteriosus)?
This is necessary to establish the circulatory channels that must take over at birth.
What mistake is the seventh grade health teacher making when teaching that arteries carry high-oxygen blood and veins transport low-oxygen blood?
This is true of the vessels in the systemic circulation but the opposite of this occurs in the pulmonary circulation.
artery
a blood vessel that carries blood away from the heart. There are three size-related categories of arteries: elastic arteries, muscular arteries, and arterioles.
Vein
a blood vessel that carries blood toward the heart. Four pulmonary veins carry blood to the left atrium from the lungs and two venae cavae (superior and inferior) carry blood from the rest of the body to the right atrium.
aneurysm
a pathologic focal enlargement of an artery or vein. These may continue to enlarge and ultimately rupture catastrophically.
Portal system
a portion of the systemic circulation where blood serially flows through two different capil-lary beds in different organs before it returns to the heart. The two present in mankind are the hepatic and hypothalamohypophysial portal systems.
Circulatory (cardiovascular) shock
a precipitous drop in blood pressure which may result from endo-toxemia, significant blood loss, catastrophic heart failure, or from neurogenic causes. This significantly endangers the brain because of greatly reduced blood flow to it.
Varicose vein
a superficial venous channel in an inferior limb that enlarges and becomes quite tortuous. These often occur in individuals who stand for prolonged periods without moving significantly.
Precapillary sphincter
a tiny ringlet of smooth muscle tissue around the initial segment of a capillary. Rhythmic contraction/relaxation of these sphincters based on oxygen levels in the surrounding tissues governs blood flow through most capillaries.
arterial trunk
an artery that branches soon after its origin into two or three other arteries. Examples include the brachiocephalic trunk, pulmonary trunk, celiac trunk, and costocervical trunk.
blood vessel
any of the tubular organs that transport blood including arteries, veins, and capillaries. Arteries and veins are transport tubes only, but capillaries have walls thin enough to allow diffusional exchange with the interstitial fluid.
the 3 types of blood vessels
arteries, veins, capillaries
smallest arteries
arterioles - 40 microns in diameter. they supply several capillaries each and in some cases empty directly into tiny veins (venules)
3 major parts of the aorta and the branches that each one supplies
ascending part of the aorta - 2 branches - left and righ coronary artery aortic arch - 3 branches - brachiocephalic trunk, left common carotid a., left subclavian a. descending part of aorta - 2 parts - thoracic part of descending aorta, abdominal part of descending aorta
Fenestrated capillary
capillary with "pores" (fenestrations) in its walls to facilitate exchange of nutrients and waste. These vessels are found in some endocrine organs and form the glomerular vessels in renal glomeruli.
3 pairs of embryonic veins
cardinal veins, vitelline veins, and umbilical veins. cardinal veins form most of the intra-embryonic venous channels.
3 different types of capillaries and their fxn
continuous capillaries - allow exchange of small molecules between blood and interstitial fluid. Those in the CNS do not have these channels and this may be a factor in the blood-brain barrier. fenestrated capillaries - capillaries in the endocrine glands, intestines, and choroid plexuses of the brain have fenestrated endothelial cells. allow the passage of larger molecules. sinusoidal capillaries (sinusoids) - Extremely porous capillaries in some locations (bone marrow tissue, liver, spleen)
largest arteries
elastic arteries - have significant amount of elastic connective tissue in their tunica media. when the heart contracts and forces blood into them, they expand. when heart is in diastole, the elastic fibers reduce the arteries back to their original diameters. "smooth out" blood pressure.
Sinusoids
extremely porous capillaries found in the liver, spleen, and bone marrow tissue. Sinusoids have discontinuous walls which facilitate exchange of nutrients and wastes even more than fenestrated capillaries.
