the cardiovascular system blood vessels and hemodynamics
TRANSCRIPT
The Cardiovascular System
Blood Vessels and Hemodynamics
Blood Vessels and Hemodynamics
• The vessels transport and distribute blood throughout the body to deliver materials and carry away wastes
• Hemodynamics- the forces involved in circulating blood throughout the body
• 5 main types of blood vessels– Arteries, arterioles, capillaries, venules and
veins
Generalized Structure of Blood Vessels
• Arteries and veins are composed of three tunics – tunica interna, tunica media, and tunica externa
• Lumen – central blood-containing space surrounded by tunics
• Capillaries are composed of endothelium with sparse basal lamina
Structure and Function of Blood Vessels
Arteries carry blood away from heart. Elastic arteries to muscular arteries to arterioles to the capillaries where the exchange of substances between the blood and organs occurs. Capillaries drain into venules to veins and veins carry blood back to heart.
Structure and Function of Blood Vessels
Arteries- branch, diverge or fork as they form smaller and smaller divisions
Veins- join, merge and converge as they form larger and larger vessels as they near the heart.
Arteries
• Wall has 3 tunics– Tunica Interna– Tunica Media– Tunica Externa
ArteryTunica Interna• Closest to the lumen (hollow
center through which blood flows)
• Lining of simple squamous epithelium called Endothelium
• Endothelium is a continuous layer of cells that line the inner surface of entire cardiovascular system, only tissue that normally makes contact with blood
• Basement membrane• Internal Elastic Lamina
ArteryTunica Media• Thickest layer
• Elastic fibers and smooth m. fibers that circle the lumen
• External Elastic Lamina
• Sympathetic neurons of ANS distributed to these smooth m. fibers
ArteryTunica Media• Increased sympathetic stimulation,
muscles contracts squeezing vessel wall, decreasing the diameter of lumen-Vasoconstriction
• Decreased sympathetic stimulation, smooth m. relaxes, increased diameter of lumen- Vasodilation
• Smooth m. relaxes in presence of NO, H+ and lactic acid
• Smooth m. contracts in vasospasm when wall is damaged
Artery
Tunica Externa• Mainly elastic and
collagen fibers
Arteries
• High Compliance due to high elastic fiber content, walls stretch easily and expand without tearing in response to increases in pressure
Elastic Arteries
• Conducting arteries, conduct blood from heart to muscular arteries
• Largest diameter, greater than 1cm
• Walls are relatively thin in proportion to overall diameter
• Tunica media has high proportion of elastic fibers
• Internal Elastic Lamina is incomplete and external elastic lamina is thin
• Function to help propel blood onward while ventricles are relaxing so have smooth, continuous flow of blood
• Aorta, brachiocephalic, common carotid, subclavian, vertebral, pulmonary and common iliac arteries
Elastic Arteries
• Blood ejected into elastic arteries whose walls stretch to accommodate the surge of blood. The elastic fibers stretch storing energy or acting as a pressure reservoir. When the elastic fibers recoil they convert the potential energy into kinetic energy causing blood to flow. Thus blood continues to flow through the arteries even while the ventricles are relaxed.
Muscular Arteries
• Medium in size, diameter 01.-10mm• Tunica media has more smooth m. and fewer
elastic fibers, thin internal elastic lamina, prominent external elastic lamina
• Walls are relatively thick• Capable of greater vasoconstriction and
vasodilation, adjusting rate of blood flow• Distributing arteries, distribute blood to various
parts of the body
Arterioles• Small, diameter 10-100µm• Bring blood to capillaries• Near arteries
– Tunica interna, tunica media of smooth m. and few elastic fibers and tunica externa of mostly elastic and collagen fibers
• Near capillaries– Ring of endothelial cell surrounded by few scattered smooth m.
