Unit 2 - Blood Flow & Blood Pressure PART F Flashcards
Movement of material across the capillary wall.
- Protein channels (intracellular channels) in endothelial cell membranes and intercellular pores between adjacent cells (paracellular pathway)
- Serves two purposes
- Starling forces
- Net Filtration Pressure
- Net colloid osmotic pressure
Movement of material across the capillary wall.
1. Protein channels (INTRACELLULAR CHANNELS) in endothelial cell membranes and intercellular pores between adjacent cells (PARACELLULAR PATHWAY) are:
a. EXCHANGE SITES FOR H2O, Na+, K+, GLUCOSE, AMINO ACIDS
b. mostly impermeable to macromolecules:
i. E.g. PLASMA PROTEINS (e.g. ALBUMIN) are so large they remain in capillary
ii. Any larger proteins that can be exchanged do so using transcytosis
Transcytosis
= endocytosis into endothelial cell from plasma/ISF, transport across cell, ending with exocytosis from cell into the ISF/plasma.
Tissues with a ↑ metabolic rate req. more O2 & nutrients. Therefore,
have MORE capillaries per unit area
- subcutaneous tissue & cartilage have the LOWEST capillary density
- muscles & glands have the HIGHEST
Movement of material across the capillary wall.
2. Serves two purposes
What are they?
a. Exchange of material between the blood and the cells.
b. Maintain fluid balance between plasma and interstitial fluid.
Movement of material across the capillary wall.
- Serves two purposes
a. Exchange of material between the blood and the cells.
- Rapid DIFFUSION of small solutes across capillary walls –> i.e. down PARTIAL PRESSURE, electrochemical, and concentration gradients
- Certain proteins are selectively transported across endothelial cells by a slow, energy-requiring process (TRANSCYTOSIS)
Movement of material across the capillary wall.
- Serves two purposes
b. Maintain fluid balance between plasma and interstitial fluid.
- Distribution of ECF between plasma and ISF in a state of dynamic equilibrium due to FILTRATION and ABSORPTION of fluid by capillaries
- Via the BULK FLOW of protein-free plasma through channels and pores driven by HYDROSTATIC and COLLOID OSMOTIC PRESSURE gradients (termed ‘STARLING FORCES’) that augment diffusion processes
Filtration
the fluid movement, if the direction of flow is OUT of the capillaries
- caused by hydrostatic pressure that forces fluid out of the capillary through leaky cell junctions
ANALOGY: think of garden “soaker” hoses whose perforated walls allow water to ooze out
Absorption
the fluid movement, if the direction of bulk flow is INTO the capillaries
Bulk Flow
mass movement of fluid as the result of hydrostatic or osmotic pressure gradients
Hydrostatic Pressure
lateral pressure component of BF that pushes fluid out through the capillary pores
Colloid Osmotic Pressure
a measure of the osmotic pressure created by proteins
- higher in the plasma (πcap = 25 mm Hg) than in the interstitial fluid (πIF - mm Hg)
- therefore, the osmotic gradient favours water movement by osmosis from the interstitial fluid into the plasma
- constant along the length of the capillary, π = 25 mm Hg
Osmotic pressure
is determined by solute concentration of a compartment
- the main solute diff. b/t plasma & interstitial fluid is due to proteins
- the osmotic pressure created by the presence of these proteins is known as colloid osmotic pressure (π)
Most capillaries show a transition from net _____ at the arterial end to net ______ at the venous end
filtration
absorption
Movement of material across the capillary wall.
3. Starling forces
a. PC = capillary hydrostatic pressure (HP)
b. PI = interstitial fluid hydrostatic pressure
c. πC = plasma colloid-osmotic pressure
d. πI = colloid-osmotic pressure of interstitial fluid
PC = capillary hydrostatic pressure (HP)
- HP inside capillary favouring FILTRATION of plasma
- declines as blood moves from arteriole (~37 mm Hg) to venule side (~17 mm Hg) of capillary (as energy is lost to friction)
PI = interstitial fluid hydrostatic pressure
- HP outside capillary favouring ABSORPTION of ISF
- ~1 mm Hg
πC = plasma colloid-osmotic pressure
- osmotic pressure within capillary due to NONPENETRATING solutes that favours ABSORPTION of interstitial fluid
- ~25 mm Hg
πI = colloid-osmotic pressure of interstitial fluid
- osmotic pressure due to nonpenetrating solutes within the
ISF that favours plasma filtration - ~0 mm Hg –> means that water movement due to hydrostatic pressure is directed out of the capillary, with the pressure gradient decreasing from the arterial end to the venous end
Movement of material across the capillary wall.
4. Net Filtration Pressure
a. PC - PI = AT ANY GIVEN POINT ALONG THE CAPILLARY.
- Force favouring plasma FILTRATION along entire length of
capillary
- = 37 – 1 = 36 mmHg
Movement of material across the capillary wall.
- Net colloid osmotic pressure
a. πC – πI.
- force favouring ABSORPTION of ISF along entire length of capillary
- = 25 – 0 = 25 mmHg
Net fluid movement at any given point in capillary:
= Net P – Net π
= 36 mmHg – 25 mmHg = +11 mmHg (filtration)
= 16 mmHg – 25 mmHg = -9 mmHg (absorption)
Positive values indicate _______, negative values indicate ________
FILTRATION
ABSORPTION
If the point at which filtration =’s absorption occurred in the middle of the capillary, there would be…
NO net movement of fluid
- all volume that was filtered at the arterial end would be absorbed at the venous end
- however, filtration is usually greater than absorption, resulting in bulk flow of fluid out of the capillary into the interstitial space
If filtered fluid couldn’t be returned to the plasma, the blood would turn into…
a sludge of blood cells & plasma