Pulmonary circulation
hat part of the circulatory pathway involving blood flow to, through, and back from the lungs. It is considered to begin in the right ventricle of the heart and end in the left atrium.
dual blood supply
having both high and low oxygen blood inputs - liver, pituitary gland, lungs, heart
dorsal aortae supplies which 3 vessels
intersefmental artery (to the body wall), vitelline arteries (to the yolk sac), and umbilical arteries (to the placenta)
4 unpaired branches of the abdominal aorta
large celiac trunk, inferior and superior mesenteric a.a.
arteries seen with the naked eye
muscular arteries. 100 microns in diameter and have mainly smooth muscle tissue in their tunica media. involuntarily contracted by the ANS to elevate the blood pressure, or relaxed to lower it
2 fetal pecularities of fetal biology
shrunken lungs (which preclude the passage of significant volumes of blood through the pulmonary circulation) and nutrient/waste exchange at the placenta (which is physically distinct and remote from the fetus).
capillary wall
single layer of simple squamous epithelium (endothelium) surrounded by a thin layer of CT Proper. Typical capillaries are 10 μ or less in diameter.
capillaries
smallest vessels only vessels with walls thin enough to allow nutrient and waste exchange within surrounding tissues. positioned between atrioles and venules
Systemic circulation
that part of the circulatory pathway from the heart to most of the body and then back to the heart. The systemic circulation begins in the left ventricle and ends in the right atrium.
Why does blood in superficial vessels (i.e., the metacarpal veins on the posterior metacarpal region) appear blue in color?
the blood in the superficial veins is dark red and the overlaying tissue translates this to a bluish color.
Stroke
the clinical syndrome related to catastrophic arterial rupture in the brain. A cerebral vascular accident (CVA) damages brain tissue and can cause numerous sensory, motor, and/or cognitive deficits.
Tunica externa (of a vessel)
the dense connective tissue proper that forms the outermost layer of a vessel wall.
patent foramen ovale vs patent ductus arteriosum
the pressures in the atria are not significantly different postnatally, and so blood does not typically flow through a patent foramen ovale. This condition causes a characteristic heart murmur which occurs continuously (throughout the cardiac cycle) and is commonly called a machinery murmur. PDA often requires surgical correction by ligation because of the circulatory disturbances that it causes. It is a major cause of "blue baby" syndrome.
Tunica media (of a vessel)
the relatively thick middle layer of an artery, vein, or lymphatic vessel. It is sandwiched between the lining layer (tunica intima) and the outer coat (tunica externa).
Endothelium
the simple squamous epithelium that forms the lining layer of the tunica intima of vessels and the lining layer of the endocardium.
capillary
the smallest of the blood vessels. They join the smallest arteries (arterioles) to the smallest veins (venules) and are the only vessels with walls thin enough to allow nutrient/waste exchange
Name the four major systemic veins, what coalesces to form each one, what parts of the body each drains, and where (into what) each one empties.
the superior vena cava - returns blood to the right atrium of the heart from the head, neck, thorax, and superior limbs. It is a short vessel which forms by the convergence of the left and right brachiocephalic veins. The inferior vena cava returns blood to the right atrium from the abdomen, pelvis, and inferior limbs. It forms within the abdominal cavity by the convergence of the left and right common iliac veins. some of these coalesce to form the great saphenous vein which ascends in front of the medial malleolus of the tibia. The hepatic portal vein (also called the "portal" vein) is the large venous channel that receives most of the venous blood that has passed through capillary beds in the stomach, spleen, pancreas, and the small and large intestines The azygos vein is the major venous channel draining the thoracic wall. It empties into the superior vena cava just before the latter enters the right atrium.
Tunica intima (of a vessel)
the thin lining layer of a vessel wall. It is formed by simple squamous epithelium called endothelium and some connective tissue proper.
continuous capillary
the typical type of capillary found in the tissues of most organs.
general flow schemes in the pulmonary and systemic circulations
they are identical. blood leaves the heart in a single artery which supplies smaller arteries. This pattern (arteries-arterioles-capillaries-venules-veins) is the standard flow scheme for almost all sets of vessels in the body. Blood normally flows through only one set of capillaries before returning to the heart. *EXCEPT portal systems - hypothalamohypophysial portal system and the hepatic portal system - the blood courses through 2 different sets of capillaries instead of 1 before returning to the heart
precapillary sphincters of arterioles.
tiny rings of smooth muscle tissue encircle capillaries as they leave arterioles. They alternately and rhythmically open and close allowing blood to flow into only a fractional part of the capillaries at any one time. If these are paralyzed by a circulating toxin, too many capillaries will be open simultaneously resulting in a precipitous drop in blood pressure
3 named layers of artery walls
tunica intima, tunica media, tunica externa
fetal-specific anatomic structures and what they become after birth
umbilical arteries - medial umbilical ligaments, umbilical vein - round ligament of the liver, foramen ovale - fossa ovalis, ductus arteriousus - ligamentum arteriosum
