Chapter 20- Cardiovascular Flashcards
Layers of the Arteries
Lumen (middle, blood flow) Tunica Intima (Endothelium, subendothelial layer, internal elastic membrane) Tunica Media (Smooth Muscle and elastic fibers, external elastic membrane) Tunica Externa (Vasa Vasorum)
Elastic Arteries
Large thick-walled arteries with elastin. Aorta and major branches. Large lumen offers low-resistance pressure reservoirs, Expand and recoil as blood ejected from heart.
Muscular Arteries
Distal to elastic arteries. Thick tunica media with more smooth muscle. Active in Vasoconstriction.
Arterioles
Smallest arteries leading to capillary beds, control flow into capillary beds via vasodilation and vasoconstriction.
Capillaries
Microscopic blood vessels, walls of thin tunica intima
Pericytes support their walls, control permeability, in all tissues except for cartilage, epithelia, cornea and lens
Function of Capillaries
Exchange of gases, nutrients, wastes, hormones between blood and interstitial fluid.
Venules
Formed when capillary beds unite,porous, allow fluids and WBC’s into tissues, consist of endothelium and a few pericytes
Veins
Return blood to heart for reoxygenation, thinner walls, larger lumens, lower blood pressure than arteries
Venous Sinus
Flatten veins with extremely thin walls
Venous Valves
Prevent backflow of blood
Anastomoses
Alternative pathway to given body region
Factors that affect the physiology of circulation
Blood Flow
Blood Pressure
Resistance
Blood Flow
Volume of blood flowing in a given period of time (ml/min) = (CO cardio output) Relatively constant when at rest
Blood Pressure
Force per unit area exerted on wall of blood vessel by blood - provides driving force that keeps blood moving from higher to lower pressure area.
Resistance
Opposition to flow measure of amount of friction blood encounters with vessel walls, generally in peripheral (systemic) circulation.
3 Factors that affect Resistance
Viscosity
Blood Vessel Length
Blood Vessel Diameter
Viscosity
Stickiness of blood due to formed elements and plasma proteins, increased viscosity = increased resistance
Blood Vessel Length
Longer Vessel = Greater resistance encountered
Blood Vessel Diameter
Greatest influence on resistance, varies inversely with 4th power of radius (if radius is doubled, resistance is 1/16) Vasoconstriction = increased resistance.
Pericytes
support walls, control permeability
Arterial blood pressure reflects two factors of arteries close to heart
Elasticity (compliance or distensibility)
Volume of blood forced in them time
Blood pressure near heart is pulsatile
Systolic pressure
pressure exerted in aorta during ventricular contraction
Averages 120 mm Hg in normal adult
When DR. Listening, is pressure when sounds first occur
Diastolic pressure
lowest aortic pressure
When Dr. Listening, is pressure when sounds disappear as blood flows freely
Pulse pressure
systolic - diastolic
Mean arterial pressure (MAP)
pressure that propels blood to tissues
MAP = diastolic pressure + 1/3 pulse pressure (BP = 120/80; MAP = 93 mm Hg)
Capillary blood pressure
Low is desirable because high BP would rupture thin-walled capillaries
Most very permeable, so low pressure forces filtrate into interstitial spaces
Venous blood pressure
Small gradient; about 15 mm Hg
Low pressure due to cumulative effects of peripheral resistance
Factors Aiding Venous Return
Muscular pump: contraction of skeletal muscles “milks” blood toward heart; valves prevent backflow
Respiratory pump: pressure changes during breathing move blood toward heart by squeezing abdominal veins as thoracic veins expand
Venoconstriction under sympathetic control pushes blood toward heart
Maintaining Blood Pressure
Requires • Cooperation of heart, blood vessels, and kidneys • Supervision by brain Main factors influencing blood pressure • Cardiac output (CO) • Peripheral resistance (PR) •Blood volume
Chemoreceptor Reflexes
Chemoreceptors in aortic arch detect increase in CO2, drop in pH or O2
Cause increased blood pressure by
Signaling cardioacceleratory center increase CO
Signaling vasomotor center increase vasoconstriction
Hormonal Controls
Cause increased blood pressure
Epinephrine and norepinephrine increased CO and vasoconstriction
Angiotensin II stimulates vasoconstriction
High ADH levels cause vasoconstriction
Direct Renal Long-Term Mechanism
Alters blood vol. independent of hormones
•Increased BP or blood volume causes elimination of more urine, reducing BP
•Decreased BP or blood volume causes kidneys to conserve water, and BP rises
Indirect Renal
Angiotensin II increases blood volume Stimulates aldosterone secretion Causes ADH release Triggers hypothalamic thirst center Causes vasoconstriction increasing blood pressure
Pulse
expansion and recoil of arteries
Pressure points
arteries close to surface
How is Pressure Measured
BP measured indirectly by auscultatory method using a sphygmomanometer
Pressure increased in cuff until it exceeds systolic pressure in brachial artery
Pressure released slowly and listen for sounds of Korotkoff with a stethoscope
Hypertension
Sustained elevated arterial pressure of 140/90 or higher
Prehypertension
if values elevated but not yet in hypertension range
May be transient or persistent.
