The Cardiovascular System: Blood Vessels and Hemodynamics



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The Cardiovascular System: Blood Vessels and Hemodynamics

Name the main blood vessel types.
There are three main categories of blood vessels that form a closed system of tubes leading from the heart, to the tissues, and back to the heart: arteries, capillaries, and veins.
A. Anatomy of blood vessels

1. Arteries
What is an artery?
An artery is a blood vessel that carries blood away from the heart and toward the tissues. Each artery has a wall consisting of three layers surrounding a hollow center called the lumen.
Identify each of the following as pertains to arteries:
Tunica intima -- The innermost coat of all blood vessels is the

tunica interna (intima). It consists of a simple squamous endothelium lying on its basement membrane and a thin connective tissue component called the internal elastic lamina.


Tunica media -- The middle coat, known as the tunica media,

consists of two components: elastin fibers, and smooth muscle. These tissues are arranged circumferentially about the artery and longitudinally through the length of the vessel.


Tunica adventitia -- The outermost coat, the tunica externa

(adventitia) consists of two components: elastin fibers and collagen fibers. These fibers form a connective tissue wrap for the vessel.


Name and define the two functional properties of arteries.
The structure of arteries gives them two important functional properties: elasticity and contractility.
Elasticity is the ability to return to a resting length after being stretched.
Vascular smooth muscle is innervated by the sympathetic nervous system. In response, the cells shorten and thicken. This is contractility.
What is elastic recoil?
As blood is ejected into the large arteries, their walls expand to accommodate the increased blood flow. As the ventricles relax, the elastic recoil of the arterial wall forces the blood forward through the system. It is this pressure exerted by arteries during diastole of the ventricles that maintains blood pressure and therefore blood flow through the body.
Distinguish between vasoconstriction and vasodilation.
In response to a sympathetic message, smooth muscle of the artery contracts and narrows the vessel lumen diameter (vasoconsriction).
When the sympathetic message is removed, the arterial smooth muscle relaxes and vessel lumen diameter increases (vasodilation).
Name the two types of arteries based on size and function.
There are two types of arteries based on size:

1. elastic (conducting) arteries

2. muscular (distributing) arteries
a. Elastic (conducting) arteries
Describe and name the elastic arteries.
Elastic arteries contain primarily elastin fibers in the tunica media. They are also called conducting arteries because they are large arteries that conduct blood away from the heart.
Elastic arteries are the ones that accommodate blood surging from the left ventricles during systole. They are the:

