2.19 Intercellular Fluid and Lymph
The Lymphatic System
The lymphatic system is a vital component of the circulatory and immune systems. It comprises:
- Lymph: The fluid circulating within the lymphatic vessels.
- Lymphocytes: A type of white blood cell crucial for immune responses.
- Lymphatic vessels: A network of tubes that carry lymph.
- Lymph nodes: Small, bean-shaped organs that filter lymph and house lymphocytes.
- Tonsils, Spleen, and Thymus gland: Lymphoid organs with specific immune functions.
Interstitium and Interstitial Fluid
The Interstitium
The interstitium refers to the collective term for the spaces between cells in the body, which make up approximately one-sixth of the body's volume.
Interstitial Fluid (Intercellular Fluid)
The interstitial fluid is the fluid found within the interstitium.
Formation: Interstitial fluid is formed by filtration and diffusion of fluid and small solutes from blood capillaries into the interstitial spaces. This process is driven by hydrostatic pressure at the arterial end of capillaries.
Composition: Interstitial fluid contains almost the same constituents as blood plasma, but with significantly lower concentrations of proteins. This is because large protein molecules do not easily pass through capillary walls. It primarily consists of water, along with electrolytes, sugars, salts, acids, hormones, neurotransmitters, and cell wall products.
Function: Interstitial fluid acts as an essential medium for exchange between blood and body cells. It allows rapid transport of water molecules, electrolytes, nutrients, oxygen, and hormones from the blood to the body cells. It also facilitates the removal of cellular excreta (waste products) and carbon dioxide from the body cells back into the blood. Materials must pass through the interstitial fluid to travel between blood and cells.
Diagram:
Lymph
Formation: Lymph is essentially interstitial fluid that has entered lymphatic capillaries. It is collected from the interstitial spaces and carried through the lymphatic vessels. If these vessels are blocked, it leads to a condition called Edema.
Composition: Similar to interstitial fluid, lymph contains high water content, various solutes like ions, nutrients, gases, hormones, enzymes, and waste products. It also contains some proteins (more than interstitial fluid but less than plasma). As lymph circulates through the lymphatic system, it accumulates a significantly higher concentration of white blood cells, particularly lymphocytes. Lymph in the lacteals (digestive system) is specifically rich in fats.
Movement: Once formed, lymph passes through lymphatic vessels, eventually returning to the blood circulation. The flow is unidirectional, moving from tissues toward the heart.
Diagram:
Comparison: Intercellular Fluid and Lymph
| Feature | Intercellular Fluid | Lymph |
|---|---|---|
| Major Electrolytes | High potassium, high phosphate, moderate magnesium, low sodium | High sodium, high chloride, moderate potassium, moderate phosphate |
| Proteins | Lower concentration of proteins (much less than plasma, slightly less than lymph) | Higher concentration of proteins than interstitial fluid, but still lower than plasma |
| White Blood Cells | Absent (or negligible) | Significantly higher, particularly lymphocytes |
| Red Blood Cells | Absent normally | Absent normally |
| Water | High water content | High water content |
| Fats | Absent (or negligible) | Can be high in lymph from the digestive system (called chyle) |
| Origin | Derived from blood plasma filtered out of capillaries; surrounds body cells | Derived from interstitial fluid that has entered lymphatic capillaries |
Biological Significance
The continuous formation of interstitial fluid and its drainage as lymph are critical for maintaining fluid balance in tissues, transporting nutrients and wastes, and enabling immune surveillance by circulating lymphocytes throughout the body. This system works in tandem with the heart's activity.