Ch. 19 Cardiovascular System: Blood Vessels A&P II
Overview of cardiovascular physiology
- *The purpose of CV regulation is the maintenance of adequate blood flow through the capillaries in peripheral tissues and organs.* - Under normal circumstances, *blood flow is equal to cardiac output* - regulate blood flow in same way we regulate CO
Blood flow through capillaries
- *Vasomotion* 1. *Slow and intermittent flow* 2. Reflects the on/off opening and closing of *precapillary sphincters*
Elastic Arteries
- *conducting arteries* Large lumens - conduits Contain a lot of elastin in tunica media Examples: Aorta Carotid Pulmonary artery - need to expand with force of blood
Muscular Arteries
- *distributing arteries* Deliver blood to specific body systems Less elastin in tunica media Examples: Mesenteric arteries Renal arteries Hepatic arteries - main vessels that take blood to specific areas
Circulatory shock
- Condition in which cells of the body receive inadequate amounts of oxygen secondary to changes in perfusion. (not enough blood getting to them) - Systolic blood pressure < 90 mmHg - Types of Shock 1. Hypovolemic 2. Cardiogenic 3. Vascular
If blood volume increases, which hormone increases?
ANP
Types of Capillaries: Continuous
Abundant in skin, muscles, lungs, and CNS Often have associated pericytes *Pinocytic vesicles ferry fluid across the endothelial cell* Brain capillary endothelial cells lack intercellular clefts (breaks in between cells) and have tight junctions around their entire perimeter: Blood brain barrier - only allows movement of liquid
arterioles and regulation of blood flow
Alterations in vascular diameter directly control the volume of tissue blood flow. - *arteriolar tone* (amount of constriction at arteriole) is regulated by numerous factors
Capillaries
Are smallest vessels with thin walls Microscopic capillary networks permeate all active tissues Extensive branching network dramatically increases the surface area. 3 types: *continuous, fenestrated, sinusoid*
Veins
Collect blood from venules and lead way back to heart. Very *little smooth muscle*. *Larger lumen* than arteries *Capacitance vessels* (hold blood) Very *low, non-pulsatile pressures* - not much ability to generate blood flow
Venous Valves
Prevent backflow of blood in veins Prevent "pooling" of blood Most common in extremities - goal is to *maintain one way flow of blood* - similar to SL valve but not as much pressure
Start here for Quiz 6
Start here for Quiz 6
Stop for quiz #6
Stop for quiz #6
Types of Capillaries: Fenestrated
Occur in areas of active filtration or absorption and areas of hormone secretion Fenestrations are Swiss cheese-like holes that tunnel through endothelial cells Fenestrations usually covered by thin diaphragm made of extracellular glycoproteins Number of fenestrations increases during active absorption of nutrients in some digestive organs - glands, kidneys, gut - *fluid and ions move through*
Types of Capillaries: Sinusoid
Occur in liver, bone marrow, spleen, and adrenal medulla Have large intercellular clefts as well as fenestrations; few tight junctions Incomplete basement membranes Irregularly shaped and have larger lumens Allow *large molecules and cells to pass across their walls*. Blood flows slowly through their tortuous channels Macrophages may extend processes through the clefts to catch "prey" or, in liver, form part of the sinusoid wall - move cells in/out of blood stream
Components of vasculature
*Arteries* carry blood away from the heart *Arterioles* are the smallest branches of arteries; resistance vessels *Capillaries* are the smallest blood vessels; exchange vessels *Venules* collect blood from the capillaries *Veins* return blood to the heart - arteries branch, veins converge - arterioles control how much blood goes thru
Capillary Beds
*Capillary bed*: interwoven network of capillaries between arterioles and venules *Microcirculation*: flow of blood through bed - Capillary beds consist of two types of vessels *Vascular shunt*: channel that connects arteriole directly with venule (metarteriole-thoroughfare channel) *True capillaries*: actual vessels involved in exchange
Short-Term Blood Pressure Control - Hormones
