Lymph

[Ref: KB2:p19-20; WG21:p595-596]

Lymph = Interstitial fluids that enter the lymphatic vessels.

The only way by which protein that leaves IVF can be returned back

Content of lymph

• No RBC and platelets
• Low in fibrinogen
• Fibrinogen content in lymph is low because of its low permeability to capillary membrane (much lower than plasma protein)
  * Still enough to clot in tubes
• Protein generally lower than plasma
• High in fat content

Protein concentration in lymph

For most of the body = 20g/L

Hepatic lymph = 60g/L

Average in thoracic duct = 50g/L

 

Lymph capillaries

Two types:

• Initial lymphatics
  * Lack valves and smooth muscles
• Collecting lymphatics

Lymph capillaries are

• Blind-ending
• Flap valves
• Almost all pass through lymph nodes
• No visible fenestration
• Very little if any basal lamina
• Junctions between endothelial cells are open
• No tight intercellular connection

Lymphatic vessels present in all tissues except:

• Cartilages
• Bone marrow
• CNS

 

Production of lymph

• Liver contribute 50% of body lymph at rest
• Production of lymph
  = 2mLs/min
  = 120mLs/hr (thoracic duct = 100mL/hr)
  ~ 3 L/day

 

Note:

• Lymph production increases with exercise

Filtration and lymph

[KB2:p20]

• Net filtration at arterial end of capillaries = 20mLs/min
• Resorption at venous end = 18mLs/min
• Net production = 2mLs/min

[WG21:p594]

• 24L filtered through capillaries per day
  * i.e. 16mLs/min (as opposed to 20mL/min per KB)
• 85% of the filtered fluids are reabsorbed.
• Rest goes into lymph.

 

Draining of lymph

Lymph returns to circulation via

• Thoracic duct (at the junction of LEFT subclavian and internal jugular veins)
• Right lymphatic duct

Flow movement

Movement is produced by

• Intrinsic factors
  * Smooth muscles in walls
  * One-way valve in lumen
• Extrinsic factors
  * External compression by e.g. muscles
• Other factors
  * Suctioning effect of blood flow in veins where lymph vessels drain into
  * Negative thoracic pressure during inspiration

 

Functions

• Return of protein and fluids from ISF to IVF
  * 25-50% of circulating proteins are returned per day
• Transport of fat (as chylomicrons) from small intestines
  --> Into systemic circulation via thoracic duct
  --> No passage through portal vein
  --> Liver is bypassed
• Immunological roles
  * Filtration and removal of bacteria by macrophages
  * Lymphocyte circulation
  * Activation and proliferation of lymphocytes in lymph nodes