aorta


brachiocephalic

right and left common carotid arteries

right and left subclavian arteries

right and left vertebral arteries



right and left common iliac arteries.
What are pressure reservoirs?
The stretched elastic fibers of the elastic arteries momen-tarily store some of this energy and therefore function as pressure reservoirs. During diastole, the elastic fibers recoil, converting their stored energy into kinetic energy and thus pushing blood forward in the vascular the system in a more-or-less continuous fashion.
b. Muscular (distributing) arteries
Describe muscular arteries?
Muscular arteries are medium-to-small arteries that have more smooth muscle than elastin in their tunica media and are thus more capable of vasoconstriction and vasodilation than are elastic arteries. They are also known as distributing arteries since they branch from the larger, more elastic arteries and distribute blood to the various regions of the body.
What is blood shunting?
Because of their greater contractility, muscular arteries are used to direct blood flow to various parts of the body accord-ing to their moment-to-moment needs. This is called blood shunting. For example, vasoconstricting arteries in the skin while vasodilating those in the gut would cause an increased blood flow to the gut.
c. AnastomOSes
What are anastomoses?
There is usually more than one muscular artery supplying a particular region or tissue. Union of these vessels is called an anastomosis.
Anastomoses provide alternative routes for blood to reach a tissue, so that if one source is injured, another source continues to supply the area.
What is collateral circulation?
The alternate route of blood flow to a particular tissue, flowing through anastomoses, is called that tissue’s collateral circulation. This alternate route would not be used unless the preferred route was compromised in some way (i.e.--coronary artery blockage).
2. ArterioleS
What are arterioles?
An arteriole is a very small, generally microscopic artery that delivers blood to the capillaries. Arterioles branch from the muscular arteries and begin to lose wall thickness. The smallest of arterioles consists of nothing more than the endothelium and a single layer of scattered smooth muscle cells.
Describe why arterioles play the key role in blood distribution to the cells of the body?
During vasoconstriction, blood flow through capillaries serviced by a particular arteriole will decrease and may even cease completely, depending upon intensity of contraction. During times of high metabolic demand within a tissue, the arterioles supplying the tissue may be completely vasodilated, allowing a large blood flow through the capillary bed.
3. Capillaries
What are capillaries?
Capillaries are countless microscopic vessels that connect the arterial tree with the venous tree and allow for exchange of substances between the blood and the interstitial fluid. They are composed of a single layer of simple squamous epithelium (the endothelium) resting on a thin basement membrane.
Where are capillaries found?
Capillaries are found in the immediate vicinity of almost every cell of the body, but their distribution varies with the activity level of the tissue.
Tissues with high metabolic rates (brain, muscle, liver, kidneys, etc.) require more oxygen and nutrients and therefore have more extensive capillary networks.
In tissues with lower metabolic needs (tendons, ligaments, etc.) There are fewer capillaries. (This is one of the reasons that sprained or torn tendons and ligaments heal slowly.)
Some tissues (cartilage, epidermis, visceral epithelia, cornea, lens) have no blood supply at all. (This helps explain why a paper cut does not bleed and why cartilage injuries do not heal.)
What is the primary function of capillaries?
The primary function of capillaries is to permit the movement of nutrients and wastes between the blood and tissue cells via the interstitial fluid.
What anatomical features help them to accomplish this?
First, since capillaries consist of only a simple squamous layer of epithelium and a basement membrane, a substance need pass through only a single layer to move from one fluid compartment to another.
Secondly, capillaries branch from arterioles, forming extensive networks (beds), allowing for greatly increased surface area for diffusion and filtration and thereby allowing for the rapid exchange of large quantities of materials.
What is a precapillary sphincter?
At the origin of each capillary from an arteriole, there is a ring of smooth muscle called the precapillary sphincter that controls the flow of blood entering it. Sympathetic nerves innervate it. In this way, the nervous system can control blood flow through individual capillaries.
Describe vasomotion?
Blood flows through capillaries in an intermittent fashion rather than continuously because of alternating contraction and relaxation of the precapillary sphincters. This process is called vasomotion. It occurs at a resting rate of 5 - 10 times/minute and can be made faster or slower, according to the needs of the tissue at any given moment.
Describe and give locations for each of the following types of capillaries.
continuous -- Continuous capillaries have a continuous, uninter-

rupted endothelium with intercellular clefts found between adjacent cells. They are found in skeletal muscle, connective tissues, and the lungs.


fenestrated -- Fenestrated capillaries are the same as continuous,

except that the endothelial cells have 70 - 100 nm (10-9) fenestrae or pores in their plasma membranes. They are located in kidneys, small intestinal epithelium, and endocrine glands.


sinusoid -- Sinusoids have larger diameters, more tortuous routes,

large gaps separating cells, and a lining of cells adapted to the functions of the organ where they are found (phagocytic cells in liver and spleen, etc.)


4. Venules

5. Veins
How are veins formed?
When several capillaries unite, they form venules that collect their blood. Venules ultimately unite to form veins, which are composed again of the same three basic tunics as the arteries.
List 3 differences between a vein and its companion artery.
The primary differences are, when compared to the companion

artery:


1. The lumen is larger.

2. The tunica media is thinner, and

3. The tunica externa is thicker.
What does this structure allow?
The structure of veins allows them to be very distensible and capable of accommodating a large volume of blood, again based on the body’s need at the moment. This makes the venous system an important blood reservoir.
Why do veins have valves?
Because blood pressure has dropped greatly in the venous tree, veins are equipped with semilunar valves, particularly in the lower extremities. These valves prevent backflow of blood and keep it moving toward the heart.
6. Blood distribution
What are blood reservoirs?
At any given time, blood of the body is distributed in different proportions to the different areas. At rest, about 60% of total blood volume is located in the veins. Because veins contain so much blood at rest, they are known as blood reservoirs, storage depots for blood that can be rapidly moved to the heart for redistribution throughout the circulation.

