Cardiovascular system: Blood Vessels

tunica externa

- Composed of dense irregular collagenous connective tissue that supports the blood vessel and prevents it from overstretching.

Hormones that control resistance

- Epinephrine and Norepinephrine: cause vasoconstriction and increase peripheral resistance, elevating blood pressure. - Angiotensin II: very powerful vasoconstrictor that sharply increases peripheral resistance and blood pressure. - ANP: is produced by the atria in response to increased blood volume. ANP causes vasodilation, especially of the vessels supplying the kidney. Causes a mild decrease in peripheral resistance and blood pressure

Capillaries

-Are generally found in clusters called capillary beds that wind their way through the cells of most tissues in the body - exceptions include cartilage, the sclera and cornea of the eye, and epithelial tissue. -extremely thin vessels, with walls that are only about 0.2 micrometers in thickness. Each capillary consists of an endothelium rolled into a tube and a small amount of basal lamina secreted by the endothelial cells -The walls of most capillaries consist of one to three endothelial cells joined by tight junctions that curl around the capillary's entire circumference. In such capillaries, blood cells must pass through in a single file.

systolic pressure

-Blood pressure in the arteries during contraction of the ventricles. -averages about 110-120

Veins

-Blood vessels that carry blood back or towards the heart -large lumen -most veins have a thin tunica media with few smooth muscle cells, and their diameter changes only slightly with vasodilation and vasoconstriction -tunica externa is thick

Vessel compliance

-ability of vessels to stretch -veins are the most compliant vessels and they stretch to accommodate added fluid when blood volume increases -when blood volume increases in arteries, overall blood pressure rises -when compliance decreases, even small increases in blood volume can raise blood pressure

essential hypertension

-also known as primary hypertension -no cause is identifiable -far more common than secondary hypertension, accounting for 95% of cases -current evidence indicates its development is based on multiple factors including genetics, ethnic heritage, advancing age, dietary factors such as excessive salt intake, excess sympathetic tone, and abnormal activity in the RAAS

arteriovenous anastomosis

-an artery empties directly into a vein without passing through a capillary bed. - Example: found in fetal circulation, where blood needs to be shunted between the arterial and venous systems to bypass certain organs.

decreased cardiac output

-another cause of hypotension -a decrease in either stroke volume or heart rate can result in hypotension. Are generally due to medications prescribed to treat hypotension. -A decrease in stroke volume is most commonly caused by heart failure, or the inability of the heart to function efficiently as a pump. -Severe heart failure can dramatically reduce cardiac output, and may result in a condition called cardiogenic shock.

excessive vasodilation

-another cause of hypotension -many conditions can excessive vasodilation, including anti-hypertension medications, acidosis, and ANS dysfunction. -Another potential cause of excessive vasodilation is the release of histamine into the blood during a severe allergic reaction, a condition called anaphylactic shock. -Similar excessive vasodilation occurs with certain bacterial infections of the blood; this is called septic shock.

effects of parasympathetic nervous system on blood pressure

-axons of the parasympathetic nervous system, via the vagus nerve, release acetylcholine primarily onto certain cardiac pacemaker cells (particularly the sinoatrial and atrioventricular nodes) and cardiac muscle cells -this slows the heart rate and has a mild effect on contractility, which decreases cardiac output and so blood pressure

effects of sympathetic nervous system on blood pressure

-axons release the neurotransmitters norepinephrine and epinephrine onto cardiac muscle cells and the smooth muscle cells of blood vessels. this produces two immediate changes: 1. an increase in heart rate and contractility, which increases cardiac output 2. vasoconstriction of all types of vessels but especially arterioles, which increases peripheral resistance -primarily blood vessels serving and draining the digestive and urinary organs and the skin are constricted -vessels serving the active skeletal muscles and heart actually dilate.

carotid sinus massage

-baroreceptor reflex may be initiated artificially in an emergency setting to reduce dangerously high blood pressure this is performed by massaging the skin over both carotid sinuses simultaneously.

elastic arteries

-conductive arteries -largest-diameter -have very extensive elastic lamina -conduct blood under high pressure to organs -are nearest to the heart and therefore are under the highest pressure of any vessel -need to be extremely distensible as they are greatly stretched with each ventricular systole -diameter does not change significantly with stimulation from vasomotor nerves

hypotension

-considered to be any abnormally low blood pressure. -Technically, hypotension is defined by a systolic pressure lower than 90 mm Hg and/or a diastolic pressure lower than 60 mm Hg, but is generally diagnosed as such only if the individual shows symptoms. -When symptoms do arise, they vary greatly with the severity of the hypotension.

