capillaries

• Capillaries: exchange of gases, nutrients, wastes, hormones, etc., between blood and interstitial fluid
• capillaries supply almost every cell, except for cartilage, epithelia, cornea, and lens of eye
• capillary walls are just thin tunica intima; in smallest vessels, one cell forms entire circumference
• capillaries are Microscopic vessels; diameters so small only single RBC can pass through at a time
• All capillary endothelial cells are joined by tight junctions with little gaps called called intercellular clefts

  • allows for fluids and small solutes to pass
• continuous capillaries

  A) least permeable

  B) most common

  C) pericytes

  D) pinocytotic

  E) tight junction

  F) endothelial

  G) pinocytotic

  
• fenestrated capillary: in areas involved in active filtration( kidneys), absorptions( intestines), or endocrine hormone secretion
• sinusoidal capillaries: found only in liver, bone marrow, spleen, and adrenal medulla

  • fewer tight junction; usually has many intercellular clefts, an incomplete basement membrane and larger lumens
  • blood is sluggish which allows time for large molecules and blood cells to modify to pass between blood and tissue
  • contains macrophages in lining
• capillary bed: knit together capillaries between arterioles and venules
• microcirculation: flow of blood through bed
• Two Types of Vessels:

  Vascular shunt: channel that connects arteriole directly with venule(metarteriole-thoroughfare channel)

  true capillaries: actual vessels involved in exchange
• Vascular shunt: metarteriole-thoroughfare channel starts with

  1. terminal arteriole
  2. metarteriole: intermediate between arteriole and capillary
  3. thoroughfare channel: intermediate between capillary and venule
  4. post capillary venule: drains bed
• true capillaries: 10 to 100 exchange vessels per capillary bed

  • branch off metarteriole or terminal arteriole
  • true capillaries branch from metarteriole and return to thoroughfare channel
  • regulated by local chemical conditions and vasomotor
• precapillary sphincters: regulate blood flow into true capillaries

  • blood may go into true capillaries or to shunt
• • fenestrations: endothelial cells contain swiss cheese pores
  • allow for increased permeability
  • fenestrations usually covered with thin glycoprotein diaphragm
• capillary blood pressure

  regularly 35 mmHg at the beginning and then 17 mm Hg at the end of the bed

  • High BP would rupture fragile, thin-walled capillaries
  • capillaries are usually very permeable to filtrate would be forced out into intersitional space
• Blood flows the...

  slowest in the capillaries
• Speed is related to the function of the capillaries


  it gives adequate time for capillaries for exchange between blood and tissue
• vasomotion


  continuous flow of blood through the capillaries

  • due to the on and off opening and closing of precapillary sphincters
• molecule pass by...

  diffusion between blood and interstitial fluid

  • move down their concentration gradient
• ways that molecules pass through capillaries

  1: through fenestration

  • water soluble solutes

  2: pass right through endothelial membrane

  • lipid-soluble molecules like respiratory gases

  3: pass through clefts

  • water-soluble solutes XAEZ

  4: active transport via pinocytotic vessels or caveolae

  • larger molecules like proteins
• movement of molecules

  meow meow

  A) membrane

  B) intercellular cleft

  C) fenestrations

  D) pinocytotic

  E) caveolae

  
• Fluid is forced out...but returns at...

  clefts of capillaries of arterial end

  venous end into blood

  • determines relative fluid volumes in blood and interstitial space
• Giant influx of fluid into capillary walls causes...

  mixing of plasma and interstitial fluid

  • maintains interstitial enviroment
• hydrostatic pressure

  force exerted by fluid pushing against walls
• Capillary hydrostatic pressure

  capillary blood pressure that forces fluids out of capillary walls

  • Greater at arterial end (35 mm Hg) of bed than at venule end (17 mm Hg)
• Interstitial fluid hydrostatic pressure

  pressure that pushes fluids back into vessel

  • assumed to be zero because lymphatic vessels drain interstitial fluid
• Capillary colloid osmotic pressure (oncotic pressure, OPc)

  Sucking pressure that the non diffusible plasma proteins create pulling the water back into capillary

  OPc ~26 mm Hg
• Interstitial fluid colloid osmotic pressure (OPif)

  not needed pressure because interstitial fluid has barely any proteins

  • 1 mm Hg
• net filtration pressure

  includes all forces acting on the capillaries

  
• Net fluid flow out at arterial end (filtration)

  Net fluid flow in at venous end (reabsorption)

  • more fluid leaves out through arterial end then returned because lymphatic system returns it to the blood