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Lecture 26: Cardiovascular System IV
Blood Vessels, Capillaries and the Control of Blood Pressure
62
Anatomy
Undergraduate 1
12/09/2010

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Term
Blood Vessels - Capacity and Resistance
Definition
The various blood vessels of the cardiovascular system exercise an important influence on its capacity to hold fluid resistance to blood flow. The arterioles are mainly involved in determining the resistance segment of the circulation, while the veins determine the capacity.
Term
Resistance and capacity are variable. Sympathetic can:
Definition

-increase the resistance by causing vasoconstriction of the arterioles

-decrease the capacity of the system by causing vasoconstriction of the veins

Term
Aterioles; Total peripheral resistance
Definition
The arterioles control the flow into the capillary beds and into the collecting muscular and non-muscular venules and veins. They also control the total peripheral resistance. In consequences, the aterioles are sometimes referred to as the resistance vessels of the cardiovascular system, and total peripheral resistance depends on whether they are undergoing vasoconstriction or vasodilation.
Term
Hypertension
Definition
A chronic elevation of the total peripheral resistance by narrowing of the lumens of the arterioles appears to be the important factor that elevates the blood pressure in hypertension. Many drugs for treating hypertension act by reducing the total peripheral resistance. That is, they dilate the arterioles.
Term
Mean arterial pressure =
Definition
cardiac output X total peripheral resistance
Term
Capacity of the Circulatory system - Importance of the Veins
Definition
The systemic veins and venules contain about two thirds of the blood in the body. Therefore, the veins and venules control the capacity of the circulation. The venous capacity is relatively small in muscles and the kidney, but relatively large in the skin and in the gastrointestinal tract.
Term
Hemorrhage
Definition
The veins and venules are important in hemorrhage. loss of blood usually leads to activation of the sympathetic nervous system and arteriolar vasoconstriction, which can cause a decrease in pressure downstream in the veins. Loss of distending pressure will allow the natural elastic recoil of the veins to cause venous capacity to decrease passively. Additionally, there may be active constriction of the veins via the sympathetic nervous system, which can further reduce the capacity of the circulation and assist in maintaining the blood pressure
Term
After a hemorrhage
Definition
the capacity of the veins and venules in the systemic circulation (particularly in the skin and gastrointestinal tract) is reduced. This compensates for the reduction of blood volume, and assists in the maintenance of cardiac output and arterial blood pressure.
Term
Venous pump
Definition

The venous pump operates in three regions of the body.

-The first is the plantar pump, which is driven by changing pressure on the soles of the feet.

 

-The second is the muscular pump of the legs. This is made possible because the long veins of the arms and legs contain flap-like valves that project inwards from their linings. These valves have two semilunar leaflets that are arranged so that the direction of blood flow can only be toward the heart. When the muscles of the legs or arms contract, they massage the veins and help to propel the blood toward the heart.