*Norepinephrine and epinephrine* are secreted from the adrenal medulla and promote widespread vasoconstriction. *Angiotensin II* - functions in short-term blood pressure control by promoting vasoconstriction; also functions in long-term blood pressure control. *Antidiuretic hormone (ADH)* - causes intense vasoconstriction in cases of extremely low BP. *Endothelium-derived factors* - endothelin and prostaglandin-derived growth factor are both vasoconstrictors. (from endothelium that lines blood vessels) *Atrial and brain natriuretic peptides (ANP; BNP)* - functions in short-term blood pressure control by promoting vasodilation; also functions in long-term blood pressure control. (constriction=increase BP, dilation= decrease BP)
Blood pressure homeostatic regulation
*Short-term neural and hormonal control* Counteract fluctuation in blood pressure by altering peripheral resistance and CO directly *Long-term renal regulation* Counteract fluctuations in blood pressure by altering blood volume contributes indirectly to alterations in peripheral resistance and CO
Capillary Beds: sphincters open
*True capillaries*: 10 to 100 exchange vessels per capillary bed - Branch off metarteriole or terminal arteriole - True capillaries normally branch from metarteriole and return to thoroughfare channel *Precapillary sphincters* regulate blood flow into true capillaries - Blood may go into true capillaries or to shunt - Regulated by local chemical conditions (gas need of organ) and vasomotor nerves (innervated/control by NS)
Capillary Beds: Sphincters closed
*Vascular shunt*: metarteriole-thoroughfare channel starts with: *Terminal arteriole* that feeds into *Metarteriole* (intermediate between arteriole and capillary) that is continuous with *Thoroughfare channel* (intermediate between capillary and venule) that feeds into *Postcapillary venule* that drains bed
Monitoring Circulatory Efficiency
*Vital signs*: pulse and blood pressure, along with respiratory rate and body temperature *Pulse*: pressure wave caused by the expansion and recoil of arteries Radial pulse (taken at the wrist) routinely used
baroreceptor reflexes involved in blood pressure homeostasis
*know picture* *Low blood pressure*: ↓ Baroreceptor firing ↑ Sympathetic nerve activity ↑ Contractility (SV), HR, CO Vasoconstriction (those two together raise BP) *High blood pressure*: ↑ Baroreceptor firing ↓ Sympathetic nerve activity ↓ Contractility (SV), HR, CO Vasodilation (those two together lower BP)
Body Sites where pulse is easily palpated
*superficial temporal artery *common carotid artery
Hormonal control of blood flow
- *Angiotensin II* triggers vasoconstriction, resulting in reduced blood flow - High levels of *antidiuretic hormone* triggers vasoconstriction, resulting in reduced blood flow. - *Epinephrine* can trigger either vasoconstriction (in most cases) or vasodilation (in vessels serving the liver and skeletal muscles) --> due to binding at different receptors
Heart hormones lower blood pressure
- *Atrial Natriuretic Peptide (ANP)* is secreted by atrial muscle cells. - *Brain Natriuretic Peptide (BNP)* is secreted by ventricular muscle cells. - Increase Na+ excretion by kidneys (lose water) - Increase volume of urine produced - Reduce thirst - Block release of ADH, aldosterone (lose Na+), epinephrine, and norepinephrine (allows for vasodilation and & decr. HR) - Stimulate vasodilation (decrease BP) *see print out*
Fluid movements: bulk flow
- *Fluid leaves capillaries at arterial end; most returns to blood at venous end* - Extremely important in determining relative fluid volumes in blood and interstitial space - *Direction and amount of fluid flow* depend on the balance of two opposing forces: *hydrostatic and colloid osmotic pressures* - *water always flows from low to high concentration of solutes*
Abnormal Arterial blood pressure
- *Hypertension*: abnormally high blood pressure Often defined as a blood pressure greater than 140/90 1. Significantly increases the workload on the heart 2. Results in LV enlargement 3. Excess stress on blood vessel walls can promote atherosclerosis - *Hypotension*: abnormally low blood pressure
Local myogenic control
- *Myogenic* responses keep tissue perfusion constant despite most fluctuations in systemic pressure. - *Vascular smooth muscle DIRECT responses to stretch* 1. Passive stretch (increased intravascular pressure) promotes increased tone and vasoconstriction 2. Reduced stretch promotes vasodilation and increases blood flow to the tissue - incr. blood flow = incr. stretch = constrict (vice versa)
Pulmonary circulation
- *Pulmonary circuit unusual* -- Pathway short -- Arteries/arterioles more like veins/venules (thin walled, with large lumens) -- Arterial resistance and pressure are low (24/10 mm Hg) - *Autoregulatory mechanism* opposite that in most tissues -- Low O2 levels cause vasoconstriction; high levels promote vasodilation --- Allows blood flow to O2-rich areas of lung