How are they used in normal physiology?
For example, as muscular activity increases, there is increased sympathetic stimulation to veins. This results in vasoconstriction, reducing the volume of blood they hold and redirecting it back to the heart for redistribution to the muscles, whose arteries are vasodilated. The principal blood reservoirs are the veins of the liver, spleen, and skin.
B. Capillary exchange

1. Diffusion
Blood flow is slowest in the capillaries for 2 reasons. Name them.
1. greatest cross-sectional area of the vascular tree

2. greatest total length of the vascular tree and therefore the



greatest resistance to flow
What is the function of blood capillaries?
The 5% of blood that is in the capillary beds at any one time is the only blood that exchanges materials with interstitial fluid.
Name the 3 basic ways nutrients, wastes, gases, etc. move between the plasma and the interstitial fluid.
diffusion, vesicular transport, and bulk flow
What is the most important method of capillary exchange of solutes?
simple diffusion
Name the types of solutes that can cross the capillary membrane (endothelium) and how they move?
All plasma solutes (oxygen, carbon dioxide, glucose, amino acids, ions, hormones, wastes, etc.) freely pass the endothelium by following their concentration gradient.
Name the types of solutes that cannot cross the capillary membrane (endothelium). Why not?
Proteins, particularly plasma proteins, are too large to cross the endothelium.

2. Vesicular transport
Describe vesicular transport.
Vesicular transport is responsible for a small quantity of materials that pass through the capillary membrane. Some large substances in plasma are taken up by capillary endothelial cells by endocytosis (active transport processes), and then passed into the interstitial fluid by exocytosis (secretion).
This type of transport across capillaries is important mainly for what?
This method is used primarily for large, lipid-insoluble molecules like antibodies. For example, movement of antibodies from maternal blood to fetal blood crosses the placenta by vesicular transport.
3. Bulk flow (filtration and reabsorption)
What is bulk flow?
Bulk flow is the mechanism whereby regulation of the relative volumes of blood and interstitial fluid occurs. It is a passive process that involves the movement of large numbers of ions, molecules, or particles in the same direction.
The substances move in unison in response to various pressures and move at rates far greater than can be accounted for by diffusion and vesicular transport alone.
What causes bulk flow to occur?
Bulk flow occurs because some forces (pressures) push fluid (water and solutes) out of capillaries into the interstitial space, resulting in filtration of the fluid.
As a result of filtration, why does fluid not accumulate in the interstitial space?
Fluid does not build up in the interstitium because opposing pressures draw the fluid, with its solutes, back into the capillary. This process is known as reabsorption of the fluid.
What is the normal balance between filtration and reabsorption? What is this relationship called?
Normally, filtration is almost equal to reabsorption, a state of near equilibrium known as Starling’s law of the capillaries.
What would happen to total blood volume, and therefore blood pressure if filtration and reabsorption were not nearly equal?
Blood volume, and therefore blood pressure, would decrease.
Name the four pressures that are used to determine bulk flow.
Bulk flow is dependent upon 4 pressures that determine the direction of net fluid flow (filtration or reabsorption):

1. blood hydrostatic pressure

2. blood osmotic pressure

3. interstitial fluid hydrostatic pressure

4. interstitial fluid osmotic pressure
Describe each of the following.
blood hydrostatic pressure – Blood hydrostatic pressure (BHP) is

the fluid pressure within capillaries, tending to push fluid out.


On the arterial end it equals 30 mmHg.
On the venous end it equals 10 mmHg.
interstitial fluid hydrostatic pressure – Interstitial fluid hydrostatic

pressure (IFHP) is the pressure of interstitial fluid pressing on the outside of the capillary wall. This pressure can become an inward force, but it is a negligible pressure under normal circumstances, so consider it to be equal to 0 or -3 mmHg (suction).


blood colloid osmotic pressure – Blood colloid osmotic pressure

BCOP) is generated by plasma proteins trapped within the capillaries (albumin plays major role). This pressure acts to move fluid into the capillary by osmosis. It equals 28 mmHg.


interstitial fluid osmotic pressure – Interstitial fluid osmotic pressure

(IFOP) is the force of osmosis created by small amounts of protein that have leaked into the interstitium and pull fluid out of the capillary. It equals only about 8 mmHg.