fibers• Regulate blood flow from arteries to capillaries by regulating
resistance– Resistance is opposition to blood flow– Vessel diameter smaller, increased friction, increased resistance– Increased vasoconstriction of arterioles increases BP– Vasodilation of arterioles decreases BP
Capillaries• Microscopic vessels, diameter 4 to 10µm• Wall is a single layer of endothelial cells and a
basment membrane, lack tunica media and externa• Connect arterioles to venules-microcirculation• Exchange vessels, function in the exchange of
nutrients and wastes between blood and tissue cells through interstitial fluid
• Form extensive branching networks that increase surface area for rapid exchange of materials
• Absent in few tissues: covering and lining epithelia, cornea and lens of eye and cartilage
Capillaries• Metarteriole arises from
an arteriole to supply a network of 10-100 capillaries in a capillary bed
• Its proximal end is surrounded by scattered smooth m fibers and distal end empties into venule. Thoroughfare channels has no smooth m and blood flow here bypasses capillary bed
Capillaries• Precapillary Sphincters at
junction of metarteriole and capillaries control flow of blood through capillary bed
• Vasomotion-normal intermittent flow due to alternating contractions and relaxations
• At any given time, blood flows through only 25% of capillary bed
Capillary Beds
Figure 19.4a
Blood Flow Through Capillary Beds
• Precapillary sphincter– Cuff of smooth muscle that surrounds each true
capillary– Regulates blood flow into the capillary
• Blood flow is regulated by vasomotor nerves and local chemical conditions
Capillary Beds
Figure 19.4b
Arteries, Capillaries, and Venule
Types of Capillaries
• Continuous capillaries
• Fenestrated capillaries
• sinusoids
Continuous Capillaries
• Continuous capillaries are abundant in the skin and muscles– Endothelial cells provide an uninterrupted
lining– Adjacent cells are connected with tight
junctions– Intercellular clefts allow the passage of fluids
Continuous Capillaries
Figure 19.3a
Fenestrated Capillaries
• Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys)
• Characterized by: – An endothelium riddled with pores
(fenestrations)– Greater permeability than other capillaries
Fenestrated Capillaries
Figure 19.3b
Figure 19.3b
Fenestrated Capillaries
• Plasma membranes have many small pores, fenestrations
• Kidney, villi of small intestine, plexuses of ventricles of brain and some endocrine glands
Sinusoids
• Highly modified, leaky, fenestrated capillaries with large lumens
• Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs
• Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues
• Blood flows sluggishly, allowing for modification in various ways
Figure 19.3c
Sinusoids
• Wider and more winding
• Unusually large fenestrations
• Incomplete or absent BM
• Large intercellular clefts
• Liver, spleen, red bone marrow, endocrine glands
Sinusoids
Figure 19.3c
Portal System
• Blood passes from one capillary network to another capillary network through a vein called a portal vein
Venules
• Small veins, diameter 10-100µm• Smaller venules, closer to capillaries have
tunica interna of endothelium and tunica media that has only a few scattered smooth m. fibers. These have porous walls and WBC emigrate here
• Larger venules have tunica externa characteristic of veins
Veins• Diameter 0.1-1mm• Same tunics but thickness
different than arteries• Tunica interna thinner• Tunica media thinner,
little smooth m. and elastic fibers
• Lack External and Internal Elastic Lamina
• Larger lumen• Not designed to withstand
high pressure
Figure 19.1a
Veins
• Many veins have valves, thin folds of the tunica interna that form flaplike cusps that prevent the backflow of blood
• A vascular or venous sinus is a vein with a thin endothelial wall that has no smooth m. to alter its diameter
Venous Valves
Anastomoses
• Most tissues receive blood from more than one artery
• Union of branches of 2 or more arteries supplying the same region of the body
• Provide alternate routes for blood to reach a tissue or organ, collateral circulation
• Arteries that do not anastomose called End Arteries
Blood Distribution
• Veins and venules are the blood reservoirs from which blood can be diverted quickly if need arises
• Venoconstriction, constriction of veins reduces the volume of blood in reservoirs
Capillary Exchange
• Movement of substances between the blood and interstitial fluid