Local metabolic control of blood flow involving metabolism
- Active tissues produce a variety of *metabolites* that exert a paracrine effect on local arterioles. - A significant increase in blood flow in response to an increase in tissue metabolism is called *active hyperemia* - incr. metabolism (demand) = incr. supply
Hypovolemic shock
- Acute sudden hemorrhage (low blood volume)
The kidneys directly raise blood pressure
- Alters blood volume independently of hormones - Increased BP or blood volume causes the kidneys to eliminate more urine, thus reducing BP - Decreased BP or blood volume causes the kidneys to conserve water, and BP rises - low atrial pressure -> decreased filtration by kidneys--> decreased urine formation -> increased blood volume -> increased mean arterial pressure
Blood flow is all about oxygen supply/demand
- Arterial pressure "myogenic", humoral factors, neural control, endothelial factors, and local metabolic factors = affect *vascular resistance* - vascular resistance affects *blood flow* - blood flow & O2 carrying capacity (# of RBCs) affects *supply* - organ activity & oxidative metabolism (activity) affect *demand* - increase vascular resistance, decrease blood flow - vascular resistance is vasodilation (decr. resistance) and vasoconstriction (incr. resistance)
Reactive hyperemia
- Assess tissue response to a brief episode of *ischemia by occluding artery.* - *Dilator metabolites accumulate in ischemic tissue* - result in marked increase in flow when occlusion is released. - *no flow* in the drop down part, *excess flow* in the hill part, *peak flow* at the top of the hill - *reactive hyperemia* = react to inadequate blood flow
Baroreceptor reflexes
- Baroreceptor: Pressure receptors in the aorta and carotid sinuses - If blood pressure goes up, baroreceptors are excited 1. Inhibit vasomotor center (sympathetic NS) -> vasodilation of blood vessels (decrease BP) 2. Inhibit cardioacceleratory center (sympathetic NS) -> reduce heart rate and contractile force (decrease HR and contractile force) 3. Stimulate cardioinhibitory center (parasympathetic NS) -> reduce heart rate and contractile force (decrease HR and contractile force)
long term blood pressure control
- Baroreceptors quickly adapt to chronic high or low BP (change set point) - *Short-term mechanisms* function primarily by altering cardiac output and peripheral resistance - increasing blood flow and decreasing blood vessel diameter. - *Long-term mechanisms* step in to control BP by altering blood volume - Kidneys act both directly and indirectly to raise arterial blood pressure - Hormones secreted by the heart lower arterial blood pressure. - know short-term vs. long term
Blood flow through the body tissues
- Blood flow (tissue perfusion) is involved in - Delivery of O2 and nutrients to, and removal of wastes from, tissue cells - Gas exchange (lungs) - Absorption of nutrients (digestive tract) - Urine formation (kidneys) - Rate of flow is precisely the right amount to provide for proper function - shunting of blood to skeletal muscles during exercise
Brain circulation
- Blood flow to brain constant as neurons intolerant of ischemia; averages 750 ml/min *Metabolic controls* --Decreased pH of increased carbon dioxide cause marked vasodilation *Myogenic controls* -- Decreased MAP causes cerebral vessels to dilate -- Increased MAP causes cerebral vessels to constrict
Dermal (skin) circulation continued
- Blood flow to venous plexuses below skin surface regulates body temperature -- Varies from 50 ml/min to 2500 ml/min, depending on body temperature -- Controlled by sympathetic nervous system reflexes initiated by temperature receptors and central nervous system
Hypertension (two types)
- Blood pressure in excess of normal range for age and gender (> 140/90 mmHg) - Afflicts about 20% of adults - *Most common type is primary or essential* caused by *complex/poorly understood* processes - *Secondary hypertension is caused by known disease processes*
Developmental aspects continued
- Blood vessels are trouble-free during youth - Vessel formation occurs: -- As needed to support body growth -- For wound healing -- To rebuild vessels lost during menstrual cycles - With aging, varicose veins, atherosclerosis, and increased blood pressure may arise
Net filtration pressure (what does it determine? two forces?)
