Module 6: section 3 - arterioles and total peripheral resistance

what else is arteriolar resistance also called

total peripheral resistance; not a constant value due to arterioles being very dynamic in the regulation of their radius

what are arteriole radius regulated to do

increase or decrease

why do arterioles adjust their radius

to distribute cardiac output to the various organs depending on the body’s needs

to help regulate arterial blood pressure

how do arteriolar adjustments of radius help distribute cardiac output

by shunting

what does shunting mean

arterioles increasing and decreasing their radius to change the amount of blood organs are getting to control where the blood is going and how much

how do arteriolar adjustments of radius help regulate arterial blood pressure

if an increase of blood pressure is needed, arterioles can decrease their radius to increase resistance which then increases arterial blood pressure

what to think arterioles as

control valves, if they decrease in size, it tightens the valve to increase resistance (which lowers the flow of blood) and vice versa

what does vasoconstriction do to blood flow

decreases it due to more pressure working against it (closes/decreases radius)

what does vasodilation do to blood flow

increases it due to less pressure working against it (opens/increases radius)

what does vascular tone mean

the state of partial contraction that the smooth muscle in the walls of the arterioles are in; important as it allows them to either dilate or constrict when needed

what is regulation of vascular tone categorized by

intrinsic control and extrinsic control

what does intrinsic control mean

local controls due to happening within the organs, the organs themselves regulate their own blood supply

how does intrinsic control work

either chemically or physically; important in regulating the distribution of cardiac output

what are local chemical influences normally

local metabolic changes and histamine

what are physical influences normally

chemical responses to shear stress and myogenic response to stretch

what is extrinsic control

all the non-local or metabolic factors that affect vasodilation/constriction

what does extrinsic control include

both neural and hormonal inputs; important in regulating blood pressure

what metabolic changes result in vasodilation (increase need for more blood supply)

• Decreased oxygen

• increased carbon dioxide

• increased acid

• adenosine release

• increased potassium

• increased osmolarity

• prostaglandin release

when does decreased oxygen happen

when there is an increase in oxidative metabolism

when does increased carbon dioxide happen

due to increased oxidative metabolism

when does increased acid happen

when anaerobic metabolism produces more lactid acid and when carbon dioxide production makes carbonic acid

how does adenosine release happen

when metabolic activity increases or deprivation of oxygen

how does increased potassium happen

when very active muscle’s rate of action potentials exceeds the ability of the sodium-potassium pump to restore normal concentration gradients

when does increased osmolarity happen

when high metabolically active tissues release osmotically active particles)

when does prostaglandin happen

when local chemical messengers made via fatty acid chains in the plasma membrane of cells

what do local chemical influences not normally do

directly act on smooth muscle cells; instead they trigger endothelial cells lining the vessels to release chemical messengers that in turn will target smooth muscle cells

what is the best example of chemical messengers that local metabolic factors trigger

nitric oxide: an important cellular signalling molecule involved in many physiological and pathological processes (vasodilation, etc.)

what else does endothelial cells release

constricting chemicals: endothelin (vasoconstrictor)

*along with smooth muscle relaxation triggers

what is endothelin

peptides released by endothelial cells of vasulature

how does nitric oxide cause dilation

by reducing the entry of calcium into the muscles

what are the three principle physical influences on the arteriolar tone

• temperature

• shear stress

• myogenic response to stretch

how does temperature influence arteriolar tone

when heat is applied to an area, vasodilation and increased blood flow happens.

When coldness is applied to an area, vasoconstriction and reduced blood flow happens

how does shear stress influence arteriolar tone

when it increases, the endothelial cells release nitric oxide, which causes vasodilation (to try and reduce it again)

what is shear stress

the external force acting on a surface parallel to the slop in which it lies; the frictional force at the endothelial surface made by flowing blood

how does myogenic response to stretch influence arteriolar tone

vasoconstriction is initiated when smooth muscle cells are passively stretched to oppose the stretch and keep lumen diameter (this is an impulse that starts in the muscle rather than nerves)

how is neural regulation of arteriolar tone acheived

solely by sympathetic activity

what does increasing sympathetic activity do

increase arteriolar tone

what does decreasing sympathetic activity do

decrease arteriolar tone

what is important to know about neural regulation of arteriolar tone

it does not occur in a singular organ, it is generalized so happens to all arterioles; changing sympathetic activity globally affects arteriolar resistance/total peripheral resistance and can regular blood pressure

what makes up MAP (mean arterial pressure)

total peripheral resistance (TPR) times cardiac output (CO)

*this shows how important total peripheral resistance is in regulation of mean arterial pressure/sympathetic NS activity at the level of arterioles is essential for blood pressure regulation

how to keep blood flow to perfuse the tissues and organs

by keeping the pressure gradient

*by increasing or decreasing sympathetic activity

when can local control override sympathetic system control

when using certain organs enough to really increase their need/demand for blood flow

identify a reason that explains why the adrenal glands would release E and why it has a preference for B-2 adrenergic receptors

when “fight-or-flight” mode is happening and the body needs to prepare for activity in a stressful or emergency situation (needs more blood)

what other hormones play important roles in the regulation of arteriolar tone

vasopressin, angiotensin II

*are potent vasoconstrictors

why is vasopressin and angiotensin II important

they play crucial roles in times of massive blood volume loss

*help keep blood pressure despite decrease volume