• Substances enter and leave capillaries by– Diffusion– Transcytosis– Bulk Flow
Diffusion
• Simple diffusion based on the concentration gradients between blood and tissues
• Water soluble substances (glucose, amino acids) diffuse through intercellular clefts and fenestrations
• Lipid-soluble (O2, CO2 and steroid hormones) diffuse through plasma membrane of endothelial cells
Transcytosis
• Substances in blood plasma that enter cell via endocytosis, move across the cell and exit on the other side of cell by exocytosis
• Large, lipid-insoluble molecules
• Insulin enters blood stream via transcytosis
Bulk Flow
• Passive process in which large numbers of ions, molecules or other particles in fluid move together in the same direction
• Occurs from an area of higher pressure to an area of lower pressure and continues as long as a pressure difference exists
• Moves substances faster than by diffusion• Important for regulation of the relative volumes of
blood and interstitial fluid
Bulk Flow
• Filtration– The pressure-driven movement of fluid and solutes
from blood capillaries into interstitial fluid
• Reabsorption– Pressure-driven movement from interstitial fluid into
blood capillaries
The volume of fluids and solutes reabsorbed normally is almost as large as the volume filtered, near equilibrium-Starlings Law of Capillaries
Filtration
• Two pressure that promote– Blood Hydrostatic Pressure BHP
• Pressure generated by pumping of heart• Due to the pressure of water in blood plasma against blood
vessel walls• Pushes fluid out of capillaries• 35mmHg at arterial end of capillary • 16mmHg at venous end of capillary
– Interstitial Fluid Osmotic Pressure IFOP• Pulls fluid out of capillaries into interstitial fluid• 1mmHg
Reabsorption
• Promoted by– Blood Colloid Osmotic Pressure BCOP
• Caused by colloidal suspension of large proteins in the plasma
• Pulls fluid from interstitial space into capillaries
• 26mmHg
Also Interstitial Fluid Hydrostatic Pressure IFHP pushes fluid back into capillaries. 0mmHg
Net Filtration Pressure• The balance of these pressure called Net Filtration
Pressure NFP determines whether the volume of blood and interstitial fluid remains steady or changes, determines if fluid leaves capillaries
• NFP=(BHP + IFOP) – (BCOP +IFHP)
• = pressure that promote filtration-pressures that promote reabsorption
• Arterial end of capillary= (35+1)-(26+0)= 10mmHG– Net outward pressure, fluid out of capillary
• Venous end of capillary= (16+1) – (26 +0)= -9mmHg– Net inward pressure, fluid moves into capillary
Capillary Exchange
• 85% of fluid filtered out is reabsorbed
• Excess fluid and few plasma proteins that do escape blood enter lymphatic capillaries and eventually return to blood
Hemodynamics
• Blood Flow- the volume of blood that flows through any tissue in a given time period
• Total Blood Flow is Cardiac Output CO• CO= heart rate x stroke volume• How the CO is distributed into circulatory routes
that serve various body tissues depends on– The pressure difference that drives blood flow through
a tissue– The resistance to blood flow in specific blood vessels
Blood Flow
• Blood flows from regions of higher pressure to regions of lower pressure
• Greater the difference in pressure the greater the flow
• The higher the resistance the smaller the flow
• Contraction of ventricles generates blood pressure BP
Blood Pressure• The hydrostatic pressure exerted on walls of a blood vessel• Systolic BP
– Highest pressure in arteries during systole– 110mmHg
• Diastolic BP– Lowest arterial pressure during diastole– 70mmHg
• Pressure falls progressively as distance from the left ventricle increases– 35mmHg at arterioles end of capillaries, 16mmHg at venous end
of capillaries and 0mmHg as blood flows into right ventricle
Blood Pressure
• Mean Arterial Pressure (MAP)– The average BP in arteries– Roughly 1/3 way between diastolic and systolic
BP– MAP= diastolic BP + 1/3 (systolic BP-
Diastolic BP)= 83mmHg = 70 + 1/3 (110-70)
MAP
MAP = CO X resistance
If CO increases due to increased HR or SV than MAP also rises as long as resistance stays the same.
If CO decreases than MAP also decreases as long resistance remains steady.
Blood Pressure
• Blood pressure also depends on total volume of blood in cardiovascular system– Decrease in blood volume of greater than 10%
and BP drops– Any increase in blood volume (water retention)
increases BP
Resistance
• Vascular Resistance is the opposition to blood flow due to friction between blood and walls of blood vessels.