- Bulk fluid flow across capillary walls causes continuous mixing of fluid between the plasma and the interstitial fluid compartments, and maintains the interstitial environment. - *Net filtration pressure (NFP) determines the direction of fluid movement.* - - Two kinds of pressure drive fluid flow: 1. hydrostatic pressure - Due to fluid pressing against a boundary - HP "pushes" fluid across the boundary - In blood vessels, is due to blood pressure 2. osmotic pressure - Due to nondiffusible solutes that cannot cross the boundary - OP "pulls" fluid across the boundary - In blood vessels, is due to plasma proteins
Essential Hypertension
- CO and HR are elevated - Chronically elevated secretion of renin, angiotensin II, and aldosterone - Risk Factors: Heredity Diet high in salt, saturated fat, cholesterol Diet lacking K+, Ca2+, Mg2+ Obesity Age Diabetes mellitus Chronic stress Smoking - Prolonged high BP causes thickening of arterial walls, resulting in atherosclerosis
Capillary pressures
- Capillary ranges from 15 to 35 mm Hg - *Low* capillary pressure is desirable because high BP would rupture fragile, thin-walled capillaries - Most are *very permeable*, so low pressure is *sufficient to force filtrate into interstitial spaces*
Velocity of blood flow
- Changes as it travels through the systemic circulation - Is *inversely related to the total cross-sectional area* - Is *fastest in the aorta, slowest in the capillaries, increases again in veins* - Slow capillary flow allows adequate time for *exchange* between blood and tissues - aorta is larger but total cross section area is small because there is just one vs. capillary bed cross sectional area is 2000x that of aorta which *decreases blood velocity in capillary bed* - think about putting finger over end of hose
Chemoreceptor reflex
- Chemoreceptors are located in the Carotid sinus Aortic arch Large arteries of the neck - Chemoreceptors respond to rise in CO2, drop in pH or O2 - Increase blood pressure via the vasomotor center and the cardioacceleratory center - More important in the regulation of respiration - constriction = incr BP & incr CO *see drawing*
Age-Related Changes in Cardiovascular System
- Decreased hematocrit - Constriction/blockage of veins by blood clots (thrombi) - Blood pools in legs due to venous valve deterioration - Reduced maximum cardiac output - Changes in nodal and conducting cells - Reduced elasticity of fibrous skeleton - Progressive atherosclerosis - Replacement of damaged cardiac muscle cells by scar tissue
Capillary exchange of respiratory gases and nutrients
- Diffusion of 1. O2 and nutrients from the blood to tissues 2. CO2 and metabolic wastes from tissues to the blood - *Lipid-soluble* molecules diffuse directly through endothelial membranes - *Water-soluble* solutes pass through clefts and fenestrations - *Larger molecules*, such as proteins, are *actively transported* in pinocytotic vesicles.
Respiratory pump for veins
- During inhalation intrathoracic pressure becomes more negative, like suction - Pressure in the abdominal cavity increases (like SM contracting) This action serves to force blood back to RA - During exhalation the valves will prevent any backflow of blood in the venous system. - *decrease intrathoracic pressure during inhalation because chest expands created vacuum, pressure increases in gut due to skeletal muscle contraction and descent of diaphragm, blood flows from high to low pressure so blood flows up*
Placental supply
- Fetal blood flow to the placenta is supplied via paired umbilical arteries - A single umbilical vein drains from the placenta to the ductus venosus - Collects blood from umbilical vein and liver - Empties into the inferior vena cava
Cerebral auto regulation
- Flow constant over a WIDE pressure range ~ 60 - 160 mmHg - Protects brain against changes in arterial pressure -- If pressure drops below 60 mmHg, the function of the brain is severely affected (mental confusion, syncopy) -- Pressure > 160 mmHg permeability of blood brain barrier cerebral edema -- Mechanism unknown
Cardiogenic shock
- Heart fails to pump blood adequately (myocardial infarction)
Cardiovascular response to hemorrhagic shock
- Initial neural responses increase cardiac output and peripheral vasoconstriction - Stress and anxiety trigger higher brain center activation of the cardiovascular center. - Hormones, ADH, Ang II, aldosterone, and EPO contribute to increased blood volume
dermal (skin) circulation
- Large organ, 10-15% of total body mass - Functions of skin: Protection from external world Conserve or dissipate heat during body temperature regulation - Arteriovenous anastomoses Shunts blood from arterioles to venules, bypassing capillary bed Sympathetic control - Lowest metabolic rate of organs in body -- low O2, nutrient requirements
regional functional hyperemia
- Local supply of blood flow to local demand -- regional cortical flow is associated with regional neural activity --- move hand --- increase flow only in the "hand area" of the contralateral sensorimotor and premotor cortex.