Term
Accumulation of edema in legs and arms
Definition
If you stand absolutely still, the pumping doesn’t occur. Pressure in the veins and capillaries then increases. The feet and legs swell because fluid leaks out from the capillaries into the tissue spaces (edema). this effect also be observed during periods of sitting without leg movement on long airline trips.
Term
The third venous pump:
Definition
in the respiratory pump, which is driven by changing pressures in the thoracic cavity during respiration
Term
Vericose Veins
Definition
the valves in the longs veins of the legs do not function properly and pressure in the veins increases. Increased venous pressure leads to leakage of fluid from the capillaries, which causes edema in the legs when one has been standing for a few minutes. THere is progressive distortion of the veins. The condition prevents adequate diffusion of nutritional materials from the capillaries to the muscle and skin cells, leading to painful and weak muscles and skin ulceration.
Term
The capillary bed
Definition
The capillary bed is the place in the circulatory system where exchange of substances can occur between the blood and interstitial fluid that fills the space around the cells of the body.
Term
Structure of a Capillary Bed
Definition
Capillaries branch off from terminal arterioles. THe terminal arterioles are intermediate in structure between arterioles and capillaries, and some some muscle in their walls. At that point where the true capillaries origante from terminal arterioles, one or two smooth fibers encircle the capillary. This is called the precapillary sphincter, which can open or close the entrance to the capillary Blood from the capillaries is collected by the venules. At first, the venules have little or no muscle in their walls, and are call non-muscular venules. As they approach the veins, however, the amount of muscle increases, they become larger in diameter and they called muscular venules.
Term
How materials move in and out of capillaries
Definition
capillaries in various parts of the body differ in the structure of their endothelial walls.
Term
Endothelial walls in the brain and retina
Definition
the vascular endothelial cells are relatively tall, and connected by tight junctions that prevent many substances from passing through them. They constitute a barrier called the blood-brain barrier
Term
Endothelial walls: In the liver, bone marrow and spleen,
Definition
the vascular endothelial cells have enormous gaps between them. these gaps allow proteins and other large molecules to move through them.
Term
Endothelium walls in the skin, muscle, lungs, gastrointestinal tract, many glands, and in the kidneys
Definition
the walls of the capillaries lie between the two extremes
Term
Diffusion in Capillaries
Definition
Substances such as carbon dioxide and oxygen are lipid-soluble and diffuse through the endothelial cell wall of the capillary, carbon dioxide inwards and oxygen outwards. There is no net transfer of water by this process Other, water-soluble substance diffuse out from the capillary through pores that are actually clefts betwe two endothelial cells. In general, no protein escapes from the capillaries, except when the pores are very large (in liver, spleen, and bone marrow)
Term
blood-brain barrier
Definition
ther are no pores in the capillaries of the cerebral and retinal circulations, where there is a blood-brain barrier that prevents many small molecules (e.g. certain hormones) from getting into the brain. Special carrier molecules in the membrane ensure that essential nutrients such as glucose and amino acids can be delivered to the brain’s cells.
Term
Pinocytosis
Definition
Small quantities of substances with large, lipid-insoluble molecules are transferred across the capillary wall by pinocytosis. pinocytotic are formed by pinching off of the plasma membrane
Term
Filtration
Definition
Filtration is the principle mechanism for controlling balance between interstitial fluid volume and plasma volume. The filtrate passing out of a capillary wall consists mainly of water, ions and small solutes. It contains negligible quantities of plasma proteins. Net protein osmotic and hydrostatic forces determine the extent and direction of capillary filtration.
Term
Blood pressure
Definition
The main force that drives fluid OUT of a capillary is the blood pressure (hydrostatic pressure) inside the capillary. THe pressure inside the capillary is opposed by pressure of the interstitial fluid, which is normally negligible but can be increased in inflammation
Term
Osmotic pressure
Definition
The main force that draws fluid INTO a capillary is blood osmotic pressure, which is higher in the blood than in the interstitial fluid the contains proteins which are lacking in the interstitial fluid. The blood protein osmotic pressure (= blood oncotic pressure) is opposed by the interstitial fluid protein osmotic pressure, which is normally less than that of the blood unless there has been extensive leakage of proteins from the blood into the interstitial space, as could happen in inflamation.
Term
Balance between blood pressure (hydrostatic pressure) and osmotic pressure
Definition
The balance between these forces determines whether fluid enters the capillary from the the interstitial space or leaves the capillary and enters the interstitial space. Generally speaking, the blood pressure at arterial end of capillary is greater than blood pressure. Therefore, fluid leaves the capillary at its arterial end. On the other hand, the blood pressure at the venous end is less than the blood osmotic pressure. Therefore, fluid is withdrawn from the interstitial space into the blood at the venous end of a capillary.
Term
Lymphatics
Definition
Normarlly, a little more fluid leaves the capillaries than returns to them. About 10% remains in the interstitial space. The excess is collected as lymph by a network of lymphatics, blind-ended vessels with valves, and returned to the venous circulation near the junction of the subclavian and internal jugular veins.
Term
Edema
Definition
is tissue swelling caused when an abnormal excess of fluid builds up in interstitial space
Term
Causes of Edema: hydrostatic pressure
Definition
1) Increased venous hydrostatic pressure (standing for a long time, failing heat Simply standing for a long time without moving the leg muscles can impair the venous pump and raise the pressure at the venous end of the capillaries in the legs and feet, causing edema in this part of the body. Edema can also be caused by increased pressure in the veins produced by a failing heart.
Term
Cause of Edema: osmotic pressure
Definition
2) reduced osmotic pressure (malnutrition, liver disease) Loss of plasma proteins in malnutrition and liver disease cause edema because of the decrease in the plasma protein osmotic pressure.
Term
Causes of Edema: interstitial fluid
Definition
3)Increased interstitial fluid osmotic pressure -buildup of protein in the interstitial space Substances that increase the premeability capillary walls to plasma proteins cause edema. The plasma proteins accumalate in the interstitial fluid, and therefore increase the interstitial fluid protein osmotic pressure. histamine, which is released during inflammation, is an example of substance that has this effect on capillary walls.
Term
Edema causes: lympatics
Definition
Blocked lymphatics -surgery, tumor, parasitic infection
Term
Pulmonary Edema
Definition
Efficient gas exchange in the lungs requires that the surfaces of the air sacs in the lungs (the alveoli) be kept dry and free of accumulated fluid. As with other tissues, there is a small, continuous leak of fluid out of the lung capillaries into the interstinal space surrounding the cells in the lung. This fluid is normally removed by a gentle suction effect by the lymphatic vessels, which also suck the air sacs dry.
Term
Causes of Pulmonary edema
Definition
Pulmonary edema can be caused by a rise in pressure in the capillaries. One cause is failure of the left ventricle - the right ventricle continues to pump blood into the lungs, but the left ventricle cannot pump it away at the same rate. Therefore, there is a rise of pressure in the pulmonary veins, and therefore in the pulmonary capillaries. Because efficient exchange cannot occur across the swollen respiratory membrane, death due to pulmonary edema can occur very quickly.
Term
Low regulation of Blood Flow
Definition