treatment for hypotension

-depends largely on its severity and cause. -Mild hypotension may be managed simply by increasing fluid intake or changing a person's medication. -Severe hypotension and circulatory shock require aggressive management aimed at the underlying cause of the shock. A patient may be given fluids and/or blood to increase fluid volume, medications to raise cardiac output, ad systemic vasoconstrictors.

treatment of hypertension

-depends on which stage it is diagnosed -two areas of key treatment in all stages are: - Lifestyle modifications, including smoking cessation, weight loss if needed, limited alcohol intake, and increased physical activity -Dietary modifications, including decreased salt, cholesterol, and saturated fat intake.

muscular arteries

-distributing arteries -generally intermediate in diameter -contain a well-developed tunica media composed primarily of smooth muscle cells. -diameter of muscular arteries does change significantly with vasoconstriction and vasodilation. -allows the nervous and endocrine systems to adjust local blood flow to different organs by changing the vessel diameter. -regulate blood pressure -vessels most likely to be blocked

blood vessel radius

-dramatically affects resistance -resistance varies inversely with vessel's radius; as a radius increases (dilates) resistance to blood flow decreases, and vice versa

venous valves

-extensions of tunica intima that overlap and prevent blood from flowing backward in venous circuit -especially numerous in veins of legs, where blood flow toward the heart is strongly opposed by gravity

Veins

-function as the collection system of the vasculature. The drain blood from capillary beds and return it to the heart - They follow the opposite pattern of arteries-small veins merge with other veins to become progressively larger as they get closer to the heart. In the pulmonary circuit, veins transport oxygenated blood, whereas in the systemic circuit, they transport deoxygenated blood.

cardiac output

-heart rate x stroke volume -When cardiac output increases, blood pressure increases. When cardiac output decreases, blood pressure decreases.

Hypertension

-high blood pressure -associated with coronary artery disease, stroke, heart failure, certain types of dementia, kidney disease, and vascular disease

tunica intima

-innermost layer of a blood vessel -consists of endothelium, basal lamina, sub endothelial connective tissue, and internal elastic lamina -FUNCTION: allows for the exchange of gases and nutrients through capillary walls. Able to stretch when subjected to increased pressure and able to recoil back to original size.

sinusoidal capillaries

-leakiest capillaries. -Have a discontinuous sheet of endothelium, an irregular basal lamina, and very large pores in their endothelial cells. -Are typically three to four times larger in diameter than are other capillaries, and often have an irregular shape because their boundaries are determined by the organ in which they reside. - -Located in organs and tissues such as the liver, spleen, lymphoid organs, and bone marrow. -Size, shape, and sluggish blood flow allow them to facilitate the transfer of large substances such as blood cells and large proteins between the interstitial fluid and the blood.

central chemoreceptors

-located in the medulla of the brainstem - respond to the pH of the interstitial fluid of the brain. When the pH of this fluid decreases, another feedback loop is stimulated that indirectly increases the activity of sympathetic neurons, resulting in vasoconstriction and a rise in blood pressure.

peripheral chemoreceptors

-located near the baroreceptors in the aortic arch and the carotid artery. - primarily play a role in the regulation of breathing, but they also affect blood pressure. These receptors respond mostly to the level of oxygen in the blood. - A significant decrease in the blood oxygen concentration triggers a series of feedback loops that indirectly stimulate an increase in heart rate and cause vasoconstriction.

continous capillaries

-majority of capillaries in the body. -located in the muscles, skin, and most nervous and connective tissues. -Are the least "leaky"- they permit the fewest substances to enter or exit the blood by the paracellular route because their endothelial cells are joined together by tight junctions. Contain small gaps between endothelial cells through which some small substances diffuse, but the majority move into or out of the capillaries by diffusion through the endothelial cells or by transcytosis. -Capillaries in the brain that form the blood brain barrier are a modified type of continuous capillary, with specialized tight junctions that prohibit many materials from crossing into or out of the interstitial fluid around brain cells.

tunica media

-middle layer of blood vessel -two components: a layer of smooth muscle cells and elastic fibers called the external elastic lamina FUNCTION: control the diameter of the blood vessel and so the amount of blood that flows to organs. help regulate blood pressure and determine blood flow to organs and tissues.

reduced blood volume (hypovolemia)

-most common cause of hypotension. -Can result from blood loss; fluid losses from diarrhea, vomiting, or overuse of diuretics; or insufficient fluid intake. -Severe blood loss can lead to hypovolemic shock, which is fatal unless blood volume is restored.

venous anastomoses

-most common type of anastomosis. Neighboring veins are connected by small collaterals. Occluded veins rarely block blood flow. -Smaller veins are often so interconnected by collaterals that they form complex, weblike patterns that are sometimes visible beneath the skin, particularly in individuals with pale skin.