Cardiovascular changes at birth
- Lungs and pulmonary vessels expand - Vestigial structures -- Ductus arteriosus constricts and becomes ligamentum arteriosum -- A valvular flap closes the foramen ovale leaving the fossa ovalis -- Ductus venosus becomes ligamentum venosum
Arterial blood pressure
- Maintains blood flow through capillary beds - Rises during ventricular systole and falls during ventricular diastole - *Pulse pressure = systolic pressure - diastolic pressure* - *MAP* changes throughout the systemic circuit *𝑀𝑒𝑎𝑛 𝑎𝑟𝑡𝑒𝑟𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (𝑀𝐴𝑃)=𝑑𝑖𝑎𝑠𝑡𝑜𝑙𝑖𝑐 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒+ (𝑝𝑢𝑙𝑠𝑒 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒/3)* - divided by 3 because heart spends more time in diastole 1. Pulse pressure and MAP decline with increasing distance from the heart. 2. Is highest in the aorta 3. Declines throughout the vasculature 4. Is ~0 mmHg in the right atrium
Autoregulation
- Mechanisms by which a *tissue or organ regulates its own blood flow.* - Alteration of the *diameter of local arterioles* feeding the capillary bed of a particular organ or tissue (not global) - Controlled by local metabolic factors, endothelial factors (secreted by endothelium), and myogenic mechanisms (arteriole response to stress) - *Independent of MAP* --> it is the average BP globally, autoregulation maintains blood flow (O2 and pressure) in certain areas regardless of global change, within limits
Blood Pressure homeostasis
- Must be able to "fine-tune" blood pressure during movement, etc. - Cooperation of heart, blood vessels, kidneys; supervision by brain - Three key variables: Cardiac output (blood flow of entire circulation) Peripheral resistance Blood volume - measures BP and changes resistance - get sick after run because blood is going to skeletal muscles and away from stomach
How do the pressures drive fluid flow across a capillary? *Net filtration*
- Net filtration occurs at the arteriolar end of a capillary. - To determine the pressure driving the fluid out of the capillary at any given point, we calculate the net filtration pressure (NFP)--the outward pressures (HPc and OPif) minus the inward pressures (HPif and OPc). NFP = (HPc + OPif) - (HPif + OPc) = (35 + 1) - (0 + 26) = 10 mm Hg (net outward pressure) - as a result, fluid moves from the capillary into the interstitial space *see pg.725*
Endothelial control of local blood flow
- Numerous vasoregulatory factors are produced by endothelium and affect layer of vascular smooth muscle. *Relaxing Factors (vasodilation)* - nitric oxide (NO) - prostaglandins (PGI2) - hyperpolarizing factor(s) *Contracting Factors (vasoconstriction)* - thromboxane A2 - endothelin-1 *NO and endothelins balanced unless blood flow inadequate, then NO wins; antagonistic effect, causes vasodilation*
Long-term auto regulation
- Occurs when short-term autoregulation cannot meet tissue nutrient requirements -> becomes long-term - *Angiogenesis* 1. Number of vessels to region increases and existing vessels enlarge 2. Common in heart when coronary vessel occluded, or throughout body in people in high-altitude areas
Treatment of essential hypertension
- Often includes *lifestyle changes* such as cessation of smoking, moderation in alcohol intake, weight reduction, exercise, reduced Na+ intake, increased K+ intake - Drug treatments include *diuretics to reduce fluid volume*, *beta-blockers to decrease HR by blocking sympathetic innervation*, *calcium blockers*, *ACE inhibitors to inhibit formation of Angiotensin II*, and *Angiontensin II-receptor blockers*
Dangers of Hypertension
- Patients are often *asymptomatic until substantial vascular damage occurs* - Contributes to atherosclerosis - Increases workload of the heart leading to ventricular hypertrophy and congestive heart failure - Often damages cerebral blood vessels leading to stroke (overflow blood vessels) - These are why it is called the "silent killer"
Measuring blood pressure
- Systemic arterial BP - Measured indirectly by the auscultatory method using a sphygmomanometer - *Pressure is increased in the cuff until it exceeds systolic pressure in the brachial artery* - Pressure is released slowly and the examiner listens for *sounds of Korotkoff* with a stethoscope - Sounds first occur as blood starts to spurt through the artery (systolic press., normal 110-140 mm Hg) - Sounds disappear when the artery is no longer constricted and blood is flowing freely (diastolic press., normal 70-80 mm Hg)
Blood pressure
- The closer the blood is to the pump, the greater the pressure. - Steepest pressure drop occurs in the arterioles *mean pressure is in between systolic and diastolic* - regulate blood pressure to regulate blood flow - arterioles responsible for creating pressure drop coming out of them (but does build pressure in front of them)
Developmental aspects
- The endothelial lining of blood vessels arises from mesodermal cells, which collect in blood islands - Blood islands form rudimentary vascular tubes through which the heart pumps blood by the fourth week of development - Fetal shunts (foramen ovale and ductus arteriosus) bypass nonfunctional lungs - The ductus venosus bypasses the liver - The umbilical vein and arteries circulate blood to and from the placenta
The relationship between pressure and resistance (Ohm's Law)
- The relationship between pressure and resistance *determines blood flow and affects rates of capillary exchange.* - *Blood flow (F) is directly proportional to the difference in blood pressure (ΔP)* between two points in the circulation. - *Blood flow is inversely proportional to resistance (R).* *F = delta P/R* - *vascular resistance is the key regulated variable* - decr R = incr BF - incr delta P = incr BF
Local metabolic control of blood flow involving O2
- When tissues use up the their oxygen (oxygen consumption, *O2 decr.*), they release metabolites. - This system allows tissues to balance oxygen supply with oxygen need.