The smooth muscle fibers of the terminal arterioles and precapillary sphincters are only sparsely innervated by the autonomic nervous system. They are controlled local factors with the tissue local factors have vasodilator effect and increase blood flow through a capillary bed include the following. 1) high carbon dioxide

2) Low oxygen

3) Low pH (e.g. from accumulation of lactic acid in tissue

Term
Active hyperemia
Definition
An increase in metabolic rate would generate more carbon dioxide, reduce the amount of oxygen, and reduce the pH because of formation of lactic acid. Therefore, as the metabolic rate of a tissue increases, we see an increase in the blood flow through that tissue. For example, the blood flow through a muscle increases during exercise, when the metabolic rate of the muscle increases. Also, the blood flow through regions of the brain increases when those regions become very active during mental activity. The increase in blood flow through a tissue that has become more metabolically active is called active hyperemia (metabolic hyperemia)
Term
Hormones and Other Chemicals
Definition
There are many compounds that have important vasoconstriction and vasodilation effects on blood by the adrenal gland.
Term
Vasoconstrictors:
Definition
a) Norepinephrine and epinephrine (catecholamines) - norepinephrine is released by sympathetic nerve terminals, and both norepinephrine and epinephrine are released into the blood by the adrenal gland.
b) Angiotension II (an eight amino acid peptide involved in the renin-angiostensin-alsosterone system)
c) Vasopressin (released from the pituitary gland)
d) Endothelin (secreted by vascular endothelial cells)
Term
Vasodilators
Definition
a) Nitric Oxide - Like the vasoconstrictor endothelium and may important in the maintenance of normal blood pressure. It is also important in penile erection.
b) Histamine is released from mast cells in allergic reactions (histamine also increases the porosity of the capillary wall, an effect that is unrelated to its vasodilator action).
Term
Sympathetic vasoconstriction
Definition
Sympathetic arteriolar vasoconstriction is epecially potent in the kidneys, gut, spleen and skin. It is less important in the blood vessels supplying the skeletal muscles, brain, and myocardium (coronary circulation), all of which are under overriding local control related to their metabolic activity.
Term
Sympathetic vasoconstrictor ton
Definition
The sympathetic sends a constant stream of nerve impulses to the blood vessels, causing a continual sympathetic vasoconstrictor tone. Sympathetic tone maintains the total peripheral resistance. This tone is lost if the spinal cord is svered or if transmission in the spinal cord blocked with a local anesthetic. This cases a large drop in the total peripheral resistance, and the blood pressure can drop dangerously.
Term
Sympathetic stimulation causes...
Definition
release of epinephrine and norepinephrine by the adrenal glands. These substances pass into the circulation and cause vasoconstriction in blood vessels that lack a sympathetic innervation (e.g. terminal arterioles)

