Phys- Local control of Blood flow Flashcards
most potent vasoconstrictors known
Endothelin
Myogenic Theory/ response
increasing pressure in a vessel causes vasocontriction (inreased resistance)
Law of La Place
T=P x r. (tension = transural pressure x radius) ——– Increase pressure and keep T (flow) contant by decreasing the radius ( = myogenic response)
Purpose of the myogenic response
protecting the capillaries from sudden increases in blood pressure
Intrinsic neurons (local control)
located within the walls of arterioles- may be involved in rapid, conducted vasicdilation and/or vasoconstriction (communication within the vascular tree)
Autocoids
locally produced vasoactive substances - generally potent vasodilators
resistance equation
1/r^4
Resting tone
tonic sympathetic activity (extrinsic) creates a level of vasconstriction
Reactive Hyperemia
after a period of ischemia, blood flwo increases above control level ( the magnitude and duration of reactive hyperemia correlates with the duration of the ischemic period)
what determines the distribution of the cardiac output
local control
Give examples of disorders of local control
1.) Diabetes 2.) Anaphylactic Shock 3.) Septic Shock (complete breakdown of local control)
Autoregulatory zone
80-150 mmHg - flow remains relatively constant in this area
Passive Vasodilation
due to decrease in active vasoconstriction. Decreases vascular resistance
Principle functions of blood flow to the tissues
1.) Delivery of metabolic substances to capillaries 2.) Removal of metabolic biproducts from the tissues (waste products, carbon dioxide, ions, urea, heat )
importance of autoregulation
1.) Maintains tissue flow at low perfusion pressures 2.) Protects the tissue from over perfusion and possible damage at high perfusion pressure
Metabolic Vasodilation
an increase in tissue metabolism causes the release of substances in the tissue that relax vascular smooth muslce and produce metabolic vasodilation (vasoregulation)
what conditions set us up for suitable exchange in the capillaries
1.) low pressure (due to resistance in arterioles) 2.) increased cross sectional area from arterioles to capillaries - results in a marked fall in flow velocity - suitable for exchange
Vasomotion (definition)
describes the normal pattern of rhythmic changes in flow within a capollary bed
Passive Vasoconstriction
due to the removal of dilatory influence. Increases vascular resistance
Extrinsic control
Systemic - mediates outside the tissues (ANS and humoural) IRRESPECTIVE OF METABOLISM
**Autoregulation (define) **
intrinsic tendency of an organ or tissue to alter local vascular resistance to maintain constant blood flow despite changes in perfusion pressure
Vasoconstrictors
very few of our local metabolic factors (most are vasodilators)
what causes vasomotion
spontaneous changes in the pre-capillary resistance in response to the changes in tissue environment (subtle chanes in the local concentration of vasoactive substances in the interstitium)
Temperature (local control)
Increases in temperature causes vasodilation - Decreases in temperature cause vasoconstriction
When do Mygenic Factors dominate
predominate when MAP IS INCREASED suddenly- prevent increases in pressure from reaching the capillaries
Intrinsic control
Local control - originates from WITHIN the tissues. Involves changes in vascular resistance at the tissue level. CLOSELY COUPLED TO METABOLIC DEMAND
Name an example of vasomotion
Reactive hyperemia
Endogenic vasoregulation
sheer stress of the endothelium causes release of nitric oxide (NO ) - vasodilator
Indirectly acting vasodilators
endothelium dependent - substances that trigger nitric oxide production/release (increased shear stress etc)
where do we see the greatest drop in arterial pressure
greatest drop in pressure corresponds to the vascular resistance in the arterioles
Examples of indirectlt acting vasoconstrictors
1.) Endothelial-derived contracting factor (ECF) 2.) Endothelin
purpose of metabolic vasodilation
increase tissue blood flow and the delivery of exygen and substrates as well as the removal of metabolic waste products in proportion to tissue metabolism
When do Metabolic Factors dominate
repdominate when MAP IS DECREASED and flow is low or when tissue metabolism is elevated relative to flow
Active Hyperemia vs sympathetic input
active hyperemia (activity of the muscles) tends to override the sympathetic vasoconsrtiction
Mechanical Muscle Pump (local control)
Rhythmic contractions in skeletal muscle contribute to blood flow through the muscle pump. During contraction the increase in untramuscular pressure compresses veins. During relaxation the surrounding pressure on both the arterial and venous side is lower which enhances blood flow and perfusion
most local metabolic factors act as
vasodilators
Active Vasoconstriction
due to neural or humeral influence. Increases vascular resistance (above resting tone)
Gap junctions (local control)
between endothelial cells - involved in rapid cell to cell conduction of both constrictor and dilatory signals along vessel segments
Relationship between local control of blood flow and metabolic capacity
the degree of local contrl of blood flow exhibited by a tissue tends to be proportional to its metabolic capacity (ex: heart has high metabolic activity - high local control) - makes sense: flow can be maintained at a level adequate to support the metabolic demands of the tissue while not imposing excessive demands on the cardiovascular system.
Endothelial-derived contracting factor (ECF)
may be superoxide ion produced by endothelial cells in response to oxidative stess - VASOCONSTRICTOR
Vasoregulation
ability of the tissues to control blood flow to meet the metabolic needs and waste disposal of the tissue while not over-perfusing it
Active vasodilation
due to a “dilatory influence” - Vascular resistance falls below resting and/or basal tone
Resting tone is (greater/less than) basal tone
Resting tone is greater than basal tone
What is the driving force for blood flow
Mean arterial pressure (MAP)
Active Hyperemia
during increased tissue metabolic activity the rate of substrate utilization and waste production increases. There is a coincident and proprotional increase in tissue blood flow due to vasodilation
Vasodilators
Products of cellular metabolism or locally released vasoactive substances (autocoids) - NOT INVOLED IN THE SYSTMEIC REGULATION OF BLOOD PRESSURE
Basal tone
exhibited in blood vessels (even when they arent innervated). Blood vessels possess an INTRINSIC myogenic tone. INDEPENDENT of extrinsic influences (sympathetic influence, humeral effects, drugs)
Discuss perfuion of capillary beds
In resting state not all capillaries within a capillary bed are uniformly perfused (some have high flow while adjacent have no or sluggish flow)
what determines the magnitude and duration of reactive hyperemia? Why?
duration of the ischemic period - presumably due to the accumulation of metabolic byproducts during the ischemic period