Atherosclerosis
- buildup of fatty plaque in the blood vessel wall - decreases the size of the lumen - increases BP
Changes in blood vessel diameter have the greatest effect on vascular resistance
- changes blood flow most drastically - *Resistance varies inversely with the fourth power of the vessel radius* - *Because resistance is inversely proportional to blood flow, reducing blood vessel radius by ½ reduces blood flow 16x & you would increase resistance 16x* - Ex: increase radius of blood vessel 2x = decrease resistance 16x and increase blood flow 16x - increase vessel diameter = decrease peripheral resistance
Overview of circulation
- composition of blood vessels walls changes - Ex. aorta is right next to ventricle and has to withstand lots of pressure from heart pump (lots of elastic, smooth muscle, and collagen fibers)
Skeletal muscle pump for veins
- contraction of skeletal muscle pushes blood down and up, the down blood closed the valve below it and the up blood caused blood to be pushed up toward the heart - reason for not locking knees
Neural control of blood flow
- extrinsic factor - Sympathetic neurons release *norepinephrine*, which promotes vasoconstriction - During exercise, for example, sympathetic neurons release norepinephrine, resulting in *global vasoconstriction*, allowing local mechanisms in skeletal muscle to signal *local vasodilation*, resulting in the directing of blood flow to skeletal muscle (blood shunting) - Sympathetic innervation of the adrenal medulla triggers release of *epinephrine*, contributing to hormonal control of blood flow. - *Parasympathetic neurons* have very few direct synapses with arterioles; rather, they synapse with, and *antagonize sympathetic neurons (cause vasodilation)*
Varicose Veins
- leaky valves caused by being on feet and putting too much pressure on veins as they try and get blood to the heart
The kidneys indirectly raise blood pressure
- low blood pressure/volume trigger the kidneys to secrete renin - Renin cleaves angiotensinogen to generate angiotensin I - Angiotensin I is cleaved by angiotensin-converting enzyme (ACE) released by lungs to generate angiotensin II. - Angiotensin II contributes to raising blood pressure incr. CO = incr. vasconstrict - do as much or as little as needed to maintain homeostasis (gradation) - EPO increases RBC = increase BV = increase BP - aldosterone= increase Na+ and water retention, increase BV, increase BP - ADH = increase water retention = increased BV = increased BP *see print out*
How do the pressures drive fluid flow across a capillary? *Net reabsorption*
- occurs at the venous end of a capillary - Again, we calculate the NFP: NFP = (HPc + OPif) - (HPif + OPc) = (17 + 1) - (0 + 26) = -8 mm Hg (net inward pressure) - Notice that the NFP at the venous end is a negative number. This means that reabsorption, not filtration, is occurring and so fluid moves from the interstitial space into the capillary. *see page 725*
Generalized structure of arteries and veins
- tunica media is thicker in arteries than veins 1) they have to handle higher pressures 2) control pressure 3) veins have more elastic fibers
Colloid osmotic pressures
1. *Capillary colloid osmotic pressure (oncotic pressure) (OPc)* - Created by *nondiffusible plasma proteins, which draw water toward themselves* - ~26 mm Hg 2. *Interstitial fluid osmotic pressure (OPif)* - Low (~1 mm Hg) due to low protein content
Hydrostatic pressures
1. *Capillary hydrostatic pressure (HPc) (capillary blood pressure)* - Tends to force fluids *out* through capillary walls - Greater at arterial end (35 mm Hg) of bed than at venule end (17 mm Hg) 2. *Interstitial fluid hydrostatic pressure (HPif)* - Pressure that would push fluid *into* vessel - Usually assumed to be zero because of lymphatic vessels
Forces acting across capillary walls