Don’t forget the action of the ANS on the heart.
Term
Angiogenesis
Definition
Over many days and weeks, they may be be change in the number of vessels in a tissue (i.e. a change in the tissue vascularity). There can be an increase in tissue vascularity if there has been a prolonged increase in metabolic rate of a tissue. An example is a rapidly growing tumor, which becomes invaded by blood vessels. The development of new blood vessels is called angiogenesis, and the process seems to be regulated by a class of peptides called angiogenic factors
Term
Angiogenic Factors
Definition
Angiogenic factors work on the vascular endothelial cells that make up the inner walls of blood vessels, notably the capillaries. An example of one of these factors is vascular endothelial growth factor (VEGF). Endothelial cells always have the capacity to multiply under suitable conditions, and will even migrate and multiply to line plastic tubing used surgically to replace damaged blood vessels.The cells that surround capillaries - pericytes - seem to have an important role to play here, because they normally prevent the endothelial cells from multiplying.
Term
Importance of Angiogenesis
Definition
1) Development of blood vessels in ischemic tissue (blood-deficient tissue)
2) In deep wounds where there is little oxygen (“hypoxic wounds”: one of the angiogenic factors is now being used clinically to promote angiogenesis in such wounds, and accelerate healing)
3) During embryonic development
4) During the menstrual cycle
Term
Diseases from unregulated angiogenesis
Definition
- in tumors, where the new vessels not only provide nourishment for the growing tumor but also provide gateways of metastasis. Quantitation of angiogenesis in biopsy specimens of breast cancer has been used as an indication of future metastatic risk.
-In arthritis, new blood vessels invade the join and destroy the cartilage.
-In diabetes (diabetic retinopathy)
-Ocular Neovascularization, such as retrolental fibroplasia, a condition of vascular proliferation in the retinas of infants that have been exposed to high oxygen conditions and placed in atmosphere that contains a normal level of oxygen.
Term
Cardiac Output =
Definition
SV x HR
Term
MAP =
Definition
CO x TPR
Term
The 4 factors that affect Blood Pressure (mean arterial pressure)
Definition
1) Cardiac Output (Starling law, Sympathetic innervation, Hormones)
2) Total Peripheral Resistance: Arterioles (Sympathetic, Hormones, Local metabolites)
3)Capacity: Veins, Venules (Sympathetic, Hormones, Passsice elastic recoil when venous pressure drops)
4) Blood Volume (Hormones via kidneys, Sympathetic via kidneys, Fluid intake (thirst).
Term
Mean arterial pressure =
Definition
cardiac output x total peripheral resistance
Term
Capacity of the venous system
Definition
affected by venous constriction and dilation. If the capacity suddenly increases, as might happenwith venous dilation, then the blood pressure will drop
Term
Volume of fluid in the circulatory system
Definition
reduced by hemorrhage, increased by antiduretics, which reduce the volume of urine. Certain hormones hace an important effect the volume of fluid in the body, as we shall see. If the volume of fluid in the circulation increases there would be an increase in blood pressure
Term
Blood pressure: The sympathetic nervous system
Definition
The sympathetic nervous system deals with changes in blood pressure over a short (seconds, minutes)