Describe how the net filtration pressure is determined and give normal values for the arteriolar and venular ends of a capillary? Show how and why filtration and reabsorption occur on either end of a capillary.
Whether fluids enter or leave the capillary depends on how the pressures relate to each other; if outward forces are greater than inward forces, the net flow is out of the capillary (filtration).
If inward forces are greater than outward forces, the net flow of fluid is into the capillary (reabsorption).
The term net filtration pressure (NFP) is used to show the direction of fluid movement. It is calculated as:
NFP = (BHP + IFOP) - (BCOP + IFHP)

Outward - Inward


On the arterial end of a capillary:
NFP = (30+8) - (28+ 0)

= (38) - (28)

= + 10 mm Hg ( net movement out = filtration)


On the venous end of a capillary:
NFP = (10+8) - (28+0)

= (18) - (28)

= -10 mm Hg ( net movement in = reabsorption)
About 85% of the fluid filtered from the arterial ends of capillaries is reabsorbed at their venous ends. The balance, including the escaped plasma proteins, is returned to the blood via the lymphatic system.
On a daily basis, about 20 liters of blood are filtered out of capillaries, 17 liters are reabsorbed, and 3 liters enters the lymphatic system.
C. Hemodynamics: physiology and circulation

1. Velocity of blood flow
Describe the relationship between the velocity of blood and the total cross-sectional area of a given section of the vascular tree. What is circulation time?
The volume (mL) of blood that flows through any tissue in a given period of time (min) is blood flow (ml./min).
The velocity of blood flow (in cm/sec) is inversely related to the cross-sectional area of the blood vessels. In other words, blood flows slowest where the total cross sectional area is greatest.
Each time an artery branches, the total cross sectional area of all the branches is greater than that of the original vessel. On the other hand, as venules combine to form veins, the total cross sectional area decreases.
aorta -- area = 3-5 sq. cm., velocity = 40 cm/sec

capillaries -- area 4500-6000, velocity = < 0.1 cm/sec

vena cava -- area 14,000 velocity = 5-20 cm/sec
Thus, the velocity of blood flow decreases from the aorta to arteries to arterioles to capillaries and increases from the capillaries to the venules to veins to the vena cava.
Because blood moves at its slowest through the capillary beds, there is adequate time for the exchange between the plasma and the interstitial fluid to occur.
Circulation time is the time required for blood to pass from the right atrium back to the right atrium. This is usually about 1 minute at rest.
2. Volume of blood flow

a. Blood pressure
How do you determine cardiac output?
CO = SV x HR

= 5.25 liters/ minute (the volume of blood circulating through systemic or pulmonary vessels each minute)


In addition to stroke volume and heart rate, what other two factors determine cardiac output.
1. blood pressure = flow from higher to lower pressures

2. resistance (opposition) = the force of friction as blood

moves along blood vessels
Show another way in which cardiac output can be determined.
CO = mean (average) arterial blood pressure (MABP)

resistance (R)


What is blood pressure (BP)?
Blood pressure (BP) (also known as blood hydrostatic pressure or BHP) is the fluid pressure exerted by the blood on the inside wall of a blood vessel.
What is the relationship between blood pressure and cardiac output, blood volume, and return of blood to the heart?
The principal determinant of blood pressure (BP) is cardiac output (CO). There is a direct relationship between BP and CO. If all other factors remain the same, an increase in CO causes an increase in BP and a decrease in CO causes a decrease in BP. Factors that influence CO, then also alter BP.
CO, and therefore BP, also depends on the total blood volume (BV). There is a direct relationship between BP and BV. If BV drops (hemorrhage, dehydration, 3rd space fluid shifts), then BP drops. If BV rises (water retention), then BP rises.
As blood leaves the aorta, passing into the systemic circu-lation, BP falls progressively to 0 mm Hg by the time it returns to the right atrium. However, as the blood is channeled into the arterial circuit, the diameters of the individual vessels decrease, total cross-sectional area increases, and as a result, resistance increases. On the venous end of the system, even though Bp is low, as capillaries become venules, then veins, diameters increase, total cross sectional area decreases, and resistance decreases, allowing blood flow to continue.
b. Peripheral resistance
Define resistance.
Resistance is the opposition to blood flow principally as a result of friction between blood and the walls of the blood vessels.
List the three factors which influence resistance.
1. blood viscosity

2. total blood vessel length

3. blood vessel radius
What is the relationship between resistance and blood pressure?
There is a direct relationship between resistance and BP: an increase in resistance will cause an increase in BP, and a decrease in resistance will cause a decrease in BP.