fenestrated capillaries

-much leakier than continuous capillaries because their endothelial cells contain fenestrations. These pores allow diffusion to take place much more quickly than it does in continuous capillaries. -Located in places where substances must rapidly enter or exit the blood such as endocrine glands, small intestine, and kidneys.

blood viscosity

-often described as the "thickness" but technical definition is the inherent resistance that all liquids have to flow. -blood has a relatively high viscosity due to the number of proteins and cells it contains. -blood viscosity remains relatively constant but can be altered by states that change either the number of cells or proteins in the blood or the amount of water in the blood -peripheral resistance is raised by conditions that increase blood viscosity and lowered by conditions that decrease blood viscosity

Diffusion through the membranes of endothelial cells

-one way of capillary exchange -lipid-soluble membranes such as oxygen, carbon dioxide, and certain lipids can generally enter and exit the capillary by diffusing across the membrane of one side of the endothelial cell and out the membrane on the other side. These substances then enter the interstitial fluid or the blood.

Diffusion and osmosis through gaps and fenestrations

-one way of capillary exchange -tight junctions between many endothelial cells are incomplete and leave small gaps between endothelial cells. -Some capillaries have small pores within their endothelial cells, called fenestrations. Water is able to move freely through these pores by osmosis, and small solutes such as monosaccharides and amino acids can move if a concentration gradient is present.

severe hypotension

-or circulatory shock -results in much more dramatic symptoms, including loss of consciousness and organ failure. --In this condition, there is insufficient blood pressure to deliver oxygen and nutrients to the cells, which can be rapidly fatal

Resistance

-other factor that determines blood flow -opposition to flow -inversely proportional to resistance: as resistance increases, blood flow decreases

blood pressure

-outward force that the blood exerts on the walls of the blood vessels -expressed in units millimeters of mercury (mm Hg) -varies dramatically in different parts of the vasculature: highest in the large systemic arteries and lowest in the large systemic veins

arterial anastomosis

-provide alternate pathways (collateral channels) to ensure continuous flow, even if one artery is blocked. -Common in joints, abdominal organs, brain, and heart; none in retina, kidneys, spleen.

myogenic mechanism

-relies on properties inherent in the vascular smooth muscle cells in the arterioles supplying capillary beds. - Increases in arteriolar pressure open stretch-sensitive channels in the arteriolar smooth muscle cells. This initiates a depolarization in the membranes of these cells so they can contract without nervous system stimulation. - For this reason, increases in arteriolar pressure lead to arteriolar vasoconstriction. The reverse also occurs- arteriolar smooth muscle relaxes when arteriolar pressure decreases. -Counters a change in blood flow by altering arteriolar resistance. The velocity of blood flow is related inversely to resistance, so if resistance increases, velocity decreases and vice versa. -The myogenic mechanism slows blood flow by increasing resistance when arteriolar pressure rises. Conversely, it speeds up blood flow by decreasing resistance when arteriolar pressure lowers.

metabolic control

-second type of autoregulatory mechanism is mediated by chemicals present in the interstitial fluid surrounding capillaries. These chemicals are the result of cellular metabolic (ATP-generating) activities. -The interstitial fluid of an actively metabolizing cell will contain a low concentration of oxygen and high concentrations of carbon dioxide and hydrogen ions. All three of these conditions cause the smooth muscle cells of local arterioles to relax, dilating the arterioles. This increases perfusion and ensures adequate oxygen and nutrient delivery to the actively metabolizing cells. -Also work in the opposite direction. Tissues whose cells are producing ATP slowly will have a high concentration of oxygen and low concentrations of carbon dioxide and hydrogen ions. These conditions cause constriction of local arterioles and a decrease in tissue perfusion.

vasa vasorum

-small vessels that supply oxygen and nutrients to outer part of the larger vessels

Arterioles

-smallest arteries -contain each of the three layers of blood vessel wall but the layers are extremely thin -both vasomotor nerves and hormones in the blood can affect the diameter of arterioles -vasoconstriction and vasodilation of these structures have a profound impact on the blood flow to individual tissues -arteriolar and/or precapillary sphincter constriction can completely cut off blood flow to tissues -feed capillary beds

Metarterioles

-smallest arterioles that directly feed capillary beds in most tissues -smooth muscle cells of metarterioles are confined mostly to a precapillary sphincter that encircles the metarteriole-capillary junction. -precapillary sphincters are mostly responsive to hormones and local tissue conditions

Venules

-smallest veins -drain blood from capillary beds -the three tunics become more distinct as venules merge to become larger venules and then veins

Arteries

-the distribution system of the vasculature. They travel away from the heart, branching into vessels of progressively smaller diameter. (carry blood away from the heart) -Arteries in pulmonary circuit carry deoxygenated blood, whereas those in the systemic circuit carry oxygenated blood.

