CFR: Blood Flow and Fluid Exchange Study Notes

1. Detailed Analysis of Oedema (Tissue Swelling)

Oedema is the accumulation of excess interstitial fluid in the tissues. This occurs when the balance of fluid exchange is disrupted by one or more of the following physiological changes:

1. Increased Capillary Hydrostatic Pressure

   • Mechanism: When the outward force ($+25.0 \text{ mmHg}$) in the capillaries increases, more fluid is forced into the interstitial space.

   • Causes: Local venous obstruction (e.g., blood clots) or systemic conditions like right-sided cardiac failure, which increases overall venous pressure.

2. Decreased Plasma Colloid Osmotic Pressure

   • Mechanism: A reduction in plasma proteins (like albumin) lowers the inward force ($-28.0 \text{ mmHg}$) that normally draws fluid back into the blood.

   • Causes: Severe burns (protein loss via skin) or nephritis (protein loss via urine).

3. Lymphatic Obstruction

   • Mechanism: Blockage of lymphatic vessels leads to protein accumulation in the interstitial fluid, raising the Interstitial Fluid Colloid Osmotic Pressure ($+5.0 \text{ mmHg}$).

   • Causes: Parasitic infections such as filariasis.

4. Increased Capillary Permeability

   • Mechanism: Damage to the capillary wall allows both plasma proteins and fluid to leak into the tissues.

   • Causes: Inflammatory responses, allergic reactions, or burns.

5. Disturbance of Negative Interstitial Pressure

   • Mechanism: The lymphatic system typically maintains a negative pressure of $-6.3 \text{ mmHg}$ by pumping proteins out of the space. If this pumping fails, fluid equilibrium is lost, leading to net inflow into the tissue.

2. Components and Anatomy of the Lymphatic System

The lymphatic system serves as an independent drainage and immune network consisting of several key components:

A. Lymphatic Vessels

• Lymphatic Capillaries: End blindly in tissues. Their thin endothelium consists of overlapping cells that act as one-way valves, allowing fluid, proteins, and bacteria to enter but preventing their exit.

• Larger Vessels: Contain internal non-return valves and act as a "lymphatic pump." Flow is driven by surrounding skeletal muscle contractions.

• Flow Rates: Rest flow is approximately $120 \text{ mL/hour}$, but this can increase 5 to 15 times during exercise.

B. Lymph Nodes and Biological Fluids

• Function: Nodes act as filters located at vessel junctions, containing lymphocytes and macrophages to neutralize pathogens.

• Specific Sites: Peyer’s patches are lymphatic nodules found specifically in the gut.

• Fluid Composition: Lymph is similar to interstitial fluid, containing proteins ($2 \text{ g/100mL}$), fat droplets, and lymphocytes.

C. Main Drainage Sites

• Thoracic Duct: Collects lymph from the lower body and the left upper body. It originates at the cisterna chyli (below the liver) and drains into the venous system.

• Right Lymphatic Duct: Collects lymph from the right side of the head, neck, thorax, and right arm, emptying into the junction of the right subclavian and jugular veins.

3. Cancer Metastasis and the Lymphatic System

The lymphatic system is a primary route for the spread of malignant cells. Understanding this pathway is vital for cancer treatment and prognosis:

• Sentinel Lymph Nodes: These are the first nodes to receive drainage from a tumor site. In breast cancer, these are usually found in the axilla (armpit).

• Sentinel Lymph Node Biopsy (SLNB):

  1. Injection: A radioactive tracer or colored dye is injected into the tissue surrounding the tumor.

  2. Mapping: The substance travels to the sentinel nodes, identifying which nodes are most likely to contain cancer cells.

  3. Removal and Examination: These specific nodes are surgically removed and analyzed.

• Clinical Significance: The presence or absence of cancer cells in these nodes provides critical prognostic information and guides decisions regarding further surgery or chemotherapy.