Define viscosity.
Viscosity is a measure of the thickness of a fluid.
Blood viscosity depends upon what two factors?
1. ratio of RBCs to plasma volume

2. number of plasma proteins in the plasma


What would happen if there were too many RBCs?
Conditions such as dehydration, polycythemia (too many RBCs) or severe burns (where there is a lot of tissue fluid loss) would increase the RBC:plasma ratio. This would increase blood viscosity and therefore resistance to flow. As a result, BP would rise.
What would happen if plasma protein concentration decreased?
A depletion of plasma proteins (liver disease) would decrease blood viscosity, and therefore resistance. As a result, BP would fall.
Describe the relationship between resistance and total blood vessel length.
Resistance to blood flow is directly proportional to the total length of the blood vessel through which blood flows. In other words, the longer the vessel, the greater its resistance to flow and vice versa.
Considering this relationship, explain why an obese person has a greater tendency towards high blood pressure (hypertension).
An obese person has a greatly increased number of blood vessels because of the amount of adipose tissue that must be serviced. As a result, the total length of his or her vascular tree is greatly increased and this person tends to have a higher blood pressure because of the greater resistance to blood flow.
Describe the relationship between resistance and blood vessel radius.
Resistance is inversely proportional to the fourth power of the radius of the blood vessel. In other words, the smaller the diameter of the vessel, the greater the resistance it offers to blood flow.
A small artery gives rise to an arteriole that is one-half the diameter of the artery. What resistance will it offer to blood flow compared to its parent artery?
If the radius of a blood vessel decreases by ½, its resistance to blood flow increases 16 times (½ x ½ x ½ x ½ = 1/16).
What group of vessels exerts the greatest amount of resistance to blood flow?
capillaries
Define systemic vascular resistance (SVR)? What is another name for systemic vascular resistance?
Systemic vascular resistance (SVR) refers to all the vascular resistance offered by all systemic blood vessels. It can also be called total peripheral resistance (TPR).
Describe the role of arterioles in control of SVR?
A major function of arterioles, because of their smooth muscle and sympathetic innervation, is to control SVR by altering their state of vasoconstriction and vasodilation.
How does vasoconstriction affect SVR and therefore blood pressure?
In vasoconstriction the diameter of arterioles decreases, resistance increase, and therefore BP increases.
How does vasodilation affect SVR and therefore blood pressure?
In vasodilation, the diameter of arterioles increases, resistance decreases, and therefore BP decreases.
3. Venous return
What is venous return?
Venous return is the volume of blood flowing back to the heart from the veins of the systemic circulation. It depends on the pressure gradient between the venules (16 mm Hg) and the right atrium (0 mm Hg).
Although pressure differences are small, venous return keeps pace with cardiac output because resistance of the veins is low. (Remember, too, that cross-sectional area is decreasing, so velocity of flow in the veins increases as they approach the heart.)
Describe the role of each of the following in venous return.
skeletal muscles -- To boost venous return, skeletal muscles work

as “pumps” because veins, particularly in the extremities, are strategically placed between opposing muscle groups. Movement of the muscles provides a “milking” action for the veins, moving blood forward towards the heart.


venous valves -- Vein, particularly those in the lower extremities,

are equipped with venous valves (semilunar- like) that allow one-way blood flow only. As the body moves, the skeletal muscles “milk” the veins, forcing blood towards the heart. When blood moves backward, the valves fill with blood and close, preventing backflow.


respiration -- The action of breathing also aids blood flow back to

the heart. During inspiration, thoracic cavity pressure is less than abdominal cavity pressure. This adds to the pressure gradient in the inferior vena cava and thus aids blood flow towards the thorax and thus the heart.


D. Control of blood pressure an blood flow
There are several interconnected negative feedback systems that control BP by adjusting SV, HR, SVR, and BV. Some work instantaneously to cope with sud-den drops in BP, while others act more slowly to provide long-term regulation of BP.
Even while BP remains steady, needs arise that require blood to be redistributed to tissues undergoing rapid metabolism.
1. Cardiovascular center

a. Input to cardiovascular center

b. Output from cardiovascular center

2. Neutral regulation

a. Baroreceptors

b. Chemoreptors
What controls blood pressure and therefore blood flow to the tissues of the body?
The cardiovascular center of the medulla as well as local systems regulates blood flow and BP to the tissues of the body.
What is the cardiovascular center and what three specific functions does it control?
The cardiovascular center consists of 3 groups of neurons in the medulla that regulates HR, contractility of the ventricles, and blood vessel diameter.
Name each of the three groups of neurons of the cardio-vascular center and give a brief description of the function of each.
cardioacceleratory center -- The cardioacceleratory center or

CAC increases heart rate and contractility, working through the sympathetic nervous system.


cardioinhibitory center -- The cardioinhibitory center or CIC

decreases heart rate and contractility, working through the parasympathetic nervous system.


vasomotor center – The vasomotor center controls blood

vessel diameter, particularly in the arterioles, using the sympathetic nervous system.