1. *NFP*—comprises all the forces acting on a capillary bed 2. *NFP = (HPc + OPif) - (Hpif + Opc)* 3. *At the arterial end of a bed, hydrostatic forces dominate & filtration dominates* 4. *At the venous end, osmotic forces dominate & reabsorption dominates* 5. *Excess fluid is returned to the blood via the lymphatic system* - *If calculation of net filtration is (+) then it is filtration; if it is (-) then it is reabsorption* A. Capillary hydrostatic pressure B. blood osmotic pressure C. arteriole D->E (gray arrow) is net filtration pressure; also leads to lymphatic vessels and return to circulation D. filtration (force fluid out, more protein/less water inside) E. Reabsorption F. venule
Factors affecting resistance to blood flow
1. *Total vessel length* - The longer the vessel, the greater the resistance - Greater resistance in adults than in infants. (not quick fix) 2. *Blood vessel diameter* - Frequently changes in arterioles - Primary physiological determinant of peripheral resistance - Greater diameter = lesser resistance (easy to regulate) 3. *Blood viscosity* - "Stickiness" of blood due to formed elements and plasma proteins. - Fairly constant (needs to be) - Greater viscosity = greater resistance (making blood thinner/thicker) 4. *Turbulence of blood flow* - Caused by high flow rates, irregular vessel surfaces, bends in blood vessels, or sudden changes in vessel diameter - Turbulence = greater resistance (plaques, not flowing efficiently)
Skeletal muscle circulation
1. Blood flow at rest - 75% of muscle capillaries closed - 1.4 - 4 ml/min/100 g (750 ml/min) 2. Strenuous exercise - *flow can increase more than 20 fold* - up to 40-100 ml/min/100 g (15-20 L/min) - Thus, during strenuous exercise *total muscle blood flow can account for 3-times the normal resting CO of 5 L/min.* - *exercise skeletal muscle -> decr. O2, incr. CO2, incr. H+, and incr. other metabolic factors in ECF -> vasodilation of arterioles (overrides extrinsic sympathetic input) -> incr. muscle blood flow (active hyperemia)*
Short-Term Blood Pressure Control - Neural Mechanisms
1. Cardiovascular center of the medulla oblongata Consists of cardiac centers and vasomotor center (regulate diameter of BV) Receives input from higher brain centers as well as from baro- (pressure) and chemoreceptors (chemicals such as gasses) 2. Reflex Arcs Respond quickly by bypassing higher brain centers
Short term blood pressure control neural mechanisms
1. Neural control is an important short-term regulator of cardiac output and peripheral resistance. *Study Tip: Review discussion of neural control of cardiac output in chapter 18* 2. Neural controls of peripheral resistance: - Maintain adequate MAP by altering blood vessel diameter on a moment-to-moment basis - Alter blood distribution to respond to specific demands of various organs.
Control of muscle circulation
1. Rest - neural, myogenic mechanisms predominate, stretching vascular muscle results in constriction 2. Exercise - sympathetic vasoconstriction of all muscle vasculature - local metabolic vasodilation in active muscles (predominates, override sympathetic constriction) *Functional sympatholysis*
1. With capillaries, which are the least permeable and which are the most permeable? 2. most important characteristic of capillaries? 3. Which vessel type sends blood back to the heart regardless of O2 level? 4. why are there valves in veins but not arteries?
1. brain, bone marrow 2. blood flows through them slowly and gas change is quick 3. vein 4. venous pressure is lower and has valves to prevent backflow
1. What is the regulator of MAP? 2. Predict what happen to peripheral resistance in Arterioles supplying skeletal muscle when PH levels drop
1. changes in cardiac output and systemic vascular resistance. 2. Vasodilation in vessels supplying skeletal muscle (to shunt blood and maintain oxygen in that area, the rest of the body's blood vessels would be in vasoconstriction)
Effects of selected hormones on blood pressure
1. epinephrine and norepinephrine: incr. BP; incr. CO (HR & contractility); incr. peripheral resistance (vasoconstriction) 2. Angiotensin II: incr. BP; incr. peripheral resistance (vasoconstriction) 3. ANP: decr. BP; peripheral resistance (vasodilation) 4. ADH: incr. BP; incr. peripheral resistance (vasoconstriction) & incr. BV (decr. water loss) 5. Aldosterone: incr. BP; incr. BV (decr. salt and water loss)
