Physiology II
Cardiovascular Physiology

MICROCIRCULATION AND LYMPHATICS

Key words: 

metarterioles:  "Terminal Arterioles" having a structure between that of arterioles and capillaries.  Guyton pg. 183 

capillaries:  Originate from metarterioles.  At that point of origination, a smooth muscle fiber usually encircles the capillary.  This is called the "precapillary sphincter".  This sphincter can open and close intermittently.  Capillaries are extremely thin structures with walls of a single layer of highly permeable endothelial cells.  Here interchange of nutrients and cellular excreta occurs between tissues and circulating blood.  Guyton pg. 183

capillary density:  10 billion capillaries.  Nearly all cells within 20-20 micrometers of a capillary.  Guyton pg. 183

vasomotion:  Blood does not flow continuously through capillaries-it flow intermittently, turning on and off.  The cause of this intermittent flow, called vasomotion, is intermittent contraction of metarterioles and precapillary sphincters.  Guyton pg. 184

transmural pressure:  Through or across a wall, as the wall of a hollow organ or cyst. (Melloni’s, p. 487)

critical closing pressure 

trans-capillary exchange 

endothelial cells:  A form of squamus epithelim consisting of flat cells that line the blood and lymph vessels.

fenestrations:  There are openings, pores, in the capillaries that allow certain particles to pass through.  In the brain these pores are much smaller than other organs such as the kidneys which have very large pores.  Guyton pg. 189

discontinuous endothelium 

Starling hypothesis:  Under normal conditions, a state of near equilibrium exists at the capillary membrane whereby the amount of fluid filtering outward from some capillaries equals almost exactly the quantity of fluid that is returned to the circulation by absorption through other capillaries.  The small difference in fluid is returned by the lymph.  Guyton pg. 192

Starling forces:  Kf * [(P_C - P_ISF) - ð(ñ_C - ñ_ISF)]  Capillary filtration constant, Reflection coefficient, Capillary pressure, Interstitial fluid pressure, Oncotic pressure.  The sum of forces that tend to move fluid into or out of the capillary wall (+0.3 mmHg). (Hall, p. 22)

reflection coefficient 

hydrostatic pressure:  Capillary blood pressure. (Physiology CB, p. 35) Averages 17 mmHg, tends to push fluid out of the pores of the capillaries into the interstitial fluid. (Hall, p. 21)

oncotic pressure:  (Colloid osmotic pressure) Osmotic force exerted by the proteins in plasma that tends to draw fluid back into the capillaries. Averages about 28 mmHg. (Hall, p. 21)

capillary filtration coefficient:  The product of the permeability and surface area of the capillaries (K_f). (Hall, p. 22).  6.67 mm of fluid per minute per mm/Hg for the entire body...or 0.1 ml/min/mm/Hg/10gm tissue.  Guyton pg. 192-193

capillary recruitment 

interstitial fluid:  Body fluid which is part of the extracellular fluid, which lies between the tissue cells. This amounts to more than three-fourths of the ECF. (Hall, p. 11).  Fluid that surrounds cells.  Not in the blood vessels.

edema:  A local or generalized condition in which the body tissues contain an excessive mount of tissue fluid.

lymphatic vessels:  Made up of endothelial cells attached by anchoring filaments to the surrounding connective tissue.  The cells overlap each other in such a way that particles may enter but not exit.  Guyton pg. 193

lymphatic flow:  Rate of 120 ml/hr=2-3 liters/day.  As interstitial pressure increases the lymph flow increases.  Lymph flow is also helped along by the lymph pump.  See below.  Guyton pg. 194-195

lymphatic pump:  Valves exist in all lymph channels.  The channels empty into "collecting lymphatics".  When the lymph vessel becomes stretched with fluid, the smooth muscle automatically contracts moving the fluid along.  There are also other intrinsic "pressures" that help "pump" lymph along...such as body muscles, movement of body parts, arterial pulsations etc.  Guyton pg. 195-196

thoracic duct:  Located at the juncture of the Left Internal Jugular vein and the Subclavian vein.  It is where all lymph from the lower body is dumped into the venous system.  Guyton pg. 193

learning objectives: 

describe the major determinants of capillary pressure and flow, and indicate how capillary pressure and flow vary with changes in arterial pressure, venous pressure, and the ratio of pre-/post-capillary resistances 

list and discuss the various factors described in the Starling hypothesis that determine the overall rate and direction of fluid movement across the capillary wall:  There are more forces acting on the fluid in the capillaries to move the fluid out as compared to in.
    Out of capillaries:  -Mean capillary pressure, Negative interstitial free fluid pressure, Interstitial fluid colloid osmotic pressure
    Into capillaries:  Plasma colloid osmotic pressure.

based on the factors identified in the Starling Equation, list and describe at least five possible mechanisms for edema formation:   Kf * [(P_C - P_ISF) - ð(ñ_C - ñ_ISF)] 

• Low oncotic pressure
• Increased capillary filtration constant (infection, burns, bug bytes)=vasodilation
• Increased Capillary pressure
• Increased interstitial oncotic pressure
• Lymph blockage

