The Capillary Flashcards
1
Q
Blood Fluid Compartments
- Blood concentrations
- To reach the intracellular compartment from plasma
A
- Blood concentrations
- Intracellular fluid > interstitial fluid > plasma
- To reach the intracellular compartment from plasma
- Molecules must diffuse across the capillary plasma membrane into the interstitial compartment
- Molecules then diffuse across the cell membrane into the intracellular fluid
2
Q
Capillaries
- Composition
- Pericytes
- Diffusion
- Aquaporins
- Transcytosis
- Fenestrated capillaries
A
- Capilalry composition
- Single layer of endothelial cells
- Basement membrane
- Pericytes
- Small intercellular clefts that separate endothelial cells
- Elongated, highly branched cells tha tform a meshlike layer b/n endothelium & interstitial fluid
- Contribute to restricting capillary permeability
- Exchange of materials through capillaries occurs through diffusion
- Small, uncharged, lipid-soluble molecules (O2, CO2) pass easily through the capillayr wall
- Large, charged molecules pass through intercellular clefts or via vesicular transport
- Aquaporins
- Water crosses the capillary through intercellular channels & through these specialized water channels in the endothelial cell membrane
- Trancytosis
- Large moleucles have a difficult time escaping from capillaries
- Some capillaries have receptors for particular proteins
- Once the protein binds, it’s carried across the membrane via this process
- Fenestrated capillaries
- In some organs, large proteins need to enter or leave circulation
- Fenestrations (large pores) facilitate this exchange
- Ex. intestine
- Contains fenestrated capillaries so that large molecules can be moved from teh GI tract to the bloodstream
- Ex. blood marrow & spleen
- Endothelial cells are discontinuous to permit RBCs to enter the circulation
3
Q
Osmolarity
A
- Measure of the absolute concentration of osmotically active particles
- A solution of 1 mole/L of non-dissociable solute = 1 osmole/L (1 Osm)
- Normal osmolarity of body fluids: 300 mOsms
- Osmotic pressure = 5107 mmHg (large)
-
Protein: only molecular species for which there’s a significant concentration difference b/n the plasma & interstitial fluid
- All other solutes (Na+, Cl-, HCO3-) are present in ~ equivalent concentrations on both sides of the membrane
- Differences in protein concentrations = only osmotic driving force at the capillary level
-
Oncotic pressure: osmotic pressure generated by protein = 25-28 mmHg
- This difference b/n capillary plams & interstitial fluid draws fluid INTO the capillary
4
Q
Hydrostatic Pressure
A
- Pressure in the blood imparted by the contraction of the ventricle
- Higher in the capillary than in the interstitial fluid
- Forces fluid OUT of the capillary
- Balance b/n oncotic pressure & hydrostatic pressure across the capillary
- Determines whether there’s a net gain or loss of fluid across the vessel
- Expressed quantitatively via Starling’s equation
5
Q
Starling’s Equation
- Equation
- Jv
- Kf
- σ
- Other factors
- Restatement of equation
A
- Jv = Kf * ( [Pc - Pi] - σ [πc - πi] )
- Jv = net fluid movement b/n compartments
- Filtration: Jv is positive, fluid leaves the capillary
- Absorption: Jv is negative, fluid enters the capillary
- Kf = filtration coefficient
- High: high capillary permeability
- Low: low capillary permeability
- Product of capillary surface area & capillary hydraulic conductance
- Usually constant
- In the kidney
- Number of aquaporin channesl in the nephron is dependent on vasopressin levels
- Low vasopresson –> few squaporins –> low Kf
- σ = reflection coefficient
- Correction factor
- Corrects for ineffectiveness of the oncotic pressure gradient
- Difference in oncotic pressures contributes to the net driving force
- Capillaries are fairly impermeable to the large molecular weight proteins
- Smaller proteins can leak across the membrane through intercellular clefts –> decreases driving force
- Usually constant from 0 to 1
- Non-fenestrated vessels: 1
- Fenestrated capillaries: lower
- Correction factor