Capillaries

-the exchange system of the vasculature. They are vessels of very small diameter that form branching networks called capillary beds. -Gases, nutrients, wastes, and other substances are quickly exchanged between cells and the blood through the capillary walls

blood vessel length

-the longer the vessel, the greater the resistance encountered - more pressure is needed to propel blood through a long vessel than a short one

peripheral resistance

-the opposition to flow that blood encounters in vessels away from the heart -as peripheral resistance increases, blood pressure increases

diastolic pressure

-the pressure in the arteries when the heart is at rest -averages about 70-80 mm Hg

what types of vessels does the microcirculation consist of:

-true capillaries, where materials are exchanged -a small central vessel

blood flow

-volume of blood flowing through vessel, organ, or entire circulation in given period -equivalent to cardiac output (CO) for entire vascular system -directly proportional to the pressure gradient, meaning that blood flow increases when the pressure gradient increases and vice versa

Hormones that control cardiac output

1. Epinephrine and Norepinephrine: produced by the adrenal medulla are identical to the effects of these chemicals produced by the sympathetic nervous system- they increase both heart rate and contractility. 2. Thyroid hormone: causes the cardiac muscle cells to produce more receptors for epinephrine and norepinephrine, which allows these two chemicals to have a greater impact on cardiac output.

what are the mechanisms that help return venous blood to the heart:

1. venous valves prevent backward flow in the veins 2. smooth muscle in the walls of veins that may contract under sympathetic nervous system stimulation to increase the rate of venous return 3. respiratory pump: pressure changes during breathing move blood toward the heart by squeezing abdominal veins as thoracic veins expand 4. muscular pump: contraction of skeletal muscle "milks" blood toward the heart; valves prevent backflow

postcapillary venules

>consist of little more than endothelium & some surrounding connective tissue. >structure enables them to exchange material with surrounding interstitial fluid

Arteries

Blood vessels that carry blood away from the heart

vasoconstriction

Reduces blood flow and heat transfer by decreasing the diameter of superficial blood vessels.

Stage 2 hypertension

Systolic over 160 Diastolic over 100

Prehypertension

Systolic: 120-139 Diastolic: 80-89

Stage 1 hypertension

Systolic: 140-159 Diastolic: 90-99

Mean Arterial Pressure (MAP)

The average pressure in the systemic arteries during an entire cardiac cycle

drug therapy for hypertension

aimed at modifying the three factors that influence blood pressure: cardiac output, blood volume, and peripheral resistance. Common targets for drug therapies include the RAAS, the tubules in the kidneys, calcium ion channels in vascular smooth muscle, and sympathetic receptors on the heart.

tissue perfusion

blood flow to a tissue through a capillary bed. Tightly regulated to ensure the metabolic needs of all tissues are always met

three main factors that influence blood pressure

cardiac output, peripheral resistance, blood volume

Pericytes

cells found around some capillaries; have contractile filaments and appear to control blood flow through capillary

pulse pressure

difference between systolic and diastolic pressure

Microcirculation

flow of blood through capillary bed

Angiogenesis

formation of new blood vessels

secondary hypertension

high blood pressure caused by the effects of another disease

vascular anastomoses

interconnections of blood vessels

Transcytosis

larger substances must cross the endothelial cells by transcytosis. During transcytosis, substances are taken into the cell by endocytosis and then leave the other side of the cell by exocytosis.

normal blood pressure

less than 120 mm Hg and diastolic less than 80 mm Hg

mild hypotension

may cause dizziness ad lightheadedness

capillary exchange

nutrients, gases, ions, and wastes must be able to cross the wall and travel between the blood in the capillary and the tissue cells.

vasomotor nerves

preganglionic sympathetic fibers that cause the constriction of blood vessels in response to signals from the cardiovascular center

baroreceptor reflex

produced by a group of mechanoreceptors that are found w/in walls of the heart. the reflex is activated when pressure rises w/in the large arteries above 60 mm Hg. peak in activity at approx 180 mm Hg. results in vasodilation secondary to inhibition of the vasomotor centers w/in the medulla as well as a decrease in heart rate and strength of contraction secondary to vagal stimulation

tunics

several tissue layers

lumen

space within a tubular part or organ, such as the space within a blood vessel

Autoregulation

the ability of tissues to regulate their own blood supply

blood volume

total volume of blood in the vessels, mainly controlled by the kidneys and hormones -directly linked to the amount of water in blood -as blood volume increases, blood pressure increases -as blood volume decreases, blood pressure decreases

vasodilation

widening of blood vessels