Mutafova Microcirculation Flashcards by michelle granstrom

At what level does the blood brain barrier occur?

At the capillary level!

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What is the rough diameter of the capillaries & the velocity of blood flowing through there & the amount of time blood spends there?

Very small diameter.
Very slow velocity. B/c large cross sectional area.
Slow velocity allows blood to spend more time there, but still not very much time at all (1-3 seconds). Amazing that exchange can happen!

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Although the velocity of blood flow isn’t constant across circulation, what is constant?

The volume of blood flow through each portion of circulation per unit time.

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Do capillaries have smooth muscle? 
Arterioles? 
Venules
Veins?
Metarterioles?

Arterioles--yes
metarterioles--yes
capillaries--no
small venules--yes
veins--yes

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What is the precapillary sphincter?

A strong muscle that restricts blood flow into the capillaries. It is regulated.

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What are the 2 main mechanisms of regulating blood flow thru the capillaries?

Constriction of the arterioles & precapillary sphincters.

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What is the direction of blood flow thru capillaries? Is there a pressure difference? Diameter difference?

Arterioles–>Metarterioles–>Precapillary Sphincter–>Capillaries–>Collecting Venules–>Small Venules–>Veins
There is a higher pressure in the arterial side of capillaries & a smaller diameter on this size. The difference is important for blood flow.

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How many blood cells can pass thru a capillary at a time? Why is this important?

1 at a time.
This way there is no competition b/w blood cells to exchange nutrients & gases.
Each cells gets to do its thing & then leave.

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What is the composition of true capillaries?

A single layer of endothelial cells w/ a basement membrane on the outside (not the luminal side).

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How do water soluble molecules pass thru the capillaries? How do lipid soluble molecules pass thru? How do large molecules pass thru? Any other mechanisms?

Water soluble molecules pass thru the intercellular (b/w cells) clefts in the endothelial cells.
Lipid soluble molecules pass thru by diffusion.
Large hydrophilic molecules (macromolecules) can pass thru w/ pinocytosis or vesicle formation.
Some molecules like ions pass thru w/ active transport, a pump.

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What are the 3 types of capillaries in systemic circulation & where are they found?

Continuous Capillary: most common form. very small, small holes. Have intercellular jcns.
Fenestrated Capillary: have fenestrae (larger holes that allow for movement of substances).
Found in kidney glomeruli, exocrine glands, Small intestine
Sinusoidal Capillary/Discontinuous: Have large fenestrations & gap jcns (allow movement of proteins). Found in the liver, bone marrow, & spleen.

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What is the most common pathway to get molecules across a capillary?

Diffusion. Lipid soluble molecules down their conc’n gradient. Explains why O2 & CO2 can move as they do.

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How is glucose transported in most systemic capillaries?

Diffusion thru the endothelial pores or clefts. In some systems (like brain) active transport.

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How are steroid hormones transported across capillaries?

By diffusion across the plasma membrane

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How are the meds for anesthesia transported across capillaries?

By diffusion across the plasma membrane

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How are amino acids transported across capillaries?

By diffusion thru endothelial pores or clefts.

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What is hydrostatic pressure? How does it relate to capillaries?

the pressure of the weight of the blood. Equivalent to blood pressure.
Hydrostatic pressure is always greater in the lumen of the capillary than in the interstitial fluid. This allows for movement of substances out of the endothelial pores.

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What does arteriolar constriction do to the hydrostatic pressure in the capillaries? In the arterioles?
What does this do to the net filtration, or reabsorption?

Arteriolar Constriction
Hydrostatic Pressure larger in the arterioles
Less blood flow to the capillaries
Hydrostatic Pressure smaller in the capillaries
Higher net reabsorption.
More interstitial fluid makes it into the vasculature.

What does arteriolar dilation do to the hydrostatic pressure in the capillaries? In the arterioles?
What does this do to the net filtration or reabsorption?

Arteriolar Dilation
Hydrostatic pressure smaller in the arterioles
More blood flow to the capillaries
Hydrostatic pressure higher in the capillaries
Higher net filtration
More fluid from vasculature will go into the interstitial space.

What is colloid osmotic pressure in capillaries?

It is the osmotic pressure created by the presence of large proteins in the blood. If there are large proteins in the blood, it makes the fluid from the interstitial space want to enter the vasculature. Large proteins like albumin contribute to this.
Normal Osmotic Pressure for the capillaries is about 25 mmHg.

If there is a significant oncotic pressure in the capillaries (about 25 mmHg normally) why don’t the capillaries continually take up fluid from the interstitial space?

B/c the oncotic pressure is opposed by the hydrostatic pressure.

At which portions of the capillaries does filtration & reabsorption take place? What determines when & where this happens?

Near the arteriolar end of the capillary there is net filtration b/c the hydrostatic pressure exceeds the constant oncotic pressure (b/c the arteriolar diameter is smaller). Here: there will be fluid & molecules pushed out of the capillaries into the interstitial fluid.
Near the venous end of the capillary there is net reabsorption b/c the hydrostatic pressure is less than the constant oncotic pressure (b/c the venous diameter is larger). Here: there will be fluid that is brought from the interstitial fluid back into the vasculature.