- Forces that contribute to the net driving force
- Pc = capillary hydrostatic pressure
- Pi = interstitial hydrostatic pressure
- πc = capillary oncotic pressure
- πi = interstitial oncotic pressure
- Jv = (Pc - Pi) - (πc - πi)
6
Q
Significance of Starling’s Equation
- Arterial vs. venule end
- Net filtration
- Effect of hematocrit on plasma oncotic pressure
A
- Arterial end of a capillary
- Jv = +8mmHg –> fluid will leave the capillary –> filtration
- As blood passes along a capillary, it loses hydrostatic pressure due to friction
- Venule end of a capillary
- Jv = -7mmHg –> fluid will enter the capillary –> absorption
- Net filtration = 1mmHg
- Lose 2-4 L of fluid per day into interstitial space due to the filtration-absorption imbalance
- Excess fluid lost into the interstitial space is collected by the lymphatic system
- Changing hematocrit doesn’t affect plasma oncotic pressure
- Blood cells have plasma membranes that place their contents in a separate osmotic compartment from the plasma
7
Q
Factors that alter osmotic relationships
- Increased venous pressure
- Hypoproteinemia
- Increased capillary permeability
- Decreased arterial pressure
A
- Increased venous pressure
- Increase venous (or arterial) pressure –> increase capillary pressure
- Greater impact from increased venous pressure b/c resistance b/n arteries & capillaries > resistance b/n veins & capillaries
- Facilitates movement of fluid out of capillaries (filtration)
- Ex. stand for a long time
- Increase venous pressure in feet
- Additional blood is present in capillary
- Raises capillary pressure
- Increases filtration
- Accumulation of fluid in interstitial space –> edema
- Increase venous (or arterial) pressure –> increase capillary pressure
- Hypoproteinemia
- Starvation, liver disease (protein metabolism deficits), & kidney disease –> protein lost in urine
- Decreased plasma & interstitial protein –> decreased πc & πi
- Increased filtration –> edema
- Increased capillary permeability
- Releae of histamine from mast cells –> expand gaps b/n endothelial cells in capillary wall
- Increase Kf –> increase net fluid movement
- Fluid accumulates in interstitial space –> edema
- Decreased arterial pressure
- Hemorrhage –> decreased blood volume –> decreased arterial pressure
- Baroreceptor reflex –> vasoconstriction to return BP to normal
- Decreased pressure in downstream capillaries
- Absorption of fluid from interstitial space
- Over time, increases blood volume –> increases BP
- Diluted concentration of RBCs –> decreased hematocrit
8
Q
Lymphatic System
- Functions
- Lymph capillaries
- Throacic duct
- Where lymphatic system makes connections w/ the CV system
A
- Functions
- Filtration > absorption in capillaries –> net loss of fluid into interstitial space
- Lymphatic system collects this fluid & returns it to circulation
- Picks up materials in the liver & intestine
- Filter: captures & destorys foreign pathogens
- Filtration > absorption in capillaries –> net loss of fluid into interstitial space
- Lymph capillaries
- Close to real capillaries in all tissues except CNS & kidney
- Thin walls of lymph capillaries are held open by attachments to surrounding cells
- Adjacent cells overlap in lymph capillaires –> provide valves that allow materials to enter (but not leave) interstitial space
- Coalesce to form larger collecting lymphatics
- Thoracic duct
- Largest collecting lymphatic
- Empties fluid from entire lower body back into the cardiovascular system
- Where lymphatic system makes connections w/ the CV system
- Near collarbones
- Near junction b/n subclavian veins & internal jugular veins
9
Q
Lymphatic System
- Major force driving fluid to enter lymph capillaries
- Factors that increase fluid flow into lymph capillaries
A
- Major force driving fluid to enter lymph capillaries
- Interstitial fluid pressure
- Lymph: name for pressure once fluid is inside the lymphatic system
- Increase insterstitial fluid pressure –> fluid enters lymph capillaries –> assures all fluid leaving the CV system is returned
- Interstitial fluid pressure
