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Physiology > Microcirculation and Lymphatics (B2: W2) > Flashcards

Microcirculation and Lymphatics (B2: W2) Flashcards

1
Q

How does branching occur typically from arterioles/venules to capillaries?

A

From first order arteriole and vein, down to 2nd, 3rd, and 4th

Then cappilaries

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2
Q

How long does the blood remain in the capillaries for exchange?

A

1 to 3 seconds

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3
Q

What percentage of capillaries are closed under normal circumstances?

A

75% are closed, and 25% are open

When different organs and tissues need more blood, capillary recruitment opens more

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4
Q

Do true capillaries have smooth muscle walls?

A

No

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5
Q

How long are capillaries?

A

0.3 to 1 mm long

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6
Q

What is the proximal and distal arrangement of a capillary bed?

A
  • Proximally: 10-100 capillaries, usually supplied by a single arteriole or metarteriole
  • Distally: capillaries transition to venules or drain into a thoroughfarechannel/preferential channel (continuation of a metarteriole)
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7
Q

What regulates blood flow through the capillaries?

A

Contraction and relaxation of the arterioles, and the precapillary sphincters

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8
Q

How are the precapillary sphincters regulated?

A

Sensitive to local factors and local conditions in the tissue

  • Concentrations of oxygen and nutrients
  • End products of metabolism
  • H+
  • etc…
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9
Q

What is the perfusion in the capillaries when the precapillary sphincters are open compared to when they are closed?

A
  • Sphincters open - capillaries well perfused
  • Sphincters closed - little to no blood flow occurs
    • Blood takes a shortcut and goes straight to venule
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10
Q

How many blood cells are able to pass through a single capillary at a particular point of time? Why does this matter?

A

One RBC passes through at a time

Important for exchange between capillaries and tissues

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11
Q

What is the composition of the capillary wall?

A
  • Unicellular layer of endothelial cells
    • Have some plasmalemmal vesicles
  • Basement membrane
  • Total thickness: 0.5 micrometer
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12
Q

How do most water-soluble substances diffuse through the capillary membrane?

A

Along intercellular clefts

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13
Q

Where are fenestrated capillaries found?

A

Small intestine

Exocrine glands

Renal glomeruli

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14
Q

Where are sinusoidal (discontinuous) capillaries found?

A

Liver

Spleen

Bone marrow

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15
Q

What are the most common form of capillaries?

A

Continuous capillaries

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16
Q

Where does exchange take place between a blood capillary and a lymphatic capillary?

A

In the interstitial space

There is always some exchange going on here

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17
Q

What are the 4 mechanisms of exchange across the capillary wall?

A
  • Diffusion: substances must be lipid-soluble (or gasses)
  • Bulk flow: intercellular clefts let water-soluble molecules through (H2O, electrolytes, glucose, amino acids)
  • Vesicular transport: larger molecules can be engulfed and transported through for release on the other side
  • Active transport: ions and small molecules can go against their concentration gradient
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18
Q

Which is the most important means for transfer of substances between the plasma membrane of the capillaries and the interstitial fluid?

A

Diffusion

(from high to low concentration)

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19
Q

What alters bulk flow across a capillary?

A

Changes in pressure gradient - either hydrostatic or colloid osmotic

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20
Q

Explain pinocytosis/transcytosis

A
  • Endothelial cells pick up matherial on one side of their membrane by pinocytosis or receptor-mediated endocytosis
  • Transport vesicles across cell
  • Discharge material on other side by exocytosis
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21
Q

What substances are transported via pinocytosis?

A
  • Fatty acids
  • Albumin
  • Some hormones
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22
Q

What will happen to the pressure and flow in the capillaries during arteriolar constriction? And during arteriolar dilation?

A

Arteriolar constriction reduces the hydrostatic pressure in the capillaries

  • Forces will exceed filtration forces and net reabsorption of interstitial fluid into the vascular bed will occur

Conversely, arteriolar dilation increases the hydrostatic pressure in capillaries

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23
Q

Which blood protein determines colloid osmotic pressure in the capillaries?

A

Albumin

24
Q

What opposes the osmotic (oncotic) pressure generated by the blood proteins?

A

The capillary blood (hydrostatic) pressure

25
Q

What is the result of the higher hydrostatic pressure on the arteriole side of the capillaries and the lower hydrostatic pressure on the venule side?

A

Opposing forces

  • High hydrostatic pressure on arteriole side pushes fluid out of capillaries
    • Net filtration
  • High colloid capillary pressure on venule side draws fluid into capillary
    • Net reabsorption
26
Q

In what situation is there continuous filtration in the capillaries?

A

Kidney!

  • Blood pressure is always a little high
    • Exceeds osmotic pressure
  • Leads to continuous filtration
27
Q

In what situation is there continuous reabsorption in the capillaries?

A

Lungs!

  • Osmotic pressure is higher
    • Pressure in capillary bed is low
  • Leads to continuous reabsorption
    • Picks up oxygen
28
Q

What is the Starling equation?

