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