Week 7: Microcirculation

Question 1

What is the purpose of micro circulation?

Answer 1

to ensure eqilibirum with the external and internal environments

it is part of the alimentary system

examples: pulmonary, gastrointestinal, renal, and bodily microcirculation

Question 2

Macro- micro transition:

How are the arteries and veins connected?

Answer 2

Arterial branches → arterioles → capillaries → venules → venous branches

Question 3

What is the graph representing optimal parameter profiles

Answer 3

Question 4

Branching

Answer 4

book

Question 5

Coupling with the microflow environment

Answer 5

book

Question 6

What is the microcirculatroy unit?

Answer 6

The unit that connects the arterioles and the venules

Question 7

What is the input and output of the microcirculatory unit?

Answer 7

input: arteriole

output: venule

Question 8

What is the “throughfare channel” and what does it allow?

Answer 8

a channel which allows blood to flow freely between an arteriole and venule

can carry nutritive flow → metarteriole, capillary network

can carry nonnutritive flow → shunt

Question 9

What is the direction of flow in the microcirculatory unit?

Answer 9

arteriole → venule

(within the capillary, the spatio-temporal pattern is more complex)

Question 10

What offers a means of overall control of flow?

Answer 10

the smooth muscle layer in the wall of the arteriole

Question 11

What offers the means of local control?

Answer 11

precapillary sphincters

Question 12

Graph displaying the different vessels v.s. pressure

Answer 12

Question 13

The pressure is autoregulated (graph)

Answer 13

Question 14

How is the diameter of the vessels adjusted?

Answer 14

vasomotion

the oscillation of vascular tone (diameter) with frequencies in the range from 1-20 min and is seen in the most vascular beds

Question 15

Where does the oscillation of vasomotion originate?

Answer 15

vessel wall

(seen in vivo & vitro)

Question 16

What must be present in order for vasomotion to occur?

Answer 16

cellular oscillator

can be modeled as a strong of events forming a feedback loop

Question 17

In order to get macroscopic oscillations of blood vessels ____

Answer 17

the oscillations in individual smooth muscle cells must be synchronized

Question 18

What are the physiological roles of vasomotion?

Answer 18

1. improve flow in the periphery of circulation
   
   • ​​flow is better through an oscillating diameter rather than a steady one
2. make peripheral flow distribution more efficient
   
   • ​​by ↑ the synchronization
3. ↑ the reactivity of a blood vessel
   
   • ​​avoiding the “latch state” (state of prolonged force in SM due to low ATP)
4. pattern alters in disease

Question 19

Describe the flow of RBC and plasma.

Answer 19

• flow in two phases in arteries and arterioles
• in the capillaries their flow behavior separates int plug and bolus flows (i.e. single file
• the plasma bolus is being stirred while flowing which provokes gas and meterial exchange betwen capillary lumen and wall

Question 20

Why can a RBC of 7 micra in diameter fit into a capillary of smaller diameter (5 micra) ?

Answer 20

special shape and remarkable membrane flexibility

Question 21

Describe the flow of white blood cells.

Answer 21

• flow is much slower than that of RBC
• tend to interfere with microcirculatory flow
  
  • therefore WBC have impact of microcirculatory flow

Question 22

What is the impact of glycocalyx on flow?

Answer 22

• the bushlike extracellular matrix structure penetrates the plasma column, capturing its edge
  
  • acts as a stagnant cell-free plasma layer
• size of this layer effects microcirculatory hematocrit
• molecular composition is complex and assists in multiple homeostatic functions

Question 23

What is responsible for the formation of ESL (endothelial surface layer) ?

Answer 23

interaction between soluble plasma components and the membrane-bound glycocalyx

Question 24

Discuss the ESL in regards to thickness

Answer 24

ESL thicker than glycocalyx

• thickness of ESL ↑ with vessel diameter
• relative thickness vise versa
• therefore the ESL is able to withstand the sheer stress of flowing blood and exclude RBC

Question 25

What are the effects of ESL on hemodynamic parameters in the microcirculation?

Answer 25

effects the flow resistance and hematocrit because of the presence of cell-free layer and thickness

Question 26

What is the result of the **loss of dynamics**?

