ICL 1.8: Microvasculature and Physiology of Capillary Diffusion

Question 1

what are the various microvessels?

Answer 1

arteries –> arterioles –> metarterioles –> capillaries –> veules –> veins

arteries and veins aren’t microvessels –> they’re larger vessels that branch into smaller vessels which increases the resistance and decreases the pressure

Question 2

what is the structure of a blood vessel?

Answer 2

1. tunica intima
   endothelium, loose connective tissue
2. internal elastic lamina
3. tunica media
   smooth muscle, elastic fibers
4. external elastic lamina
5. tunica externa
   collagen fibers

this setup is present in all vessels except capillaries!

Question 3

why do arteries have a prominent tunica media

Answer 3

the tunica media is made of smooth muscles and elastic fibers so arteries need this because they are the primary vessels involved in regulation of blood flow = vasoconstriction/dilation

Question 4

why do veins have a thick tunica externa?

Answer 4

the tunica externa is composed of collagen fibers and at any given time 70% of your blood is in the venous system so the veins need to structural support provided by the collagen to contain all that blood

Question 5

what is the function of a capillary?

Answer 5

exchange of material!

so it’s just an endothelial cell layer with a basement membrane, there’s no tunica externa/interna/media

Question 6

_______have the most increase in resistance and cause the largest drop in blood pressure as blood flows through vasculature

Answer 6

arterioles

even though capillaries are the smallest with the smallest diameter it’s actually the arterioles

there’s the most increase in resistance in arterioles because of their smooth muscle cells; the smooth muscle constriction can increase the resistance a lot so you see the most drop in BP in the arterioles

Question 7

where does the primary mechanism for the control and distribution of blood flow is in which vessel?

Answer 7

arterioles

Question 8

what are the characteristics of arterioles?

Answer 8

1. same 3 tunics as the larger vessels

tunica intima has an intact endothelial lining; tunica media is restricted to one or two smooth muscle cell layers in thickness; tunica externa is very thin

2. thickness is greatly diminished
3. they are the primary site of resistance and regulation of blood pressure

**vasoconstriction and vasodilation in the arterioles is the primary mechanism for control and distribution of blood flow!

Question 9

what are the characteristics of a venule?

Answer 9

a venue is an extremely small vein

the walls of venues consist of a tunica intimate, media and externa!

Question 10

what are precapilary sphincters?

Answer 10

they make sure the blood flow to the different capillary beds is regulated

not all the capillary beds in your body are perfused all at the same time; it’s an on/off mechanism and can be metabolically regulated –> when you’re cold and your face turns white the capillary blood flow to your face is shut off to try and conserve heat

so the metarterioles are making sure the blood doesn’t flow to all the capillary beds and instead goes straight to the venules and this is possible because of the precapillary sphincters which are smooth muscle rings and their constriction cuts off the blood from going to all the capillaries

Question 11

what are the 3 types of capillaries?

Answer 11

1. continuous capillary
2. fenestrated capillary
3. sinusoidal capillary

each of these allows for movement of different amounts/sizes of material

Question 12

what are continuous capillaries?

Answer 12

1. tight junctions between endothelial cells
2. intercellular clefts for exchange of water and other very small molecules
3. rich in transport vesicles except in brain
4. they’re part of the blood-brain barrier in the brain and prevents the movement of nearly all substances

Question 13

what are fenestrated capillaries?

Answer 13

1. have pores (or fenestrations) in addition to tight junctions
2. permeable to larger molecules
3. common in small intestine and kidneys

Question 14

what are sinusoidal capillaries?

Answer 14

1. extensive intercellular gaps, incomplete basement membranes, plus intercellular clefts and fenestrations
2. blood flow is very slow with more time for exchange
3. found in liver and spleen, bone marrow, lymph nodes, and many endocrine glands

Question 15

Aunt Rosa was diagnosed with nephritic syndrome, a kidney disorder with high levels of protein in the urine and low blood albumin concentration. She complains to you that she has actin pain in the leg. you notice there is also swelling in her legs. she seems to think it’s due to her kidney disease since she has no other health issues. what is responsible for aunt rosa’s swollen legs?

