Detailed Notes: Capillary Exchange & Edema

PART 1: CAPILLARY EXCHANGE

1.1 Introduction & Definition

• Occurs at the capillary bed - the site of exchange between the arterial and venous sides of the circulation.
• The capillary wall is composed of a single layer of endothelial cells, only 0.5 μm thick, making it extremely thin for rapid diffusion.
• Capillary beds are formed between metarterioles (small vessels branching from arterioles) and venules.

1.2 Structure of the Capillary Wall

• At the entrance of each capillary is a precapillary sphincter - a ring of smooth muscle that controls blood flow into the capillary by relaxing (vasodilation, allowing flow) or contracting (vasoconstriction, stopping flow).
• The capillary walls have small gaps called intercellular clefts, which are only 6-7 nm wide.
• These clefts allow small molecules (water, ions, small solutes) to pass through by filtration and diffusion, while excluding large molecules like proteins and blood cells.

1.3 Starling Forces - The Physics of Capillary Exchange

Force 1: Hydrostatic Pressure (HP)

• The outward force - pushes fluid OUT of the capillary into the interstitial space.
• Generated by the pumping action of the heart (blood pressure within the capillary).
• Values:
  • Arterial end: 30 mmHg (high - favors filtration/outward movement)
  • Venous end: 15 mmHg (lower - less filtration occurring)

Force 2: Plasma Colloid Osmotic Pressure (POP) / Oncotic Pressure

• The inward force - pulls water BACK INTO the capillary.
• Created by the large plasma proteins (especially albumin) that remain inside the capillary.
• These proteins carry a strong negative charge, attracting water molecules.
• Value: 28 mmHg (relatively constant throughout the capillary length)

Force 3: Interstitial Fluid Colloid Osmotic Pressure (IOP)

• An outward force - pulls fluid OUT of the capillary into the interstitium.
• Created by the small amount of protein present in the interstitial fluid.
• Value: 8 mmHg

Net Effect - Starling Equation Summary

• At the arterial end: outward forces (30 + 8 = 38) > inward forces (28) → fluid is filtered OUT into the interstitium.
• At the venous end: outward forces (15 + 8 = 23) < inward forces (28) → fluid is reabsorbed BACK into the capillary.
• Any fluid not reabsorbed is drained by the lymphatic system.

1.4 Substances Involved in Capillary Exchange

Substances Exiting the Capillaries (Blood → Interstitial Fluid → Cells)

• Oxygen (O₂) - for cellular respiration
• Water - for hydration and metabolic reactions
• Glucose - primary energy source
• Amino acids - for protein synthesis
• Fatty acids - for energy and membrane building
• Electrolytes - Na⁺, K⁺, Cl⁻, HCO₃⁻ (for osmotic balance, nerve conduction, pH)
• Hormones - chemical messengers to target cells

Substances Entering the Capillaries (Cells → Interstitial Fluid → Blood)

• Carbon dioxide (CO₂) - waste product of cellular respiration
• Water (reabsorbed) - excess fluid drawn back by oncotic pressure
• Urea - nitrogenous waste from protein catabolism (excreted by kidneys)
• Creatinine - waste from muscle creatine phosphate metabolism
• Uric acid - waste from purine (nucleotide) breakdown
• Other metabolic wastes
• Excess H⁺ ions - for acid-base balance

Substances That REMAIN in the Capillaries

• Red Blood Cells (RBCs) - too large to pass through capillary pores
• White Blood Cells (WBCs) - normally remain inside; however, during inflammation they can be signaled to leave via a process called diapedesis/extravasation
• Platelets - too large; remain in circulation to be available for clotting
• Most plasma proteins (albumin, globulins, fibrinogen) - too large for most capillary pores; their retention is what creates the oncotic pressure that draws water back in

PART 2: EDEMA

2.1 Definition

2.2 Types of Edema: Pitting vs. Non-Pitting

2.3 Etiology & Pathophysiology - The Four Main Mechanisms

Mechanism 1: Increased Hydrostatic Pressure (Pitting Edema)

• Right Heart Failure:
  • The right ventricle fails to pump blood effectively into the pulmonary artery.
  • Blood backs up: Right atrium → Superior/Inferior Vena Cava (SVC/IVC) → systemic veins and organs.
  • Increased blood volume in systemic veins = increased hydrostatic pressure → peripheral (systemic) edema affecting limbs and abdominal organs.
• Left Heart Failure:
  • Blood backs up from the left atrium into the pulmonary veins.
  • Increased hydrostatic pressure in the pulmonary capillaries → pulmonary edema (fluid accumulates in the lungs).