Describe the neural inputs to and outputs from the cardiovascular system.
The CV center receives input from higher brain centers, such as the cerebral cortex, the limbic system, and the hypothalamus.
It receives input from baroreceptors, which monitor BP in the right atrium, aorta, and common carotid arteries, and from chemoreceptors in the aorta and common carotid carotids that monitor H+, carbon dioxide, and oxygen in the blood.
Output from the CV center flows along the vagus (X) nerve to the heart (parasympathetic) and to the thoracic spinal cord where it stimulates sympathetic fibers that pass to the heart and blood vessels.
To accomplish vasomotor tone, increased sympathetic activity to blood vessels promotes vasoconstriction while decreased activity allows vasodilation. These actions alter resistance and therefore BP.
Describe the carotid sinus reflex, the aortic reflex, and the right heart (atrial or Bainbridge) reflex that use negative feedback to maintain blood pressure.
Neural regulation of the heart is dependent upon 3 reflexes

and sensory input from the baroreceptors and chemoreceptors.


In the carotid sinus reflex and aortic reflex, increased BP in the carotid arteries and aorta is detected by baroreceptors and passed to the CV center by cranial nerve IX (glossopharyngeal) or cranial nerve X (vagus).
In response the CV center increases parasympathetic outflow to the heart, via cranial nerve X (vagus), resulting in decreased HR and contractility, decreased CO, and therefore, decreased BP.
If there is decreased BP, the CV center decreases parasym-pathetic outflow to the heart and increases sympathetic outflow, resulting in increased HR and increased contrac-tility, increased CO, and therefore increased BP.
In addition, there is increased sympathetic outflow to the arteriolar smooth muscle, resulting in increased vasocon-striction, which increases SVR and therefore BP.
In the right atrial reflex, increased BP in the vena cavae and right atrium, as a result of increased venous return, stimulates baroreceptors which signal the CV center via the vagus nerve.
In response, the CV center increases sympathetic outflow to increase HR and contractility in order to handle the increased venous return.
Chemoreceptors work in much the same way: decreased oxygen, or increased carbon dioxide and H+ result in increased sympathetic outflow to increase HR and contractility, thus raising BP and blood flow to the tissues.
3. Hormonal regulation
In addition to neural effects, there are a number of chemical mechanisms that affect blood pressure. Give a brief description of the role of each of the following in blood pressure regulation.

norepinephrine and epinephrine -- Epinephrine and norepinephrine

increase HR, contractility, and vasoconstriction, all of which would increase blood pressure.


angiotensin II -- Angiotensin II increases vasoconstriction and

indirectly acts to raise BV via actions of aldosterone and thirst, thus raising blood pressure.


histamine -- Histamine, released during inflammatory responses,

causes vasodilation, and can elicit a life-threatening reduction in blood pressure if the response is body wide (anaphylactic shock, for instance).


antidiuretic hormone -- Antidiuretic hormone increases vasocon-

striction and blood volume, thus raising blood pressure.


atrial natriuretic peptide -- Atrial natriuretic peptide increases loss of

sodium and water into the urine, thus lowering blood pressure. It also promotes vasodilation, which decreases peripheral resistance and thereby reduces blood pressure.