The sympathetic nervous system exercises strong control over the cardiac output, the peripheral resistance and the capacity of the venous system. It also acts on the kidneys the affect the volume of blood in the circulation.
Term
Actions of Sympathetic nevous system raises the blood pressure, as follows
Definition
-the sympathetic nervous system increases cardiac output by increasing heart rate and the force of contraction
-The SNS raises the TPR by causing vasoconstriction of the arterioles
-The SNS system redes the capacity of the venous side of the circulation by constricting the veins. THis raises the venous pressure and increases the volume of blood returned to the heart. This causes an increase in the cardiac outpur, because the heart always pumps what it receives.
-The SNS also acts on the kidney, causing it to release into the circulation
Term
The renin-angiotensin-aldosterone system
Definition
The volume of fluid in the circulatory system comes under a very complex system of hormonal control that acts primarily in handling fluid and salt retention. the substances involved are the enzyme renin (released by the kidneys), and the hormones angiotensin II (formed by renin and angiotensin-converting enzyme) and aldosterone (released by adrenal cortex).
Term
Renin-angiotensin-aldosterone
Definition
has a particulary important role in long-term effects on the cardiovascular system, compared with other actions of the sympathetic nervous system, which deal primarily with short-term event.
Term
Actions of renin-angiotensin-aldosterone
Definition
The system operates through the kidney. It works like this. When there is a fall of blood pressure, the sympathetic nervous system is activated via the baroceptor reflex.
Term
Sympathetic stimulation of the kidney leads to:
Definition
- release of the enzyme renin from the kidneys
-angiotensin I is formed whenrenin acts on a protein called angiotensinogen, which is found in the blood
-conversion angiotensin I to angiotensin II by angiotensin-converting enzyme (ACE)
-angiotensin II-induced release of aldosterone from the adrenal glands

The effects are to cause vasoconstriction coupled with retention of water and salt (= reduction of urine volume
Term
Angiotensin II has two actions
Definition
1) It stimulates adrenal cortex to release aldosteron, which reduces volume of urine
2) It is a vasoconstrictor, and therefore raises the TPR. ACE inhibitors and angiotensin receptor blockes are in hypertensive therapies.
Term
Baroreceptor Reflex
Definition
An important part of the sympathetic control of blood pressure is the baroreceptor reflex. THe baroreceptors are specialized mechanoreceptors located in the walls of the aortic arch and in the carotid sinuses.
The baroreceptors are really stretch receptors. Censequently, most of the of the time, the baroreceptors are continually firing. A drop in aortic pressure decreases the stretch in the wall o fthe aortic arch and the carotids, and therefore the baroreceptors fire more slowly. A rise of aortic pressure stretches the walls of the arteries, and the baroreceptors increase their firing rate.
Term
If the mean arterial pressure drops below normal
Definition
the baroreceptors reduce their rate of firing. This causes decreased inhibition of the vasomotor center, increasing sympathetic vasomotor tone and causing constriction of the veins, muscular venules and arterioles. Additionally, there is inhibition of the vagal center that normarlly holds down the heart rate and an increase in sympathetic outflow to the heart.

Results:
-Increase of total peripheral resistance due to vasoconstriction of the arterioles.
-Increased cardiac output due to increased venous return caused by vasoconsctriction of mascular venules, increased heart rate and force of contraction
-Therefore, mean arterial pressure increases
Term
When the mean arterial pressure rises above normal
Definition
the baroreceptors increase their rate of firing. This inhibits the vasoconstrictor center in the medulla and diminishes the sympathetic vasomotor tone. Excitation of the vagal center sends inhibitory impulses to the heart. There is also inhibition of the sympathetic outflow to the heart.
Results:
Lowering of TPR due to reduction of sympathetic tone in the arterioles, cause them to dilate.
-Decreased cardiac output due to decreased venous return caused by vasodilation of veins and muscular venules, decreased heart rate and force of contraction
-Therefore, mean arterial pressure decreases.
Term
Orthostatic hypotension
Definition
Patients in which this reflex is impaired are liable to faint on standing suddenly, and are said to have orthostatic hypotension. This condition is seen in patients with degeneration of neurons in the autonomic nervous system, causing central autonomic failure with produces Shy-Drager syndrome.

Unfortunatley, the baroreceptor system adapts to changes in blood pressure over 1-3 day period, and therefore it is not important in controlling hypertension
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