Answer 15

her proteins are being lost in the urine so as a result of which she would have decreased albumin in the blood which would effect πc

so there would be a decreased colloid osmotic pressure in the capillaries! less of a pull force and less reabsorption

so yes it’s due to the kidney damage but it’s because the capillaries in your legs don’t have good reabsorption due to low protein concentration

there would also be an increase in lymph flow!! because there’s increased filtration and decreased reabsorption so that’s why she has edema

Question 16

what is capillary exchange?

Answer 16

exchangeof material between the blood in thecapillary and the tissues

Question 17

what are the 3 mechanisms of capillary exchange?

Answer 17

1. simple diffusion
2. vesicle transport or transcytosis
3. bulk flow

Question 18

what is simple diffusion?

Answer 18

a mechanism of capillary exchange where all nutrients, O2, CO2, wastes and other small molecules enter or leave capillaries by passive diffusion down their concentration gradient –> when the blood reaches the capillary bed, the O2 is high in the blood and low in the interstitial fluid so O2 diffuses via simple diffusion into the interstitial fluid and then to the blood

this is the predominant mechanism of exchange of materials between the blood and interstitial fluid!

lipid soluble substances diffuse through the lipid component of the endothelial cell membranes

ions and other polar molecules have to pass through the water filled channels

Question 19

what is vesicle transport?

Answer 19

a mechanism of capillary exchange where large lipid insoluble macromolecules are endocytosed by the endothelial cell into a lipid vesicle because they can’t diffuse across the lipid membrane

the vesicle moves through the cell and is then exocytosed to the other side into the interstitial fluid

if they’re smaller molecules they can go through the fenestrae/pores but if they’re large and have to be transported, they’ll be endocytosed and transported via vesicles

Question 20

what is bulk transport?

Answer 20

a mechanism of capillary exchange that involves filtration and reabsorption

it’s the movement of water and dissolved solutes as a result of a pressure gradient! so the movement is from high to low pressure

this can be bidirectional depending on the net filtration pressure unlike simple diffusion

net filtration pressure is determined by the balance of the 4 Starling forces which determine the pressure at that capillary bed

if the net filtration pressure is positive then filtration will happen and if the net filtration pressure is negative then reabsorption will happen

Question 21

what is filtration? reabsorption?

Answer 21

filtration = movement of fluid from the capillaries into the interstitial fluid.

reabsorption = movement of fluid from interstitial fluid into the capillaries

Question 22

what are the 4 starling forces?

Answer 22

FACTORS FAVORING FILTRATION
1. capillary hydrostatic pressure, Pc***

2. interstitial fluid osmotic pressure, πi

FACTORS FAVORING REABSORPTION
3. interstitial fluid hydrostatic pressure, Pi

4. capillary osmotic pressure, πc***

it doesn’t determine the extent of movement, it just determines the direction of movement –> Pc and πc are the two big factors that determine the direction

Question 23

what is Pc?

Answer 23

capillary hydrostatic pressure

it’s the force extend by the fluid against the capillary wall and is created due to the pumping action of the heart –> it pushes the fluid outward from the capillaries into the interstitial fluid!!

it’s the highest at the arteriolar end and lowest at the venular end of the capillary

favors filtration!

Question 24

what is plasma osmotic pressure?

Answer 24

it’s the osmotic pressure due to protein molecules and is called colloid osmotic pressure or oncotic pressure –> it pulls fluid inward into the capillaries!!

osmosis = diffusion of water from a region of low solute concentration to a region of high solute concentration across a semipermeable membrane (aka the plasma membrane of the endothelial cell of the capillary)

this happens because there’s proteins in the blood that can’t diffuse into the interstitial fluid because they’re too big so instead the water gets pulled into the capillaries because it wants to go to where the protein gradient is high – so the proteins are pulling the water into the capillaries and that’s why the plasma osmotic pressure is a pull force!

it favors reabsorption!