• Resistance depends on– Size of blood vessel lumen– Blood viscosity– Total blood vessel length
Resistance
• Size of Lumen– Smaller the lumen (smaller the diameter of
blood vessel), greater the resistance to blood flow
– As arterioles dilate, resistance decreases and BP falls
– As arterioles constrict, resistance increases and BP rises
Resistance
• Blood Viscosity– Thickness of blood depends mostly on the ratio of RBC
to plasma volume (to a smaller extent on concentration of proteins in plasma)
– Higher viscosity, higher the resistance
– Dehydration and polycythemia (increased RBC) increase BP
– Decreased plasma proteins or RBC (anemia, homorrhage) decreases viscosity and decreases BP
Resistance
• Total Blood Vessel Length– Longer a vessel, the greater the resistance
Systemic Vascular Resistance
• SVR = TPR• Systemic Vascular Resistance = Total
Peripheral Resistance• All the vascular resistances offered by
systemic blood vessels• Major function of the arterioles is to control
SVR, BP and Blood Flow to particular tissues by changing their diameter
Venous Return
• Volume of blood flowing back to heart through systemic veins, occurs due to pressure generated by contractions of left ventricle
• If pressure increases in r. atrium or r. ventricle , venous return decreases (leaky tricuspid)
• 2 other mechanisms pump blood from lower body back to heart– Skeletal muscle pump– Respiratory pump
Skeletal Muscle Pump
• 2 other mechanisms– Skeletal muscle pump – milks blood in 1
direction due to valves
– Respiratory pump – due to pressure changes in thoracic and abdominal cavities
Proximalvalve
Distalvalve
1
Proximalvalve
Distalvalve
1 2
Proximalvalve
Distalvalve
1 2 3
Venous Return
• Respiratory Pump– During inhalation the diaphragm moves downward
causing decrease in pressure in the thoracic cavity and increase in pressure in abdominal cavity. Abdominal veins are compressed and a greater volume of blood moves into the thoracic veins and into r. atrium
– When pressure reverses during exhalation, valves prevent backflow from thoracic to abdominal veins
Velocity of Blood Flow
• Velocity of blood flow decreases as blood flows from aorta to arteries to arterioles to capillaries and increases as it leaves and returns to the heart
• Slow flow of blood in capillaries aides capillary exchange
Circulation Time
• Time for drop of blood to travel from right atrium through pulmonary circulation to left atrium to systemic circulation, down to foot and back to right atrium.
• At rest about 1 minute
Control of Blood Pressure and Blood Flow
• Several interconnected negative feedback systems control BP by adjusting HR, SV, Systemic Vascular Resistance and Blood Volume
Cardiovascular Center
• Cardiovascular Center (CV center) in the Medulla Oblongata helps regulate HR and SV and it controls neural, hormonal and local negative feedback systems that regulate BP and flow to specific tissues
• CV Center receives input from higher brain regions (cerebral cortex, limbic system and hypothalamus) and from sensory receptors (proprioceptors, baroreceptors and chemoreceptors)
• Output from CV center flows along neurons of ANS and to smooth m. in blood vessel walls along vasomotor neurons to arterioles
– Sympathetic impulses via cardiac accelerator nerves and increase HR and contractility
– Parasympathetic impulses conveyed along Vagus (X) Nerve to decrease HR
Neural Regulation of BP
• Two reflexes– Baroreceptor Reflexes– Chemoreceptor Reflexes
Neural Regulation of BP• Baroreceptor Reflex
– Barorecptors are pressure sensitive sensory receptors that monitor changes in pressure and stretch in the walls of blood vessels
– Located in aorta, internal carotid a. and other large arteries in neck and chest
– In walls of carotid sinuses initiate Carotid Sinus Reflex that regulate BP in Brain (carotid sinus is small widening of R and L internal carotid arteries). Bp stretches wall of carotid sinus which stimulates the receptors, nerve impulses along glossopharyngeal IX nerve to CV center in Medulla Oblongata
– Baroreceptors in wall of ascending Aorta and Arch of Aorta initiate Aortic Reflex which regulates systemic BP. Impulses travel to CV via Vagus X Nerve
Neural Regulation of BP