Why Hypertension is bad
Prolonged hypertension big cause of heart or renal failure, vascular disease, stroke
Heart must work harder myocardium enlarges, weakens, becomes flabby
Also accelerates atherosclerosis
Secondary hypertension
due to obstructed renal arteries, kidney disease, endocrine disorders
Hypotension
Blood pressure below 90/60 mm Hg
Usually not a concern, only if leads to inadequate blood flow to tissues
Orthostatic hypotension
temporary low BP and dizziness when suddenly rising from sitting or reclining position
Chronic hypotension
hint of poor nutrition and warning sign for endocrine disorders
Acute hypotension
sign of circulatory shock; threat for surgical and ICU patients
Tissue perfusion
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)
Order of strongest blood pressure to lowest blood pressure
Aorta Arteries Arterioles Capillaries Venules Veins Venae Cavae
Autoregulation
Automatic adjustment of blood flow to each tissue relative to needs
Controlled intrinsically by modifying diameter of local arterioles feeding capillaries
Autoregulation: Metabolic Controls
Vasodilation of arterioles and relaxation of precapillary sphincters occur in response to:
Declining tissue O2 and substances from metabolically active tissues (H+, K+, adenosine, prostaglandins), inflammatory chemicals
Effects
• Relaxation of vascular smooth muscle
• Release of NO (powerful vasodilator)
• Endothelins released from endothelium are potent vasoconstrictors
Autoregulation: Myogenic Controls
Tissue perfusion constant despite most fluctuations in systemic pressure, stretch
• Passive stretch promotes vasoconstriction
•Reduced stretch promotes vasodilation
Intrinsic Vasodilators
-- Metabolic -- Reduced O2 Increased CO2 Increased H+ Increased K+ • Prostaglandins • Adenosine • Nitric oxide
Intrinsic Vasoconstrictors
Myogenic = Stretch Metabolic = Endothelins
Extrinsic Vasodilators
Neuronal - Decrease Sympathetic Tone
Hormonal - Atrial natriuretic peptide
Extrinsic Vasoconstrictors
[ Maintain mean arterial pressure
(MAP) & Redistribute blood during exercise
and thermoregulation ]
Neuronal - Increase Sympathetic Tone Hormonal • Angiotensin II • Antidiuretic hormone • Epinephrine • Norepinephrine
Angiogenesis
Number of vessels to region increases and existing vessels enlarge
Coronary vessel included, or in people in high-altitude areas, exercise
Blood Flow: Skeletal Muscles
At rest, myogenic and neural mechanisms predominate ~ 1L /minute
• Active or exercise hyperemia - blood flow increases in direct
• Local controls override sympathetic vasoconstriction
• Muscle blood flow can increase 10
Blood Flow: Brain
Constant as neurons intolerant of ischemia; averages 750 ml/min Metabolic controls ↓ pH of ↑ CO2 cause vasodilation Myogenic controls ↓ MAP, cerebral vessels to dilate ↑ MAP, cerebral vessels constrict
Brain vulnerable under extreme pressure changes - MAP below 60mm Hg can cause syncope (fainting) - MAP above 160 can result in cerebral edema
Blood Flow: Skin
Supplies nutrients to cells, O2 needs
Provides a blood reservoir
Helps regulate body temperature
Blood Flow: Lungs
Pulmonary circuit unusual: Path is short, arteries like veins (thin walls, large lumens), Arterial resistance and pressure are low
Autoregulatory mechanism opposite: Low O2 levels cause vasoconstriction; high levels promote vasodilation
Blood Flow: Heart
Ventricular systole: vessels are compressed
• Myocardial blood flow ceases
• Stored myoglobin supplies oxygen
During diastole high aortic pressure forces blood through coronary circulation
During Strenuous exercise
During strenuous exercise coronary vessels dilate in response to local accumulation of vasodilators
• Blood flow may increase 3-4 times
•Cardiac cells use 65% of O2 delivered, increased blood flow provides more O2
Direction and amount of flow within capillaries depends on 2 Opposing Forces
Hydrostatic and Colloid Osmotic Pressure
Hydrostatic pressure
Tends to force fluids through capillary walls
Greater at arterial end (35mmHg) of bed than at venule end (17mmHg)
Interstitial fluid hydrostatic pressure
Pressure that would push fluid into vessel, Usually 0 due to lymphatic vessels
Colloid osmotic pressure
Created by non-diffusible plasma proteins, which draw water toward themselves ~26 mm Hg
Net Filtration Pressure (NFP)
The outward pressure minus the inward pressure or Hydrostatic Pressure minus Colloid osmotic pressure
Hypovolemic shock
Hypovolemic shock is a result of severe blood loss. It will cause the blood vessels to constrict in an attempt to increase venous return and the heart will increase the rate of pumping blood as it attempts to make up for the low blood pressure and blood not getting throughout the body.
Vascular shock
from extreme vasodilation and decreased peripheral resistance
Cardiogenic shock
results when an inefficient heart can’t sustain adequate circulation
3 Types of Capillaries
- continuous - most common, in all vascular tissue, complete endothelial lining with tight junctions intermixed with intercellular clefts
- fenestrated - has pores and clefts, found in intestinal lining, choroid plexus, and endocrine glands
- sinusoid - swiss cheese and flattened, found in the liver and spleen, bone marrow, lymph nodes (where they carry lymph, not blood), and many endocrine glands including the pituitary and adrenal glands.
Ischemia
blood pressure too low to pump blood sufficiently
Hypoxia
inadequate oxygenation of tissues do to ischemia
Skeletal Muscle Pump
In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle.
