Unit 2 - Blood Flow & Blood Pressure PART C Flashcards
_____ organs require ________ amounts of blood flow both at rest and as activity changes. As such, the body needs a way to allow for changes in blood flow to different organs.
DIFFERENT
DIFFERENT
Most of the blood flow coming out of the left ventricle at rest goes to the ____.
LIVER
If cardiac output from the left ventricle is ~5.0 L/min (based on the average resting heart rate of 72 bpm and average stroke volume of 70 mL), then flow rate to the liver is…
Figure 15.12 (page 8)
27% x 5.0L/min, which is equal to 1.35L/min.
What is there to note about blood flow to the lungs for ex?
Note: Blood flow to the lungs occurs at a rate of 5.0L/min coming from the right ventricle, so the entire volume of the body’s blood passes through the lungs each minute).
At rest, the least amount of blood flow goes where?
to the heart (4%) and the skin (5%). These values will increase with increasing activity (walking, running, etc).
Kinda random: The arterioles are the main site of variable resistance in the systemic circulation & contribute…
more than 60% of the total resistance to flow in the system
____ _____ to each organ is a function of the pressure gradient in the system and the arteriolar _____.
FLOW RATE
RESISTANCE
(F = ∆P/R)
How is blood flow to organs controlled by changing arteriolar resistance?
- The radius of arterioles leading to different parts of the body can be adjusted independently of one another, allowing for specific changes in blood flow to specific organs.
- Smooth muscle surrounding the arteriole can contract (VASOCONSTRICTION) or relax (VASODILATION) to produce changes in resistance and therefore changes in FLOW.
- So rate of blood flow to each organ depends on the degree of contraction and relaxation (VASOCONSTRICTION/ VASODILATION) in the arterioles that supply blood to that organ.
Describe figure 15.13, blood flow through individual blood vessels and how it is determined by the vessel’s resistance to flow
a. 100 ml flows through 2 blood vessels both at 50 ml
b. Then 100 ml flows through 2 blood vessels, 1 is 20 ml & 1 is 80 ml
- from 50 ml –> 20 ml so resistance increased & flow decreased
- from 50 ml –> 80 ml so resistance decreased & flow increased
Blood flow entering capillary beds of each organ/tissue is further controlled by contraction/relaxation of _________.
PRECAPILLARY SPHINCTERS
Precapillary sphincters =
bands of smooth muscle at junctions between arterioles and capillaries.
When precapillary sphincters are OPEN (RELAXED), blood flows…
into the capillary bed
When precapillary sphincters are CLOSED (CONSTRICTED), blood flows…
directly from ARTERIOLE to venule though connecting vessels called METARTERIOLE, and does not enter the capillaries.
What are the 5 kinds of regulation of arteriolar vasodilation/vasoconstriction:
- Begins with vascular tone
- Intrinsic (local) autoregulation mechanisms
a. Myogenic control
b. Local Metabolic Control
c. Non-metabolic chemical mediators - Systemic (extrinsic) mechanisms
a. Neural Control
b. Hormonal Control - Example of blood flow control during exercise (local metabolic factors, neural and hormonal control)
- Example of blood flow control during severe hemorrhage (blood loss that causes MAP to decrease)
a. Local Effect (e.g. at brain, or heart)
b. Sympathetic response
Regulation of arteriolar vasodilation/vasoconstriction:
- Begins with VASCULAR TONE:
a. State of partial contraction independent of neural signaling and chemical effects (hormones, vasoactive mediators)
b. Baseline constriction of arterioles.
c. Can be modified by external signals to increase or tone
decrease the concentration of cytosolic calcium in smooth muscle cells, which in turn will affect the level of contraction/relaxation of the muscle. (Recall from 2410, increasing cytosolic [Ca2+] increases tension, decreasing cytosolic [Ca2+], decreases tension)
Regulation of arteriolar vasodilation/vasoconstriction:
- Intrinsic (local) autoregulation mechanisms:
a. MYOGENIC CONTROL
- Ability of vascular smooth muscle within vital organs to regulate its tone in response to changes in blood pressure.
- Increasing blood pressure stretches the smooth muscle in the arteriole.
- Response to stretch is the opening of mechanically gated (stretch gated) Ca2+ channels in the smooth muscle cell membranes, which allows more Ca2+ to enter the cells (DEPOLARIZES), increasing crossbridge formation and therefore tension.
What are 2 examples of Myogenic control?
EXAMPLE 1: when you stand, the arterial pressure and flow in the feet increase. The increase in pressure in the feet causes the arterioles to stretch, allowing more Ca2+ into the cells. In response the smooth muscle contracts, resulting in vasconstriction (increased tone) which reduces flow in the feet.
EXAMPLE 2: When you stand up (from a supine position), the cerebral arterial pressure decreases, which reduces the amount of stretch in the arterioles. In response, smooth muscle relaxes (less stretch, less Ca2+), resulting in vasodilation (decreased tone), which increases blood flow to the brain.
In the absence of autoregulation…
an INCREASE in BP INCREASES BF through an arteriole
Regulation of arteriolar vasodilation/vasoconstriction:
- Intrinsic (local) autoregulation mechanisms:
b. LOCAL METABOLIC CONTROL
- Many tissues can control their own blood supply by releasing paracrine signaling molecules in response to changes in the metabolic activity of the tissue.
- The release of paracrines into the extracellular fluid surrounding the arterioles can stimulate the endothelium of the arterioles to release VASOACTIVE MEDIATORS (e.g. nitric oxide, (NO)) that have direct action on the arteriolar smooth muscle and will initiate either vasoconstriction of vasodilation.
An example of Local Metabolic Control is ACTIVE HYPEREMIA. Describe this
E.g.1: Active hyperemia
- Increase in blood flow to a tissue in response to an increase in tissue activity (metabolism).
- As the tissue becomes more active, oxygen levels decrease and carbon dioxide levels increase. The effect on arteriolar smooth muscle is to cause relaxation and vasodilation, which increases flow to the organ.
Figure 15.10
An example of Local Metabolic Control is AUTOREGULATION & EXERCISE. Describe this
- Rapidly contracting skeletal (and cardiac) muscle leads to:
i. Local reductions in O2 levels (hypoxia)
ii. local increases in CO2,H+,K+, AND ADENOSINE LEVELS (metabolic waste products) - Causes vascular endothelial cells to secrete NITRIC OXIDE specifically at these sites.
- Increased [NO] reduces Ca2+ entry into adjacent smooth muscle cells thereby inducing localized VASODILATION of:
i. Arterioles – increases blood flow to active tissues
ii. Precapillary sphincters –increases number of open capillaries inactive
tissues - Result = increased O2 delivery and increased waste removal by blood.