A

Jv = Kf[(Pc - Pi) - (πc - πi)]

Jv = fluid movement, net driving force (NDF)

Kf = hydraulic conductance

Pc = capillary hydrostatic pressue

Pi = interstitial hydrostatic pressure

πc = capillary oncotic pressure

πi = interstitial oncotic pressure

29
Q

Can systemic arterial blood pressure change transcapillary water movement?

A

YES

Increased blood volume by reabsorption is one way to “correct” low arterial blood pressure

30
Q

What are the causes of edema?

A
  • Increased Pc (capillary hydrostatic pressure)
  • Decreased πc (capillary oncotic pressure)
  • Increased Kf (hydraulic conductance)
31
Q

Kwashiorkor is an example of edema. What is the mechanism?

A

There is a decreased osmotic pressure

Due to lack of protein

32
Q

How much of the fluid that is filtered during a day is reabsorbed by the capillaries, and how much is absorbed into the lymph?

A
  • Capillaries reabsorb about 85% of the fluid they filter
    • 18 L out of 20 L
  • The remaining 15% is absorbed by the lymphatic system and returned to the blood
    • 2-4 L a day
33
Q

What is the role of the lymphatic system in maintenance?

A

Critical to both hydrostatic and homeostatic maintenance

34
Q

What serves as a balance for any net differences between hydrostatic and oncotic forces?

A

Lymphatic vessels are the “safety valve” that balances forces moving fluid in and out of the capillary and in and out of the interstitial space

35
Q

Lymphatic vessels penetrate nearly every tissue of the body. Where are they absent from?

A
  • CNS
  • Cartilage
  • Cornea
  • Bone
  • Bone marrow
36
Q

What is the structure of lymphatic vessels?

A
  • Capillary wall is endothelial cells overlapping each other like roof shingles
  • Closed at one end
  • Cells tethered to surrounding tissues by protein filaments - fibrils anchored to surrounding fibrous matrix
37
Q

How do substances enter lymphatic capillaries?

A

Gaps between cells are large enough to allow bacteria and cells to enter

38
Q

How is lymph propelled back to the general circulation?

A

In a manner reminiscent of the muscular pump for venous return

39
Q

What is the function of the contraction-relaxation of lymph bulbs?

A

Removes excess water and plasma proteins from the interstitial spaces

Lymphatic pressure along the lymphatic vasculature are generated by lymphatic vessel contraction and organ movements

40
Q

What are the factors that affect lymphatic return?

A
  • Peristaltic contractions
  • One way valves
  • Movement of skeletal muscles
  • Respiratory diaphragm (decreases intrapleural pressures)
41
Q

What happens when there is too much or too little volume in the interstitial space?

A

Too little - dehydration

Too much - edema

42
Q

Would a lymphatic obstruction cause edema?

A

Yes

43
Q

What system supports interstitial solute and fluid clearance from the brain?

A

Glymphatic system

44
Q

What is the blood-brain barrier?

A

The brain’s most formidable gatekeeper

Specific case of microcirculation and capillary system

45
Q

What would happen if you administered urea into the circulation by constant IV infusion?

A
  • There would be a high concentration of urea in muscle, CSF, and spinal cord within an hour
  • Much lower concentration in the brain at the same time
    • Something is stopping urea
46
Q

What are the structural components of the blood brain barrier?

A
  • Endothelial cells
  • Very tight junctions
  • Thick basement membrange
  • Astrocyte foot processes - significant mechanical barrier
47
Q

What are the main differences betwen nonbrain systemic capillaries and brain capillaries?

A
  • Systemic
    • Leaky
    • Transcytosis
    • Thin basement membrane
  • Brain
    • Not leaky
    • Reduced transcytosis
    • Lipid soluble substances will go through capillary wall a little easier - use transport mechanisms
48
Q

What chemical property has the greatest effect on a substane’s capacity to pierce the blood brain barrier?

A

Lipid solubility

49
Q

How do some molecules needed for brain metabolism cross the barrier more readily than lipid solubility alone would suggest?

A

These compounds are carreid across the barrier by specialized transport systems

d-glucos, L-DOPA, phenylalanine

50
Q

Which molecules require active transporters to cross the blood brain barrier?

A
  • D-glucose
  • Large netural amino acids (such as phenylalanine)
51
Q

What is used in the treatment of Parkinson’s disease?

A

L-DOPA

52
Q

How does the metabolic blood brai barrier use enzyme conversion to control how much L-DOPA (or other compound) enters the brain?

A
  • L-DOPA enters and leaves the brain via transporter
  • In the capillary, L-DOPA may be converted into dopamine and then DOPAC
  • Although dopamine can leave the brain, neither dopamine nor DOPAC can cross the antiluminal membrane into the brain
53
Q

What causes the blood blain barrier to brake down?

A

Breaks down in areas of infection or injury

54
Q

How can the blood brain barrier be useful in identifying tumors?

A

Development of new blood vessels, no tight junction, and the lack of a barrier helps in identifying the location of tumors

55
Q

What can temporarily disrupt the blood brain barrier?

A

Sudden marked increases in blood pressure or by IV injection of hypertonic fluids

Physiology (29 decks)

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