A different Look:  Edema refers to the presence of excess fluid in the body tissues. Most instances occur in the ECF compartment.
Possible mechanisms for edema formation:

1. Increased capillary filtration coefficient: (K_f) Allows leakage of fluids and plasma proteins through the capillary membranes.
2. Increased capillary hydrostatic pressure: (P_isf) Can result from obstruction of a vein, excess flow of blood from arteries into capillaries, or failure of the heart to pump blood rapidly out of the veins.
3. Decreased plasma colloid pressure: (p _C) This can occur as a result of failure of the liver to produce sufficient quantities of plasma proteins, loss of large amounts of proteins into the urine in certain kidney diseases, or loss of large quantities of protein through burned areas of the skin.
4. Increased interstitial fluid colloid osmotic pressure: (p_isf) This draws fluid out of plasma into tissue spaces. This results most frequently from lymphatic blockage which prevents the return of proteins from the interstitial spaces to the blood.

(Hall, p. 23)

predict the direction of fluid movement across the capillary wall given information about the magnitude of the various Starling forces:  Study the Starling Equation.  Kf * [(P_C - P_ISF) - ð(ñ_C - ñ_ISF)] 

If the capillary hydrostatic pressure is 25 mm Hg, plasma colloid osmotic pressure is 28 mmHg, interstitial fluid hydrostatic pressure is 0 mm Hg, and interstitial fluid colloid osmotic pressure is 5 mmHg, the net force across the capillaries is:
Net pressure = 25-28-0+5 mm Hg
Net pressure = +2 mm Hg
Filtration will occur.
    (Hall, p. 22)

identify the functional significance of the lymphatic system, and describe the following:
    the physiologic anatomy of the lymphatic system:  Nearly all tissues of the body have lymphatic channels that drain excess fluid directly from the interstitial spaces.  Essentially all lymph from the lower part of the body flows up the thoracic duct and empties into the venous system at the juncture of the left internal jugular vein and subclavian vein.  Lymph from the left side of the head, the left arm, and parts of the chest region also enters the thoracic duct before it empties into the veins.  Lymph from the right side of the neck and head, the right arm, and parts of the thorax enters the right lymph duct, which then empties into the venous system at the the juncture of the right subclavian vein and internal jugular vein.  In the structure of the lymphatic capillaries, the endothelial cells of the capillary are attached by anchoring filaments to the surrounding connective tissue. At the junctions of adjacent endothelial cells, the edge of one endothelial cell usually overlaps the edge of the adjacent cell in such a way that the overlapping edge is free to flap inward, forming a minute valve that opens to the interior of the capillary. Interstitial fluid with suspended proteins push the valve open and flow directly into the lymphatic capillary. Backward flow will close the flap valve.
    the origin and composition of lymph:  Lymph is derived from interstitial fluid that flows into the lymphatics. Initially lymph has almost the same composition as interstitial fluid. The protein concentration of lymph depends upon where the lymph was formed.  The lymphatics can transport larger particles such as proteins, nutrients, and even bacteria.

• Interstitial fluid averages about 2 gm/dl
• Lymph from liver can have protein concentrations as high as 6 gm/dl
• Lymph from intestines can have protein concentrations of 3 to 4 gm/dl
• Thoracic lymph (mixture of lymph from all areas of body can have protein concentration of 3 to 5 gm/dl

    the magnitude of and the determinants of lymphatic flow:   About one-tenth of the fluid filtered from the arterial capillaries enters the lymphatic capillaries and returns to the blood through the lymphatic system.  Lymph flow is about 120 ml/hr .. between 2 and 3 liters per day.  The following with increase lymph flow

• Elevated capillary pressure
• Decreased plasma colloid osmotic pressure
• Increased interstitial fluid protein
• Increased permeability of the capillaries 

These all favor fluid movement into the interstitium , increasing interstitial fluid volume, interstitial fluid pressure and lymph flow all at the same time.

The lymphatic pump increase lymph flow. Valves exist in all lymph channels. When collecting lymphatics or larger lymph vessels becomes stretched with fluid, the smooth muscle in the wall of the vessel automatically contracts. Each segment of the lymph vessels between successive valves function as a separate automatic pump. In addition to the intrinsic contraction of the lymph vessel walls, external factors that compress the lymph vessel can cause pumping. Such factors are:

• Contraction of surrounding muscles of the body
• Movement of the parts of the body
• Arterial pulsations
• Compression of the tissues by objects outside the body

It is also possible that part of lymph pumping results from lymph capillary endothelial cell contraction (anchoring filaments; pressure inside capillary increases and causing overlapping edges to close like valves; pressure pushes lymph forward into collecting lymphatic).  Guyton, p. 193-196)

describe the nature of, and the physiologic rationale for, Osteopathic manipulative procedures designed to mobilize edema fluid and to return it to the venous blood via the thoracic duct:  Dr. Norton described the technique of compressing the chest more and more with each breath then quickly letting it go.  Theoretically this would create a negative pressure in the lymph system thus "sucking" it towards the thoracic duct.  The theory is that this will get the lymph system to move more interstitial fluid through the lymph system thus reducing the edema.  Good though anyway.

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