Answer 26

↓ phase separation in the macro-micro transition dimension of the circulation

Question 27

What is the result of the **slower dynamics**?

Answer 27

* lesser slipping over the ESL * results in ↑ cellular interaction * results in ↑ microcirculatory viscosity the combination of **loss of dynamics** and **slow dynamics** results in the verge of **collapse**

Question 28

What are capillary types defined by?

Answer 28

their endothelial lining

Question 29

Name examples of **continuous** capillary type

Answer 29

**tight:** brain retina **intercellular clefts:** skeletal muscle myocardium smooth muscle skin

Question 30

Name examples of **fenestrated** capillary type

Answer 30

GI mucosa glands certain renal vessels

Question 31

Name examples of **discontinuous** capillary type

Answer 31

liver bone marrow adrenal cortex spleen

Question 32

Different ways of passive endothelium transcapillary exchange

Answer 32

Question 33

Describe **transcellular exchange**

Answer 33

- **mechanism:** difussion - **what does it transport? →** gases (O₂, CO_2, Xe) → lipid soluble molecules → water via **aquaporin-1 channel** - depends on diffusion properties of the surrounding tissues

Question 34

Describe **paracellular (small pore pathway)** exchange

Answer 34

- **mechanism:** diffusion - **what does it transport?** → water soluble (lipophobic) molecules → small polar molecules ( \< albumin-size) - **via small water-filled pathways (pores): →** intercullular clefts → interendothelial junction complex → gaps → fenestrae

Question 35

What version do we use of Fick's Law of diffusion?

Answer 35

Question 36

What is **flow limited transport**?

Answer 36

- for small molecules - there is rapid equilibrium between the plasma and the tissue - only **blood flow** determines how much molecule can be delivered to the tissue per unit time

Question 37

What is **diffusion limited transport?**

Answer 37

- when diffusion becomes restricted and there is no equilibrium between the plasma and the tissue

Question 38

What are the complicating factors of diffusion?

Answer 38

* attractions between molecules of solute and solvent * surface charge * pore structure * glycocalyx

Question 39

What is the **paracellular (large pore pathway)** exchange

Answer 39

**mechanism:** (very slow) diffusion **what does it transport?** protein sized molecules **via:** large waterfilled pathways (large pores) → large clefts → gaps → fenestrae

Question 40

What is **transcytosis**?

Answer 40

**mechanism:** vesicular transport NOT diffusion - translocation of macromolecules - flux is NOT governed by Fick's Law

Question 41

What are the exchange pathways, mechanism, and structures of the **exchange of fluid**

Answer 41

**exchange pathways:** transcellular, paracellular **mechanism:** transcellular = diffusion paracellular = convection **structure:** transcellular = AQP1 channels paracellular = clefts, fenestrae, gap junctions

Question 42

Fluid exchange

Answer 42

book

Question 43

What are the homeostatic functions of the endothelium?

Answer 43

- regulates adhesion, rolling, and migration of leukocytes tho the endothelium - maintains vessel lining that prevents formation of blood clots (thrombi) - supresses the proliferation of VSM - regulates VSM tone

Question 44

What are 2 examples of vasoactive molecules produced by endothelium?

Answer 44

1. Prostacyclin 2. Nitric Oxide

Question 45

Name some properties of **Prostacyclin**

Answer 45

- released in response to **shear stress** - formed from arachidonic acid (AA) by the actions of cyclooxygenase (CycOx) and protacyclin synthase (PGl2Syn) **Function:** - prevent platelet aggregation - prevent platelet adhesion to endothelium - relaxes adjacent smooth muscle via ↑ cAMP

Question 46

Name some properties of **Nitric Oxide**

Answer 46

- released in response to **shear stress** - can also be stimulated by Ach - stimulated endothelium **releases NO** **Function:** - ↑ cGMP level which produces relaxation of VSM by ↓ [Ca++] sensitivty of the myofilament

Question 47

Draw the individual and net pressures in the capillary and in the interstitium

Answer 47

Question 48

What is the equation for capillary colloid osmotic pressure? List common reflection coefficients

Answer 48

Question 49

Draw the diagram representing the factors responsible for filtration and absorption across the capillary wall

Answer 49