- Factors that increase fluid flow into lymph capillaires (increase interstitial fluid pressure)
- Increase capillary pressure –> increase filtration
- Decrease plasma oncotic pressure –> decrease absorption
- Increaes interstitial fluid protein –> decrease absorption
- Increase capillary permeability
10
Q
Lymphatic System
- Movement of large molecules
- Forces inducing lymph movement
A
- Large molecules easily enter lymph capillaries in the liver & digestive system to allow large molecules to enter circulation
- Pathogens in interstitial fluid also enter the lymphatic system
- Lymph passes through immunologically-active lymph nodes
- Forces inducing lymph movement
- Compression of lymph vessels during muscular contraction
- Valves in large lymph vessels permit only unidirectional fluid
- Largest lymph vessels have smooth muscle in the walls that contract automatically when stretched
11
Q
Edema & Elephantiasis
A
- Edema
- When interstitial fluid builds up
- Often happens after injury
- Breakage of blood vessels –> increased protein in interstitial space –> pulls fluid from CV system
- Damaged lymph capillaries –> increased interstitial pressure doesn’t lead to increased lymphatic drainage
- Elephantiasis
- Lymph vessels blocked by parasites –> compromised fluid drainage
- Increased protein concentration –> decreased oncotic pressure gradient –> decreased absorption
12
Q
Ramifications of the actions of the lymphatic system
A
- Hydrostatic pressure in the interstitial space is slightly negative
- Due to the presence of the lymphatic system & the movement of fluid into this system
- As long as the interstitial pressure is negative, no edema will occur
- Lymphatic system is a conduit for protein to leave the interstitial space
- W/o the lymphatic system, the oncotic gradient b/n the plasma & interstitial space would equalize
- Would contribute to the develpoment of edema
13
Q
Changes in plasma osmolarity and cell size
- Normal capillary permeability
- Effect of intravenous hypertonic saline solution
- Effect of intravenous hypotonic saline solution
A
- Most capillaries are highly permeable to small ions (Na+, Cl-, water)
- Normally selective about the ions that enter
- Maintain low [Na]i through the Na+/K+ ATPase pump
- Freely permeable to water
- Effect of intravenous hypertonic saline solution
- Add ions to intravascular space
- Increase osmolarity of plasma relative to interstitial space
- Na+ & Cl- diffuse into the interstitial space while water diffuses into capillaries to balance osmolarity
- Lose water (by osmosis) from the interstitial space
- Na+ & Cl- diffuse into the intracellular space while water diffuses into the interstitial space to balance osmolarity
- Lose water (by osmosis) from the intracellular space
- Intracellular volume decreases –> cells shrink
- Given to patients to combat edema to lose fluid from the interstitial & intracellular spaces
- Effect of intravenous hypotonic saline solution
- Causes plasma osmolarity < interstitial osmolarity
- Na+ & Cl- diffuse into capillaries while water diffuses into the interstitial space
- Water moves into the interstitial space
- Na+ & Cl- diffuse into the interstitial space while water diffuses into the intracellular space
- Water moves into the intracellular space
- Intracellular volume increases –> cells expand/swell
- Problem: RBCs can swell & burst when plasma osmolarity is suddenly decreased via a hypotonic solution
14
Q
Osmotic relationships in lung capillaries
A
- Pressure in lung capillaries < systemic capillaries
- Interstitial hydrostatic pressure = 0
- Plasma protein content doesn’t change b/n systemic & pulmonary circulation, so πc is the same
- Differences
- Interstitial plasma concentration in the lung (πi) is high
- Rate of water movement through the pulmonary interstitial space is low
- Pulmonary Jv = 4 mmHg
- Fluid is lost in the lung into the interstitial space
- Little fluid remains in the interstitial space due to the lymphatic system
- Lympahtic system can clear the fluid from the interstitial space as long as capillary hydrostatic pressure remains less than 25 mmHg