Capillaries Flashcards
Capillary Structure
- single layer
- extensively branched
- low blood volume (large SA)
- slow velocity
Capillary Function
Sites for exchange
Capillary Structure is
ideal for exchange
Capillary density
tissue dependent
- skeletal m. and heart are capillary dense
- cartilage/fat = low capillary density
Capillaries also do..
nutritional/non-nutritional flow
heat removal
Capillary wall structure
consists of endothelium & basement membrane. thin walls & small lumen diamenter
optimal for exchange
Capillary permeability
varies depending on organ/tissue supplied
Types of capillary permeability
- Continuous
- Fenestrated
- Sinusoidal
Continuous capillary location
brain (NO pores)
muscle
adipose
lymph nodes
Fenestrated capillary location
kidney
intestines
colon
exocrine glands
Sinusoidal capillary location
liver
spleen
pituitary gland
bone marrow
Capillary blood flow controlled by…
- Arterioles
- Metarterioles
- Precapillary sphincters
Metarterioles
can bypass system if capillary bed doesn’t need blood OR permit it to flow
precapillary sphincters
- appear to be innervated sympathetcially
- high degree of myogenic tone
- sensitive to metabolic factors
Transcapillary exchange
processes of solvent & solute movement across capillary walls
Types of transcapillary exchange
- Diffusion
- Filtration/Absorption
- Pinocytosis
Diffusion
movement from high to low concentration
types of transcapillary exchange diffusion
- transcellular
- paracellular
FIltration/Absorption (bulk flow)
volume of protein-free fluid filtered out of capillary into ISF or absorbed into capillary from ISF
Purpose of Filtration/Absorption transcapillary exchange
keep plasma volume at normal level
Pinocytosis
vesicular transport
least utilized mechanism of trancapillary exchange
Things affecting transcapillary exchange
- Permeability
- Surface area
- Concentration gradient
Permeability coefficient
decreases as molecular radius increases
Lipid soluble substance diffuse by
transcellular route
Small water soluble solutes diffuse by
paracellular route
Capillary exchange of water occurs as
bulk flow
important in ECF distribution
Driving forces for fluid movement
- Hydrostatic pressures
2. Colloid osmotic or oncotic pressures
hydrostatic pressures
exerted by fluid across capillary wall
colloid osmotic/oncotic pressure
exerted by proteins across capillary wall
Fluid filtration/reabsorption dictated by
hydrostatic & oncotic pressures
Capillary pressure
pushes fluid out of capillary to decrease total plamsa volume
interstitial fluid pressure
pushes fluid into capillaries
plasma colloid osmotic pressure
pulls H2O into capillary via osmosis
Interstitial fluid colloid osmotic pressure
pulls H2O out of vascular bed
movement depends on
which pressure gradient is the highest
What determines if fluid leaves capillary beds?
hydrostatic pressure gradient is higher than colloid osmotic pressure gradient
What determines if fluid enters vascular space?
Hydrostatic pressure gradient is less than colloid osmotic pressure gradient
Absorption in capillary beds
occurs when capillary beds are in series because 2nd capillary bed has decreased hydrostatic pressure due to loss of fluid in 1st capillary
Hemodynamic forces affect
capillary hydrostatic pressure & direction/magnitude of transcapillary fluid movement
Hemodynamic forces do
^ arterial or venous pressure
^ arterial resistance
^ venous resistance
arterial resistance is
a principal determinant of capillary hydrostatic pressure
Pinocytosis
vesicular transport of MACROMOLECULES across capillary wall
Caveolae
- coated pits in capillary wall
- may form channels through cell for transport across membrane
Limits to pinocytosis
molecular radius
-molecules must fit into caveolae
Another name for pinocytosis
Transcytosis
High metabolic demand induces
capillary recruitment
When metabolic demand increases
- more capillaries open
- smaller tissue area is supplied per capillary
Capillaries serve __ amount of tissue
can be larger or smaller depending on metabolic demand
Capillary recruitment
- alternating areas of tissues have open/closed capillaries constantly.
- even under resting conditions, capillaries alternate opening and closing
Better blood supply given
when more capillaries are open
Molecular transport limited by..
blood flow to the area or diffusion from capillary to tissue/cells
Limitations in molecule transport can be overcome by
increasing capillary density