1. Why does PP disappear by time blood reaches capillary beds? 2. Why is BP higher in aorta than inferior vena cava?
1. the muscular arterioles do not exhibit elastic rebound 2. the diameter of the aorta is smaller and velocity of blood flow is greater (*lumen of veins is larger, increase velocity = increase turbulence = increase BP*)
Fluid movement across capillaries
1.Factors promoting filtration (fluid leaves capillary): - capillary hydrostatic pressure (Pc) - interstitial colloid osmotic force 2.Factors promoting reabsorption (fluid enters capillary from interstitial fluid): - interstitial hydrostatic pressure (Pif) - capillary colloid osmotic force *factors promoting filtration - factors promoting reabsorption = net filtration pressure*
Returning blood to the heart
Because of low venous pressures, veins need help returning blood to the heart Four mechanisms: 1) Venous valves 2) Sympathetic venoconstriction - Smooth muscle in tunica media contracts following sympathetic NS activation Two auxiliary "pumps" 3) Skeletal muscle pump 4) Respiratory pump (low pressure, lots of blood, not much smooth muscle)
Blood pressure homeostatic measurements
F= delta P/R because blood flow of entire system is cardiac output: CO= delta P/R So... *delta P = CO x R* - *increased SV, increased HR = increase CO* - *decreased diameter of blood vessels, increased blood viscosity, increased blood vessel length = increase peripheral resistance* - *increased CO and increased peripheral resistance = increased MAP*
Blood pressure gradients
The differences in blood pressure within the vascular system provide the driving force that keeps blood moving, always from an area of higher pressure to an area of lower pressure. - not pressure of flow but outward pressure blood puts on blood vessel wall - flows from area with lots of pressure on wall to areas with little pressure on wall
Coronary circulation
Tissue pressure/contraction affects flow High Resting O2 Extraction ~ 75% O2 extraction at rest Coronary venous PO2 ~ 20 mmHg Autoregulating vascular bed Neural-Metabolic mechanisms active
Veins serve as reservoirs
Under resting conditions, veins contain ~60% of our total blood volume. Under stressed conditions, such as hemorrhage, veins can redistribute blood into the arteries leading to delicate organs, such as the brain.
Venules
Venules are where several capillaries unite to form small veins. Collect blood from capillary bed and drain into veins. Very porous and are site were many WBCs emigrate from bloodstream to fight infection (diapedesis). - smallest veins
Arterioles
Very small arteries that deliver blood to capillaries. *Key role in regulating blood flow from arteries into capillaries by regulating resistance*. AKA - *Resistance Vessels* 1. *responsible for largest pressure drop across circulation* 2. smooth muscle contracts (vasoconstriction) decrease blood flow into capillary bed. 3. Smooth muscle relaxes (vasodilates) increase blood flow into capillary bed. - so we don't rupture capillaries - we don't always need same blood to certain area (ex. exercise needs blood to muscles) - lowers BP coming out the other end by constricting certain areas (raising BP at those areas but overall lowering BP) - helps to shunt and put blood where it needs to go
What is the main question when talking about blood vessels?
are we getting blood where it needs to go and the right amount
Vascular shock; three types?
due to inappropriate vasodilation (large ↓ in blood pressure). 1. Anaphylactic- severe allergic reaction 2. Neurogenic- trauma to head, damage CV center 3. Septic - vasodilator release by bacteria (endotoxin)
how are blood pressure and speed (flow) related?
lower blood pressure in general, creates less of a steep pressure gradient which means slower blood flow
resistance
opposition to flow; a measure of the amount of friction blood encounters as it passes through the vessels
Anastomoses
singular: anastomosis blood vessels that join together (where not as much gas is exchanged) Can be between: Artery and artery (also called collaterals) Artery and vein (Example: thoroughfare channel in capillary beds) Vein and vein (most common)
blood pressure
the force per unit area exerted on a vessel wall by the contained blood (mmHg) - *liquid contained in vessel exerts force on wall*
blood flow
the volume of blood flowing through a vessel, an organ, or the entire circulation in a given period of time (ml/min) - in heart, aorta, whole body