Capillaris and Fluid Exchange

Capillary Exchange

• two way movement of fluid

• process of transfer between bloodstream and tissue fluid through walls of capillary blood vessels

How are chemicals passed through capillary walls

1. endothelial in cell cytoplasm

2. intercellular clefts 

3. filtration pres of fenestrated capilaries

What are the mechanisms of movement through the capillary walls

• diffusion

• transcytosis

• filtration

• reabsorption

Diffusion

• most important mechanism of exchange of water and solutes

What substances diffuse out of the blood

• oxygen and glucose

what substances diffuse into the blood

• carbon dioxide and waste

• they are more concentrated in tissue fluid, so they diffuse into blood

What substances cant pass through via diffusion 

• substances insoluble in lipids

  • glucose

  • electrolytes 

  • large molecules such as proteins 

What substances can pass through via diffusion 

• lipid soluble substances

  • steroid hormones

  • O2

  • CO2

Transcytosis

• process where endothelial cells pick up material on one side and discharge material on other side via exocytosis 

What can pass through transcytosis

• fatty acids, albumin, antibodies, and some hormones

What drives capillary fluid exchange

• equilibrium between filtration and osmosis 

Hydrostatic Pressure

• physical force exerted by a liquid against a surface such as a capillary wall 

What does the negative value for hydrostatic pressure of interstitial fluid mean?

• means there is a slight suction 

• draws fluid OUT of capillary 

Colloid osmotic pressure (COP)

• portion of the osmotic pressure of a body fluid that is due to protein 

Oncotic Pressure

• difference between COP of blood and tissue fluid 

• opposes hydrostatic pressure because it tends to draw water INTO the capillary 

Net Filtration Pressure

• Hydrostatic Pressure - Oncotic Pressure

Solvent Drag

• as water crosses the capillary walls via filtration and reabsorption, chemicals disolved in water are dragged with it 

Kidney capillary network

• no reabsorption

• only filtration

Alveolar Capillary Network 

• only absorption 

How does capillary activity vary from moment to moment? 

• in resting tissues sphincters are constricted and capilaries collapse

• when tissues become more active, capillary pressure rises and filtration overrides reabsorption 

What chemical influences impact capillary permeability 

• substance P

• bradykinin

• histamine

• increase permeability and filtration 

Edema

• accumulation of excess fluid in tissues

Causes of Edema

• increased capillary filtration 

• reduced capillary reabsorption

• obstructed lymphatic drainage

What are consequences of Edema

• impaired oxygen delivery and waste removal 

• suffocation in pulmonary edema

• drop in blood volume/ pressure that can cause circulatory shock 

What can increase capillary filtration 

• kidney failure

• histamine production

• old age

• poor venous return

• failure of right ventricle of heart

What causes reduced capillary reabsorption

• deficiency of albumin 

• liver diseases

• dietary protein deficiency

• hypoproteinemia 

• severe burns 

How can obstructed lymphatic drainage cause edema

• surgical removal of lymph and obstruction can interfere with fluid drainage and an accumulation of tissue fluid distal to the obstruction