How does the model of net filtration & reabsorption in the capillary differ in the kidneys & the lungs?

In the kidneys: there is always net filtration b/c the hydrostatic (blood) pressure always exceeds the oncotic pressure of 25 mmHg.
In the lungs: there is always net reabsorption b/c the hydrostatic (blood) pressure is always below the oncotic pressure of 25 mmHg. this is good in the lungs b/c otherwise you would get fluid accumulation there. Bad.

What is the Starling equation for the capillaries?

Jv=Kf [(Pc-Pi) - (pic-pii)]
Jv: fluid movement or net driving force
Kf is a constant
Pc (pressure in the capillary)
Pi (pressure in the interstitial fluid)
PiC (oncotic pressure in the capillary)
PiI (oncotic pressure in the interstitial fluid)
Filtration if Jv>0
Reabsorption if Jv<0

What are the 2 filtration forces and the 2 absorption forces at the level of the capillary?

Filtration Forces: High capillary pressure High Interstitial Osmotic Pressure Absorption Forces: High interstitial pressure High Capillary oncotic pressure

Can systemic arterial blood pressure increase alter transcapillary water movement?

Yes. Higher BP can cause slight increase in blood vessel diameter & increase blood flow & this will increase capillary hydrostatic pressure. This will increase filtration.

What is a corrective capillary mechanism for low blood pressure?

``` Low blood pressure. Lower blood volume in the capillaries. This causes more reabsorption. This raises blood volume. Corrective Mechanism: raises blood pressure. ```

Thinking of capillaries, what can cause edema generally?

Increased Filtration.

What are 3 general forces that can cause the increased filtration that causes edema?

``` Increased capillary hydrostatic pressure Reduced capillary oncotic pressure Increased Kf (usu a constant, means that the walls have become more leaky) ```

What are some things that can cause an increase in the capillary hydrostatic pressure such that edema may result?

``` Arteriolar Dilation Venous Constriction Increased Venous Pressure Standing Heart Failure b/c of the increased blood volume. ```

What are some things that can cause a decrease in capillary oncotic pressure such that edema may result?

``` Reduced proteins basically. Hypoalbuminemia Severe liver disease Nephrotic syndrome protein malnutrition ```

What are some things that can decrease the constant Kf, such that the capillaries are more leaky & edema may result?

``` Burn Trauma Sepsis (whole body inflammation caused by severe infection) Allerigic Rxn Inflammation ```

What causes the distended bellies in children in poor situations, situations of malnutrition?

``` Protein malnutrition. Decreased Capillary Oncotic Pressure More filtration Edema Ascites ```

What happens to the fluid that is filtered out of the capillaries and goes to the interstitial fluid?

``` About 20 L per day is filtered out. 16-18 L per day is reabsorbed back in. 2-4 L per day is absorbed from the interstitial fluid into the lymph system...which eventually drops back into the circulatory system. Overall: Filtration=Reabsorption + Lymph Flow ```

Describe the lymphatic capillaries.

They are extremely thin-walled. Freely permeable to proteins etc. b/c of large gaps in their endothelial layer. Have one-way valves. Have smooth muscle surrounding the vessel once you get to the level of the collecting lymphatic.

How are the lymphatic capillaries anchored to the surrounding tissues?

By anchoring filaments attached to surrounding fibrous matrix.

Which factors affect venous return?

Peristaltic contractions & skeletal muscle movements one way valves Respiration which decreases intrapleural pressure

What is lymphedema?

It is edema--accumulation of fluid in the interstitial space from a lymphatic obstruction. This means that the 2-4 L/day that the lymphatic system is responsible for circulating just stays in the interstitial tissue.

Which organ lacks a lymphatic circulation?

The brain!!

How is the blood brain barrier different than a normal capillary structure? Which types of substances have a hard time/easy time crossing?

No clefts or pores. Very difficult for hydrophilic substances to cross over. Tight Jcns in the epithelial cells. Thick basement membrane Foot processes of the astrocytes (glial cells) forms almost another membrane/barrier. Still...hydrophobic substances (alcohol) can cross relatively easily.

Aside from the relatively easy diffusion of hydrophobic substances across the BBB...how do you get important molecules like glucose across?

Glucose & neutral AA like phenylalanine must cross the BBB thru active transport.

What is the general idea of metabolic BBB & what is its clinical implication?

Basically...could maybe treat Parkinson's w/ the inactive & hydrophobic form of dopamine. Once inside the BBB, enzymes could in a controlled & regulated manner convert it into the active dopamine form.

What are some things that can break down or compromise the BBB?

Infection or injury can break it down. Tumors have their own vascularization that is normal/leaky...so that can allow bad things to cross the BBB. A sudden increase in BP can compromise the BBB.