• With increasing BP, impulses traveling at a faster rate to CV center which will increase parasympathetic stimulation and decrease sympathetic stimulation which results in decreased HR and decreased force of contraction which decreases CO. Also vasodilation will also help lower BP
Neural Regulation of BP
• Chemoreceptors– Sensory receptors that monitor chemical composition
of blood(levels of O2, H+ and CO2)
– Located close to baroreceptors in structures called carotid bodies and aortic bodies
– Hypoxia, Hypercapnia and Acidosis stimulate receptors who send impulses to CV center resulting in increases sympathetic stimulation to arterioles and veins producing vasoconstriction and increased BP
Hormonal Regulation of BP
• 1. Renin-Angiotensin-Aldosterone (RAA) System
• 2. Epinephrine and Norepinephrine
• 3.Anitdiuretic Hormone ADH (vasopressin)
• 4. Atrial natriuretic peptide ANP
Hormonal Regulation of BP
• 1. Renin-Angiotensin-Aldosterone (RAA) System– Decreased blood volume or decreased blood flow to
kidneys, kidney cells secrete renin, resulting angiotensin II raises BP in two ways
• Acts as vasocontrictor raising BP by increasing systemic vascular resistance
• Stimulates secretion of aldosterone which increases reabsorption of Na+ and water by kidneys which increases total blood volume
Hormonal Regulation of BP
• 2. Epinephrine and Norepinephrine– Released by Adrenal Medulla in response by
sympathetic stimulation– Increase CO by increasing rate and force of heart
contractions– Also act as vasoconstrictors of arterioles and veins in
skin, abdominal organs and vasodilation of arterioles in cardiac and skeletal m. which help increase blood flow to muscle during exercise
Hormonal Regulation of BP
• 3. Antidiuretic Hormone ADH (vasopressin)– Produced by hypothalamus and released by
posterior pituitary in response to dehydration or decreased blood volume
– Also causes vasoconstriction which increase BP
Hormonal Regulation of BP
• 3. Atrial Natriuretic Peptide ANP– Released by cells in atria of heart– Lowers BP by causing vasodilation by
promoting loss of salt and water in urine which decreases blood volume
Autoregulation of BP
• The ability of a tissue to automatically adjust its blood flow to match its metabolic demand
• In capillary bed, locale changes can regulate vasomotion
• Two types of changes can stimulate autoregulatory changes in blood flow– Physical changes
– Vasodilating and vasoconstricting chemicals
Autoregulation of BP• Physical Changes
– Warming-vasodilation
– Cooling-vasoconstriction
– Smooth m in walls of arteriole exhibit Mygoenic Response-contract more forcefully when stretched and relaxes when stretching lessens
• Vasodilating Chemicals released by metabolically active tissue cells– K+, H+, lactic acid, adenosine, NO, kinins, histamine
• Vasoconstricting Chemicals– Thromboxane A2, superoxide radicals, sertonin, endothelins
Autoregulation of BP
• There is a difference in the way the Pulmonary and Systemic Circulations respond to O2 levels, difference their autoregulation– Systemic dilate in response to low O2 so O2
delivery increases– Pulmonary constrict in response to low O2 so
that blood flow goes to better ventilated areas of lungs
Pulse• Alternate expansion and recoil of elastic arteries
after each systole of L. ventricle creates a traveling pressure waved
• May be felt in any artery that lies near the surface of body that can be compressed against a bone or other firm structure
• Normally same as HR, 70-80 beats/min• Tachycardia-rapid resting pulses/HR, over 100• Bracdycardia-slow resting HR/pulse, under 50
beats/minute
Measuring Blood Pressure
• BP-pressure in arteries generated by L.ventricle during systole and pressure remains in arteries during ventricle diastole
• Usually measured in brachial a. in L. arm via a sphygomomanometer
• Systolic BP is the 1st sound and corresponds to pressure in arterial wall after ventricle contraction
• Diastolic BP when there is no sound, force after ventricle relaxes
• Difference is the pulse pressure
Shock
• Failure of cardiovascular system to deliver enough O2 and nutrients to meet cellular metabolic needs
• Due to inadequate blood flow to body tissues• Cells switch to anaerobic production of ATP.
