4- Local Control and Hormonal Control of Blood Flow by the Tissues: Local Mechanisms

Question 1

whats the main factor determining blood flow

Answer 1

metabolism (metabolic needs of each organ and tissue)

Question 2

metabolic autoregulation

Answer 2

maintains blood flow constant as blood pressure changes

coupling of blood flow and metabolism

the tissue is maintaining blood flow in proportion to the metabolic need

Question 3

myogenic autoregulation

Answer 3

• most important in smaller arterioles
• cannot regulate flow precisely
• occurs a lot in the brain to protect its capillaries from an elevation in pressure
• smooth muscle cells resist stretch by increasing active force generation
• increase in blood pressure stretches the blood vessel and stretches the smooth muscle cells, activates the smooth muscle that in turn contracts resulting in a reduction in the diameter of the vessel and a preservation of blood flow in the face of elevated pressure.

Question 4

reactive hyperemia

Answer 4

responses that pay off O2 debt

• excess tissue flow following occlusion
• size of overshoot proportional to length of occlusion

ex: like when you hold your finger tight then let go so blood can flow again

Question 5

Law of Laplace

Answer 5

Wall Tension = Pressure x Radius

• as the radius increases wall tension increases
• In order to overcome this increased tension, wall thickness can increase, a process known as hypertrophy.

Question 6

to increase cardiac output (CO), what needs to happen in the periphery?

Answer 6

arterial dilation

• our body (muscles) can get more blood flow automatically without going to the neurons to “request” more
• blood flow increase is due to local control

Question 7

cardiac output distribution

Answer 7

based on various organs and tissues in a way thats related to their metabolic need

-exception are organs that do work with blood for example the kidneys which filter blood to make urine or skin which provides cooling function for body (these organs can have relatively high blood flow compared to their metabolic need)

Question 8

functional hyperemia

Answer 8

as metabolism increases, blood flow also increases

Blood flow is generally related to rate of tissue metabolism

• first only small increase of flow (O2 extraction increases)
• larger increases in metabolism can only occur with an increase in flow
• an 8 fold increase in metabolism increases flow rate acutely 4 fold

Question 9

Needs of every tissue

Answer 9

-delivery of
O2
glucose
aa
fatty acids
-removal of CO2

• maintenance of ion concentrations
• transport of hormones, vitamins, etc.

Question 10

effect of reduction of O2 saturation

Answer 10

increase blood flow

When the demand (metabolism) exceeds the delivery (by blood) then oxygen levels in the tissue fall. The blood vessels that are imbedded in the tissue (arterioles) sense this and dilate in response to the low oxygen. This dilation results in an increase in diameter and thus a decrease in vascular resistance and an increase in local blood flow. The increase in blood flow elevates the oxygen delivery and so the metabolic needs are met

-vasculature surrounded by smooth muscle is embedded in tissue they serve

Question 11

autoregulation

Answer 11

tissues resist having too little or too much blood flow

2 types

1. Metabolic
2. myogenic

Question 12

2 theories of metabolic autoregulation

Answer 12

BOTH of these occur but we don’t know which is more important

1. Vasodilator substances
   - tissue produces waste products of metabolism (C02, adenosine, lactic acid) that can vasodilate blood vessels.
   - In the steady state, the rate of production is balanced by the rate of washout by the blood.
   - When blood flow is too low, these dilator metabolites build up in the tissue causing blood vessels to increase their diameter and thus blood flow to increase.
2. Oxygen theory
   - negative feedback: reduction of tissue PO2 relaxes precapillary sphincters to increase blood flow
   - Here it is not dilator metabolites, but instead the vasoconstrictor properties of oxygen that is regulating blood flow.

Question 13

steps of oxygen demand theory

Answer 13

1. decreased tissue blood flow
2. decreased tissue PO2
3. arteriolar dilation
4. increase tissue blood flow
5. increase tissue PO2
6. normal arteriolar diameter

Question 14

endothelial cells and nitric oxide (NO)

Answer 14

• endothelial cells produce Nitric Oxide in response to elevated shear stress
• Nitric oxide: very potent vasodilator

Question 15

nitric oxide production in vascular endothelial cells (mechanism)

Answer 15

1. L-arginine OR Nitric oxide synthase cause NO release
2. activates guanylyl cyclase
3. cGMP causes vasodilation

Question 16

hypertrophy

Answer 16

increasing wall thickness

Question 17

Capillary exchange - 4 pathways

Answer 17

1. Intercellular clefts (hydrophilic substances)
2. Pinocytotic vesicles (large hydrophilic molecules)
3. Fenestrae and aquaporins (large amounts of fluid)
4. Transcellular (lipophilic substances)