Flashcards in Microcirculation and Oedema Deck (20)
What are the three layers of blood vessel walls?
Tunica intima, media and adventitia (outer layer)
Describe continuous capillaries
Least permeable (have tight junctions) and have a sealed endothelium and continuous basement membrane
Describe the two subtypes of continuous capillaries
Those with numerous transport vesicles (caveolae) - primarily found in skeletal muscle, lungs, gonads and skin AND those with few vesicles - primarily found in the CNS (BBB)
Describe fenestrated capillaries
Have small circular pores in endothelium, have continuous basement membrane and permit relatively free passage of salts and water - found in tissues specialised for bulk fluid exchange
Where are fenestrated capillaries primarily located?
Exocrine glands, intestines, pancreas and glomeruli (involved in bulk fluid exchange)
Describe sinusoidal/discontinuous capillaries
Highest permeability, have large openinings in endothelium which allow red and white blood cells and several serum proteins to pass, has a discontinuous basement membrane
Where are sinusoidal/discontinuous capillaries primarily located?
Liver, spleen and bone marrow
Describe how movement is facilitated across a capillary
Diffusion by lipid-soluble substances (oxygen, carbon dioxide and anaesthetics), movement through intercellular clefts (water-soluble substances), movement through fenestrations (water-soluble substances) and transport via vesicles or cavaeolae for large substances
How do you calculate net filtration pressure along a capillary?
(hydrostatic pressure of capillary - hydrostatic pressure of interstitial fluid) - (capillary osmotic pressure - interstitial fluid osmotic pressure)
Describe the net filtration pressure of the lungs
The NFP is positive to ensure fluid is present to keep lungs damp (involves fluid moving out of capillaries and into lung tissue)
Describe water movement from the arterial to venous end of a capillary
At the arterial end, NFP is positive, so water is forced out of capillary in filtration, whereas at the venous end the NFP is negative so water is pulled into the capillary in reabsorption.
Why is there water reabsorption at the venous end of the capillary?
Plasma protein can't leave the capillary, so they exert an osmotic pressure which tends to draw fluid into the capillary when the hydrostatic pressure is lesser
What happens to any surplus interstitial fluid?
Taken up by the lymph ducts or produces oedema, but will eventually be returned to the circulation
Describe lymphatic capillaries
Specialised vessels that are made up of an endothelium with large intracellular gaps and surrounded by a permeable basement membrane which contains one-way valves to ensure lymph only travels away from tissues
Describe the flow into the lymphatic system
Plasma flow through capillary beds --> net movement of water from capillaries into interstitial space --> net movement of fluid entering lymph capillaries --> water reabsorbed into circulation at the lymph nodes and lymph is also returned to circulation at the subclavian veins
What is oedema?
Fluid retention, it occurs where there it too much interstitial fluid in the tissues, causing swelling to occur in the affected area and can be caused by increased secretion of fluid into the interstitium or impaired removal of this fluid will cause oedema
What are the causes of oedema?
Increased secretion of fluid into the interstitium or impaired removal of this fluid
How may prolonged standing lead to oedema?
Causes increased venous pressure in the legs --> oedema in the ankles etc.
How can increased capillary permeability lead to oedema?
Allows water to flow more freely and allows protein to leave the vessel more easily --> reduces the oncotic pressure and causes more fluid to enter the interstitial space (occurs in vascular damage e.g. burns, trauma and inflammation)