Lactic acid accumulates in body fluid. If shock persists, cells and organs become damaged and cells may die
4 Types of Shock
• 1. Hypovolemic Shock– Due to decreased blood volume which leads to
decreased venous return to the heart and thus decreased SV and decreased CO
– Internal or external blood loss, loss of body fluids or inadequate intake of fluids
4 Types of Shock
• 2.Obstructive Shock– Due to obstruction of blood flow, part of
circulation blocked– PE
• 3. Cardiogenic Shock– Due to poor heart function– MI, arrhythmias, ischemia of heart
4 Types of Shock
• 4. Vascular shock– Due to inappropriate vasodilation or arterioles and
venules• Anaphylatic shock
– Severe allergic rxn, release histamine that causes vasodilation
• Neurogenic shock– Vasodilation following head trauma that caused malfunction of
CV center
• Septic shock– Bacterial toxins produce vasodilation
Responses to Shock
• Negative feedback systems work to return CO and arterial BP to normal
• Compensatory mechanisms can maintain adequate blood flow and BP despite an acute blood loss of as much as 10% total volume
Responses to Shock• 1. Activation of Renin-angiotensin-aldosterone
system– Kidneys release renin and resulting angiotensin II
causes vasoconstriction and aldosterone causes increased reabsorption of sodium and water in kidneys
– Resulting increased vascular resistance and increased blood flow helps raise BP
• 2. ADH– Increased water reabsorption in kidneys– Vasoconstriction– Increase resistance and blood volume
Responses to Shock
• 3. Activation of Sympathetic Division of ANS– Marked vasoconstriction of arterioles and veins of skin,
kidneys and other abdominal viscera
– Increased resistance and increased venous return help maintain BP
– Sympathetics also cause increased HR and contractility which cause release of epi and norepi which intensify the vasoconstriction, HR and contractility
– All together help increased BP
Responses to Shock
• 4. Release of local vasodilators– In response to hypoxia cells release
vasodilators (K+, H+, lactic acid, adenosine, NO) that dilate arterioles and relax precapillary sphincters resulting in increased local blood flow which may restore oxygen level to normal in part of body
Responses to Shock
• If Blood Volume drops more than 10-20% or heart cannot bring blood pressure up sufficiently, compensatory mechanisms may fail to maintain adequate blood flow to tissue. Then shock becomes life threatening as damaged cells start to die.
Signs and Symptoms of Shock• Systolic BP lower than 90mmHG• Rapid HR• Weak pulse• Cool, pale, clammy skin• Altered mental state• Reduced urine formation• Patient thirsty• pH of blood low• nausea
Circulatory Routes
• Blood vessels organized into circulatory routes that carry blood to specific organs in the body.
• Routes are parallel so that each organ receives its own supply of freshly oxygenated blood
• 2 main routes– Systemic circulation
– Pulmonary circulation
Pulmonary Circulation• Carries deoxygenated blood from R. ventricle to air sacs in
lungs and returns oxygenated blood from airs sacs to l. atrium
• Pulmonary trunk emerges from r. ventricle and divides into right and left pulmonary arteries that go to right and left lungs. Only arteries that carry deoxygenated blood. These arteries have larger diameters, thinner walls and less elastic tissues than systemic arteries so there is low resistance to pulmonary flow and so need less BP to move blood through lungs
• Right and left pulmonary veins exit lungs and carry oxygenated blood to l. atrium
Systemic Circulation
• Includes arteries and arterioles that carry oxygenated blood from L.ventricle to systemic capillaries, plus veins and venules that return deoxygenated blood to r. atrium
• All systemic arteries branch from aorta• All veins of systemic circulation drain into
superior vena cava, inferior vena cava or coronary sinus which all empty into r. atrium
• Bronchial arteries that carry nutrients to lungs are also part of systemic circulation
Hepatic Portal Circulation• Carries venous blood from gastrointestinal organ and
spleen to the liver• Hepatic portal vein receives blood from capillaries of
gastrointestinal organs and the spleen and delivers it to sinusoids of liver
• Supermesenteric vein (drains small intestine, stomach, pancreas) and splenic vein (drains stomach, pancreas and large intestine) unite to form the hepatic portal vein
• Right and left gastric veins from stomach empty into hepatic portal vein and cystic vein from gallbladder drains into hepatic portal vein
• Liver gets oxygenated blood from hepatic artery
Fetal Circulation
• Fetus obtains oxygen and nutrients from and eliminates wastes into maternal blood
• Exchange of materials between fetal and maternal circulations occurs through placenta
• Normally no direct mixing of maternal and fetal blood because all exchange occur through diffusion through capillary walls