4. Autoregulation (local control)
What is autoregulation (local control)?
Autoregulation (local control) refers to a local, automatic adjustment in blood flow to a given tissue to match its needs at the moment, irrespective of nervous control.
What is the primary controller of autoregulation?
Oxygen is the primary stimulus for this action. With prolonged periods of vasoconstriction to a particular tissue, the tissue level of oxygen drops perilously low.
What do tissue cells do in response to this stimulus?
Tissue cells secrete vasoactive factors that cause the precapillary sphincters of the area to relax. This allows blood flow through the capillary bed to replenish the oxygen supply.
E. Blood vessel routes
You should be able to trace blood from the left ventricle to any given region or organ of the body through appropriate arteries, then return that blood to the right atrium via the appropriate veins. Of particular importance, be sure you can describe the great vessels of the heart, the immediate branches of the aorta, and the vessels that join to form the venae cavae. Keep in mind that most vessels are named for the body region through which they pass and that if you have learned the name for the artery supplying a body region, you have also learned the name of the pertinent vein. Use your text and the study guide in your lab manual to help you identify the vessels.
When thinking about blood vessel routes, think about the path blood takes as it flows through vessels to some destination in the body. This is like giving directions to someone on how to drive somewhere. Name the vessels through which blood flows to get to a body part. The following is by no means a complete listing of blood vessels.

UPPER EXTREMITIES
ARTERIAL BLOOD SUPPLY TO THE LEFT UPPER EXTREMITY
left ventricle  ascending aorta  aortic arch  left subclavian artery  axillary artery  brachial artery  ulnar artery or radial artery  superficial and deep palmar arches  palmar metacarpal and palmar digital arteries

DEEP VENOUS RETURN FROM THE LEFT UPPER EXTREMITY


palmar digital and palmar metacarpal veins  superficial or deep palmar venous arches  radial or ulnar veins  brachial vein  axillary vein  subclavian vein  left brachiocephalic vein  superior vena cava  right atrium

SUPERFICIAL VENOUS RETURN FROM THE LEFT UPPER EXTREMITY


palmar digital and palmar metacarpal veins  dorsal venous network  (2 paths)

1 - cephalic vein  subclavian vein  brachiocephalic vein  superior vena cava

2 - basilic vein  brachial vein  axillary vein  subclavian vein  brachiocephalic

vein  superior vena cava  right atrium

____________________________________________________________________________________
ARTERIAL BLOOD SUPPLY TO THE RIGHT UPPER EXTREMITY
left ventricle  ascending aorta  aortic arch  brachiocephalic artery  right subclavian artery  axillary artery  brachial artery  ulnar artery or radial artery  superficial and deep palmar arches  palmar metacarpal and palmar digital arteries
DEEP VENOUS RETURN FROM THE RIGHT UPPER EXTREMITY
palmar digital and palmar metacarpal veins  superficial or deep palmar venous arches  radial or ulnar veins  brachial vein  axillary vein  subclavian vein  right brachiocephalic vein  superior vena cava  right atrium

SUPERFICIAL VENOUS RETURN FROM THE RIGHT UPPER EXTREMITY


palmar digital and palmar metacarpal veins  dorsal venous network  (2 paths)

1 - cephalic vein  subclavian vein  brachiocephalic vein  superior vena cava

2 - basilic vein  brachial vein  axillary vein  subclavian vein  brachiocephalic

vein  superior vena cava  right atrium



LOWER EXTREMITIES
ARTERIAL BLOOD SUPPLY TO THE LOWER EXTREMITIES
left ventricle  ascending aorta  aortic arch  thoracic aorta  abdominal aorta  common iliac artery  external iliac artery  femoral and deep femoral arteries  popliteal artery  anterior tibial or posterior tibial arteries  dorsal pedal artery or lateral and medial plantar arteries  plantar arch  digital arteries

DEEP VENOUS RETURN FROM THE LOWER EXTREMITIES


digital and metacarpal veins  dorsal venous arch and plantar veins  dorsal pedal vein  anterior tibial vein  popliteal vein  femoral vein  external iliac vein  common iliac vein –> inferior vena cava  right atrium

SUPERFICIAL VENOUS RETURN FROM THE LOWER EXTREMITIES


digital and metacarpal veins  dorsal venous arch and plantar veins  small and great saphenous veins  femoral vein  external iliac vein  common iliac vein  inferior vena cava  right atrium

THE BRAIN
ARTERIAL BLOOD SUPPLY TO THE LEFT BRAIN
left ventricle  ascending aorta  aortic arch  left common carotid artery  left internal carotid artery  circle of Willis

left ventricle  ascending aorta  aortic arch  left subclavian artery  left vertebral artery  basilar artery  circle of Willis

ARTERIAL BLOOD SUPPLY TO THE RIGHT BRAIN
left ventricle  ascending aorta  aortic arch  brachiocephalic artery  right common carotid artery  right internal carotid artery  circle of Willis

left ventricle  ascending aorta  aortic arch  brachiocephalic artery  right subclavian artery  right vertebral artery  basilar artery  circle of Willis