• Occurs in people who stand for long periods without movement (e.g., traffic officers, surgeons, soldiers).
• Gravity causes blood to pool in the leg veins → increased blood volume in leg veins → increased hydrostatic pressure in leg capillaries → transient edema of the feet, ankles, and lower legs.
• Oncotic pressure is unchanged, so the imbalance is purely from the hydrostatic side.
• Typically resolves with elevation of the legs or walking (which activates the venous muscle pump).

• Conditions like renal disease and liver disease cause the body to retain sodium and water → increased blood volume → increased hydrostatic pressure.

• A clot in a deep vein obstructs venous return → blood and fluid build up proximal to the clot → increased hydrostatic pressure → unilateral limb edema.

• Certain drugs (e.g., calcium channel blockers, NSAIDs, corticosteroids) can cause fluid retention and edema.

Mechanism 2: Decreased Oncotic Pressure (Pitting Edema)

1. Dietary absorption via the GI tract (you eat protein → absorbed as amino acids → liver makes albumin)
2. Hepatic synthesis (the liver manufactures albumin)

• Hypoalbuminemia (low albumin) → decreased oncotic pressure → generalized edema.

Mechanism 3: Lymphatic Obstruction (Lymphedema - Non-Pitting Edema)

• Breast cancer commonly spreads via the lymphatic system to the axillary (armpit) lymph nodes before becoming systemically metastatic.
• Surgical treatment involves removing both the breast lump AND the axillary lymph nodes (lymph node dissection) to prevent future Stage IV (metastatic) disease.
• Removing these nodes eliminates the lymphatic drainage pathway for the entire arm on that side.
• Result: lymphedema of the affected arm - a recognized and significant side effect/morbidity of breast cancer surgery.

• Caused by the parasitic nematode worm Wuchereria bancrofti, transmitted by mosquitoes.
• The microscopic larvae (microfilariae) migrate through the bloodstream and lodge in the lymphatic vessels, where they obstruct lymph flow.
• Most commonly affects the legs and genitals.
• Chronic obstruction leads to massive lymphedema, causing gross thickening and hardening of the skin and tissues, making the limb resemble an elephant's leg - hence the name "elephantiasis."

• A hereditary/congenital condition (genetic mutation) in which the lymphatic vessels fail to develop functionally from birth.
• The affected individual has generalized edema throughout the body due to non-functional lymphatics from birth.
• Classified as Primary Lymphedema (vs. secondary which is acquired).

Mechanism 4: Increased Capillary Permeability (Can be pitting or non-pitting)

• This edema fluid is protein-rich and is called an exudate (as opposed to a transudate, which is protein-poor, seen in the other mechanisms).
• This is diagnostically important - analyzing the protein content of edema fluid helps identify the cause.

• Burns - heat destroys endothelial cells
• Insect bites - venom/irritants trigger mast cell degranulation → histamine release → increased permeability
• Cellulitis - bacterial infection causes local inflammation
• Allergic reactions - immune mediators (histamine, leukotrienes, bradykinin) increase capillary permeability

• Localized edema - at a specific site of injury/inflammation (e.g., ankle swelling after a sprain)
• Generalized edema - in severe systemic inflammatory states (e.g., anaphylaxis, sepsis)

2.4 Edema Grading (Pitting Edema)

2.5 Summary of Edema Types and Their Fluid Characteristics

PART 3: KEY TAKE-HOME MESSAGES

1. Capillary exchange is a balance of Starling forces - hydrostatic pressure drives fluid out at the arterial end; oncotic pressure draws it back in at the venous end.
2. The four main causes of edema (the mnemonic "HILO" can help):
   • High hydrostatic pressure (heart failure, DVT, gravity)
   • Inflammation / Increased permeability (burns, infection, allergy)
   • Lymphatic obstruction (post-surgery, filariasis, Milroy disease)
   • Oncotic pressure decreased (liver disease, nephrotic syndrome, malnutrition)
3. Pitting vs. non-pitting is a key clinical distinction:
   • Pitting = free fluid (transudate) → think cardiac, hepatic, renal, or nutritional cause
   • Non-pitting = bound fluid / fibrotic change → think lymphedema, myxedema, or lipedema
4. Albumin is the master regulator of oncotic pressure - anything that reduces albumin (liver failure, protein malnutrition, nephrotic syndrome) leads to edema.
5. The lymphatic system is the safety valve - it prevents minor imbalances from becoming clinically significant edema. Blocking it (surgery, parasites, genetics) causes non-pitting lymphedema.