L26 Microcirculation

Question 1

Small arterioles

Answer 1

Highly innervated by SNS nerves (release norepinephrine)

Contain vascular smooth muscle (VSM)

Site of resistance

Question 2

Terminal arterioles

Answer 2

Contain VSM

Less innervation by SNS nerves than small arterioles

Greater control by local vasoactive substance

Site of resistance

Divide to give rise to capillaries

Undergo vasomotion- periodic constriction and relaxation

Question 3

Metarterioles

Answer 3

In some tissues act as thoroughfare vessels from small arterioles to venules

Capillaries branch from metarterioles

Capillaries have precapillary sphincters at origin - cuff of VSM

Blood flow can bypass capillaries

Question 4

Capillaries

Answer 4

Major site for exchange

Question 5

Postcapillary venules

Answer 5

Some exchange also may occur

Question 6

Non-nutritive flow

Answer 6

Flow through metarterioles

Bypass capillaries

Little exchange occurs (walls a little too thick)

Question 7

Nutritive flow

Answer 7

Flow through capillaries

Active tissue requires greater blood flow

Arterioles and precapilary sphincters dilate

Increased flow through capillaries

Question 8

Arteriolar radius

Answer 8

Determines overall blood flow to a particular vascular bed

Question 9

Precapillary sphincters

Answer 9

Determine which capillaries are perfused

Question 10

3 types capillaries

Answer 10

Continuous

Fenestrated

Discontinuous

Question 11

Continuous capillaries

Answer 11

Most common

Found in muscle, skin, lung, fat, connective tissue, neural tissue

Tight junctions in intercellular clefts btw endothelial cells

Hydrophilic molecules must pass through clefts or small pores

Question 12

Fenestrated capillaries

Answer 12

Leaky

Found in kidney, intestines, endocrine glands, and joints

Endothelial cell later contain perforations called fenestrae

Enhances permeability to water and hydrophilic molecules

Question 13

Discontinuous capillaries

Answer 13

Found in bone marrow, liver, spleen

Wide gaps btw adjacent endothelial cells

Highly permeable for large molecules (proteins) and water (trafficking of proteins)

Question 14

Lipophilic substances

Answer 14

Can diffuse through the plasma membranes of capillary endothelial cells

Question 15

Hydrophilic substances

Answer 15

Mainly diffuse through pores in capillary wall

Question 16

Surface area for diffusion of

Answer 16

Lipophilic substances is generally much greater than the surface area for diffusion of hydrophilic Substances

Smaller subs diffuse easily (small radii relative to pore) Na+, glucose

Larger molecules (proteins) diffuse poorly

Question 17

Capillary walls are very permeable to

Answer 17

Water

Water transport across capillary walls depends on hydrostatic and osmotic forces in capillary and interstitial fluids - starling forces

Balance of forces determine direction and magnitude of flow

Question 18

Filtration

Answer 18

Movement of fluid out of the capillary

Question 19

Absorption

Answer 19

Movement of fluid into the capillary

Question 20

4 starling forces that control filtration and absorption

Answer 20

1. Capillary hydrostatic P (Pc)
2. Interstitial fluid hydrostatic P (Pi)
3. Plasma oncotic P (pic)
4. Interstitial oncotic P (pii)

Question 21

Capillary hydrostatic P

Pc

Answer 21

Depends on pre and post capillary resistance and pressures

Favors filtration - movement of fluid out of capillary into the interstitium

30-40mmHg

Pressure drops from arteriole to venule end (friction, viscosity) 1.5mmHg

Variable (other 3 constant)

Question 22

Determinants of Pc

Answer 22

Pa- arterial P
Ra- arteriolar resistance
Rv- venular resistance
Pv- venous P ( major contributor of edema)

Question 23

Plasma oncotic pressure

Pic

Answer 23

Osmotic pressure due to plasma proteins which cannot leave capillary lumen due to low permeability

25-30mmHg

Favors absorption- movement of fluid into the capillary from the interstitium

This is a major pressure , along with Pc

Question 24

Interstitial hydrostatic pressure

Pii

Answer 24

Produced by fluid located in intermolecular spaces of interstitial matrix

P depends on V of fluid in interstitial space as well as compliance of space

-5 to 10 mmHg

Think of it like a back P

Question 25

Interstitial oncotic P

Pii

Answer 25

Some plasma protein escapes the capillary lumen

Protein in interstitial space exerts an osmotic P, generally quite small

1-5mmHg, can be 0

Question 26

Net filtration pressure

NFP

Answer 26

Balance of hydrostatic and oncotic forces

Sign determines direction

Pos = filtration 
Neg = absorption

Question 27

Rate of fluid movement

Answer 27

Kf x NFP

Kf (ml/mmHg/min) - filtration coefficient and reflects the capillary permeability

Question 28

Increased capillary pressure

Answer 28

Favors filtration

Vasodilator

Question 29

Decreased capillary pressure

Answer 29

Favor absorption

Vasoconstrictor

Raise BP

Question 30

Decreased plasma protein level

Answer 30

Liver not functioning well
Passing proteins in urine (nephrotic syndrome)

Steady line decreases

Favors filtration (filtration phase much large)

Major transfer of fluid from inter vascular space to interstitial space - leads to edema

Question 31

Why doesn’t fluid accumulate in interstitial space?

Answer 31

Lymph vessels collect interstitial fluid and return it to the bloodstream through subclavian veins

Question 32

Lymphatic system

Answer 32

Network of one way vessels that returns fluid/lymph back into blood

Functions:
Returns filtered fluid
Transport of absorbed fats
Return of filtered protein

Filtration from capillaries = 20L/day
Reabsoprtion back to capillaries = 17L/day
3L/day enter lymphatic system

Question 33

What causes edema?

Answer 33

Increased filtration (filtration grossly outstripping absorption)
Or
Impaired lymphatic drainage