VENOUS RETURN FROM THE BRAIN
dural sinuses  internal jugular vein  brachiocephalic vein  superior vena cava  right atrium
THE HEAD AND NECK (not the brain)
ARTERIAL BLOOD SUPPLY TO THE LEFT HEAD AND NECK
left ventricle  ascending aorta  aortic arch  left common carotid artery  left external carotid artery  (a number of branches that supply the head and neck)

ARTERIAL BLOOD SUPPLY TO THE RIGHT HEAD AND NECK


left ventricle  ascending aorta  aortic arch  brachiocephalic artery  right common carotid artery  right external carotid artery  (a number of branches that supply the head and neck)

VENOUS RETURN FROM THE HEAD AND NECK


various branches draining the head and neck  right external jugular vein  subclavian vein  brachiocephalic vein  superior vena cava  right atrium

THE THORACIC WALL
ARTERIAL BLOOD SUPPLY TO LEFT ANTERIOR THORACIC WALL
left ventricle  ascending aorta  aortic arch  left subclavian artery  internal thoracic artery  11 anterior intercostal arteries

ARTERIAL BLOOD SUPPLY TO THE RIGHT ANTERIOR THORACIC WALL


left ventricle  ascending aorta  aortic arch  brachiocephalic artery  right subclavian artery  internal thoracic artery  11 anterior intercostal arteries

ARTERIAL BLOOD SUPPLY TO THE LEFT POSTERIOR THORACIC WALL AND THORACIC VISCERA


left ventricle  ascending aorta  aortic arch  thoracic aorta  (several branches given off as the aorta passes respective structures)

1. right and left bronchial arteries

2. 11 pairs of posterior intercostal arteries

3. a series of esophageal arteries

VENOUS RETURN OF THE THORACIC WALL AND THE THORACIC VISCERA
intercostal veins and esophageal veins  azygos veins  superior vena cava  right atrium
blood from the bronchial arteries joins the flow from the pulmonary blood flow

THE ABDOMINAL VISCERA
ARTERIAL BLOOD SUPPLY TO PAIRED ORGANS OF THE ABDOMINAL CAVITY
left ventricle  ascending aorta aortic arch  thoracic aorta  abdominal aorta  (several branches given off as the aorta passes respective structures)

1. right and left inferior phrenic arteries

2. right and left superior, middle, and inferior suprarenal arteries

3. right and left renal arteries

4. right and left gonadal arteries (gonads originated near the kidneys)

5. a series of lumbar arteries to the body wall

VENOUS RETURN FROM THE PAIRED ORGANS OF THE ABDOMINAL CAVITY
paired venous branches corresponding to the arterial supply join the inferior vena cava  right atrium

ARTERIAL BLOOD SUPPLY TO THE UNPAIRED ORGANS OF THE ABDMINAL CAVITY


left ventricle  ascending aorta  thoracic aorta  abdominal aorta  3 branches from superior to inferior:

1. celiac trunk gives rise to three branches:

a. common hepatic artery to the liver

b. splenic artery to the spleen

c. left gastric artery

2. superior mesenteric artery supplies all of the small intestine and the large

intestine to the splenic flexure

3. inferior mesenteric artery supplies the large intestine from the splenic

flexure to the rectum

VENOUS RETURN FROM THE UNPAIRED ORGANS OF THE ABDOMINAL CAVITY


blood from the large intestine from the splenic flexure to the rectum empties into the

inferior mesenteric vein  splenic vein

blood from the rest of the large intestine, the small intestine, and the pancreas 

superior mesenteric vein


the splenic vein and the superior mesenteric vein unite to form the hepatic portal vein
the gastric veins join the hepatic portal vein
the hepatic portal vein dumps its blood into the liver sinusoids for processing  hepatic

vein  inferior vena cava  right atrium




THE PELVIC STRUCTURES
ARTERIAL BLOOD SUPPLY TO THE STRUCTURES OF THE PELVIC CAVITY
left ventricle  ascending aorta aortic arch  thoracic aorta  abdominal aorta  common iliac artery  internal iliac artery  various organs and structures

VENOUS RETURN FROM THE STRUCTURES OF THE PELVIC CAVITY


various organs and structures  internal iliac vein  common iliac vein  inferior vena

cava  right atrium







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