Capillary Dynamics and Hydrostatic Pressure

Capillary Dynamics

• Exchange of gases and nutrients occurs through diffusion, accompanied by bulk fluid flow.
• Hydrostatic pressure (HP) is the force exerted by a fluid pressing against a wall, with capillary HP (HPC) equal to capillary blood pressure (filtration pressure).

Hydrostatic Pressure

• HPC is greater at the arterial end (35mmHg) than at the venous end (16 mmHg).
• HPC is opposed by the hydrostatic pressure of the interstitial fluid (HPIF), which is approximately 0mmHg due to excess fluid being reclaimed by the lymphatic vessels.

Osmotic Pressure

• Osmotic pressure (OP) is created by the presence of large, non-diffusible molecules (plasma proteins) in a fluid, drawing water towards them.
• OPC = 26mmHg, OPIF = 5mmHg.

Fluid Flow and Lymphatic System

• At the arterial end, fluid leaves the capillary because Net HP > Net OP, with a pressure differential of 14mmHg.
• At the venous end, some fluid is reclaimed by the capillary because Net OP > Net HP, with a pressure differential of 5mmHg.
• There is a net fluid loss of 1.5mL/min, which is picked up by the lymphatic system and returned to the vascular system (blood).
• The lymphatic system prevents the body from being depleted of plasma in a day.

Lymphatic System

Components and Functions

• Lymphatic vessels: transport excess fluid from interstitial spaces and return it to the venous system.
• Lymphoid tissues and organs: phagocytic cells (lymphocytes) "cleanse" lymph and provide body defense.

Lymphatic Pathways

• Fluid leaves the arterial end of a capillary, with most being reclaimed at the venule end.
• Excess fluid remaining in the interstitial space causes a temporary increase in HPIF.
• This increase in HPIF, along with the design of lymphatic capillaries, allows fluid to enter the lymphatic capillaries.

Lymphatic Capillaries

• Blind-ended (open at one end) and found almost everywhere blood capillaries occur, except in bones, teeth, bone marrow, and the central nervous system.
• Fluid moves in one direction, and backflow is prevented by valves.
• Specialized lymphatic capillaries include lacteals in the intestinal mucosa, which absorb fatty lymph (chyle).

Lymphatic Pathway

• Lymphatic capillaries → afferent lymphatic vessels → lymphatic organs (nodes) → efferent lymphatic vessels → lymphatic trunks → two lymphatic ducts (right lymphatic duct and thoracic duct).
• Thoracic duct:
  • Begins as the cisterna chyli, collecting lymph from the lumbar trunks (legs) and intestinal trunk (digestive organs).
  • Collects lymph from the left side of the head, neck, and thorax (including the left arm) and the abdomen and legs.
  • Empties lymph into the venous system at the juncture of the left subclavian vein and left internal jugular vein.
• Right lymphatic duct:
  • Collects lymph from the right side of the head, neck, and thorax (including the right arm).
  • Empties lymph into the venous system at the juncture of the right subclavian vein and right internal jugular vein.

Lymph Transport

• Pumpless, with low pressure conditions (3L/Day).
• Movement controlled by:
  • Skeletal muscle contraction
  • Actions of breathing
  • Contraction of smooth muscle in lymph vessel wall
  • Valves preventing backflow
• Lymph flow increases with physical activity, highlighting the importance of immobilizing an infected area.

Cells, Tissues, and Organs

• Lymphocytes: T cells and B cells
• Activated T cells manage the immune response and directly attack antigens.