3.2.4. Microcirculation and Local Control

Question 1

What is microcirculation?

Answer 1

• circulation of blood in the smallest blood vessels, located within organ tissues
• components include small arterioles, capillaries and small venules

Question 2

What is the primary function of microcirculation?

Answer 2

-to regulate fluid and nutrient exchange between blood and tissues

Question 3

What are the three types of capillaries?

Answer 3

1. Continuous
2. Fenestrated
3. Disontinous

Question 4

What is the function of continuous capillaries?

Answer 4

• to transfer molecules via diffusion or transport processes through the cells
• heart, skeletal muscles, skin and brain (BBB, a special case of continuous capillaries)

Question 5

What is the function of fenestrated capillaries?

Answer 5

• to transfer molecules via diffusion (including some large and charged molecules into the interstitium)
• kidney and intestine

Question 6

What is the function of discontinuous capillaries?

Answer 6

• to allow for the free exchange of very large molecules and blood cells between the blood and tissue (discontinuous capillaries have large gaps between endothelial cells)
• liver, spleen and bone marrow

Question 7

What is the process of diffusion?

Answer 7

-net movement from a region of high concentration to a region of low concentration

Question 8

What are the factors that affect diffusion rate proportionally?

Answer 8

1. solubility of molecule (constant)
2. difference in molecule concentration between compartments
3. cross-sectional (surface) area for diffusion
4. temperature

Question 9

What are the factors that affect diffusion rate inversely?

Answer 9

1. square root of MW of the molecule (constant)
2. distance of diffusion
3. molecular size (physical barriers)
4. electrostatic change

Question 10

What are the forces that favor fluid loss across a capillary membrane?

Answer 10

1. capillary hydrostatic pressure

2. interstitial fluid colloid osmotic pressure

Question 11

What are the forces that favor reabsorption across a capillary membrane?

Answer 11

1. interstitial fluid hydrostatic pressure

2. capillary colloid osmotic pressure

Question 12

What is the primary function of the lymphatic system?

Answer 12

• to remove interstitial fluid from the tissues (unabsorbed fluid)
• other functions include: remove interstitial protein, remove cellular debris, minimize interstitial fluid pressure to prevent edema

Question 13

How does the lymphatic system work to move fluid?

Answer 13

1. Intrinsic pumping: smooth muscle of lymph vessels contract. When vessels are distended by internal pressure they move fluid across a valve
2. External compression: when muscle surrounding lymph vessels collapse squeezing contents across valves. Thereby, propelling lymph centrally

Question 14

What is the primary controller of blood pressure?

Answer 14

the sympathetic nervous system (via modulation of vasoconstriction)

Question 15

What is local control?

Answer 15

-the property of a tissue to modulate its own blood flow independent of neural influences

Question 16

What is autoregulation?

Answer 16

-the maintenance of constant blood flow in the face of change in perfusion pressure (metabolic demand is constant, no neural influence)

Question 17

What causes edema?

Answer 17

1. occurs when volume of interstitial fluid exceeds the capacity of the lymphatics to return it to circulation
2. caused by excess filtration or blocked lymphatics

Question 18

How do we achieve local control at the organ level?

Answer 18

1. Autoregulation
2. Functional hyperemia
3. Reactive hyperemia

Question 19

What is the myogenic theory?

Answer 19

• autoregulation is an intrinsic property of a blood vessel

- when the vessel is stretched by pressure, it tends to contract and compensate for the inc. pressure

Question 20

Describe lymphatic anatomy:

Answer 20

1. Lymphatic capillaries are thin walled, dead-end vessels
2. collecting lymphatics are larger bore thin walled vessels with small amounts of smooth muscles
3. Lymphatics have gaps (pores) between endothelial cells, which allow large molc. and other cells to enter capillaries
4. Within collecting vessels the endothelium flaps form valves, which facilitate propulsion of lymph centrally
5. Walls of capillaries are tethered to the ECM via anchoring filaments which prevent the collapse of capillaries and facilitate filling

Question 21

What is the metabolic theory?

Answer 21

-there are vasodilator metabolites produced by the tissue that regulate vascular resistance to assure blood flow is adequate for metabolic demand

Question 22

When does active (functional) hyperemia occur? (higher blood flow)

Answer 22

-functional hyperemia is seen when functional activity (metabolic work) of a tissue or organ is increased

Question 23

When does reactive hyperemia occur?

Answer 23

• reactive hyperemia is observed after blood flow to a tissue is constricted
• blood flow increases after a period of blood flow restriction to pay back the O2 deficit