Question 25

what is the starling equation for fluid filtration?

Answer 25

Jv = Kf ([Pc - Pi) - σ[πc - πi])

usually σ is constant an [Pc - Pi) - [πc - πi]) is the net filtration pressure so really the equation simplifies to Jv = Kf*netfiltration

Jv = volume flow of fluid across the capillary wall

Kf = filtration coefficient of the capillary wall = capillary permeability

Pc = capillary hydrostatic pressure

Pi = interstitial fluid hydrostatic pressure

σ = osmotic reflection coefficient of all plasma proteins

πc = oncotic pressure of the plasma in the capillaries

πi = oncotic pressure of the interstitial fluid

Question 26

a patient arrives at the ER with dangerously low BP. his blood colloid osmotic pressure is normal. how would you expect this situation to affect his net filtration pressure?

Answer 26

it would decrease

low BP means mean arterial pressure is low and the Pc would be low which is the major factor determining net filtration pressure

Question 27

what’s the significance of net filtration pressure?

Answer 27

fluid moves out of the capillaries into the tissues if the NFP is positive:
[Pc – Pi] > [πc – πi]

fluid moves from the tissues into the capillaries if the NFP is negative:
[Pc – Pi] < [πc – πi]

Question 28

for a 76 year old man with left heart failure, Pc at the venous end was high (35 mmHg). the rest of the values were Pc arteriolar end 30, πc 28, Pi 0 and πi 4. what is the net filtration pressure at the venous end?

Answer 28

= (25-0)-(28-4)
= 25-24
= 1 mmHg

Question 29

an increase in mean arterial pressure would result in:
A. increased filtration of fluids at the arterial end

B. increased reabsorption of fluids at the venous end

C. decreased filtration at the arterial end

D. no net effect since changes in filtration will be balanced by the changes in reabsorption

Answer 29

A. increased filtration of fluids at the arterial end

increased in mean arterial pressure would mean increase in Pc so fluids will be filtered more at the arterial end

if the Pc is higher at the arterial end it’ll also be higher at the venous end so it wouldn’t increase the reabsorption of fluids at the venous end, it would actually probably decrease it

Question 30

what is edema?

Answer 30

filtered fluid that isn’t reabsorbed returns tot eh circulation through the lymphatic system

if that doesn’t happen and filtration > reabsorption there is collection fo excess fluid within a tissue or organ!

if there is more reabsorption than filtration then it leads to HTN

Question 31

one of the obvious acute features of cutaneous thermal injury is the swelling of he involved tissue. which among the following changes can be attributed to this?

A. increased plasma hydrostatic pressure

B. decreased plasma oncotic pressure

C. increased capillary permeability

D. increased osmotic reflection coefficient

Answer 31

C. increased capillary permeability

injury releases TNF-alpha and histamine which increases capillary permeability

there will be no change in hydrostatic or oncotic pressure

Question 32

a 55 year old obese woman with HTN and DM2 presents to the hospital with shortness of breath, increasing fatigue and worsening lower extremity edema. what is the most probably reason for edema in this case?

A. increased capillary permeability

B. decreased plasma oncotic pressure

C. increased capillary hydrostatic pressure

D. venous congestion

Answer 32

C. increased capillary hydrostatic pressure

she has HTN so the pressure at which the blood reaches the capillaries is going to be higher

Question 33

kwashiorkor, a severe form of malnutrition is seen in certain parts of the world. it is often associated with edema of the abdomen. changes in which the starling forces contributes to the edema in these patients?

Answer 33

plasma oncotic pressure

kids have less protein in the blood due to malnutrition

Question 34

what is the pathophysiology of edema?

Answer 34

1. impaired venous flow –> heart failure
2. increased permeability –> tissue injury, viruses
3. hypoproteinemia –> liver and kidney diseases
4. decreased lymphatic drainage –> tumor, inflammation