Notes: Microcirculation, Capillaries, and Lymphatics

Structure of the Microcirculation

• Components in order from arteriole to venule: arteriole → precapillary shunt → metarteriole → true capillaries → postcapillary venules → venules → veins
• Key vessel sizes and typical pressures (from the slides):
  • Arteriole: 10–15 μm
  • Metarteriole: 10–20 μm
  • Capillaries: 5–10 μm
  • Venules: 10–100 μm
  • Pressure references (approximate, as given):
  • Metarteriole: 40–50 mmHg
  • Arteriole: 50–60 mmHg
  • Venule: 30–40 mmHg
  • True capillary segments: 5–12 mmHg
• Functional components include precapillary sphincters that regulate perfusion of capillary beds and shunts that bypass certain regions as needed
• The muscular layers around arterioles and venules contribute to dynamic changes in resistance and flow distribution

Structure of the Capillary Wall

• Capillaries are organized as continuous, fenestrated, or discontinuous endothelium, with different permeability characteristics
• Continuous capillaries (most tissues):
  • Tight junctions between endothelial cells
  • Restricted paracellular diffusion; selective passage of molecules
  • Leukocyte (e.g., neutrophil) transmigration occurs primarily through postcapillary venules where the endothelium is more permissive
• Fenestrated capillaries: contain fenestrae (pores) that may have diaphragms, allowing greater exchange of small solutes
• Discontinuous (sinusoidal) capillaries: large intercellular gaps and discontinuous basement membranes, permitting free exchange of large and small molecules
• The capillary wall structure governs what leaves the blood and what remains intravascularly
• Capillary permeability varies by organ, and endothelial cells can differ markedly between tissues

Capillaries of Certain Organs

• Brain (Blood–Brain Barrier):
  • Continuous capillary endothelium with tight junctions
  • Regulated transcytosis and a specialized basement membrane
  • Very restricted paracellular transport
• Kidney (Glomerulus and Peritubular Capillaries):
  • Fenestrated endothelium with diaphragms in some segments
  • Highly selective ultrafiltration in glomeruli; solute exchange and resorption in peritubular capillaries
• Lung (Alveolar-Capillary Barrier):
  • Thin barrier enabling gas exchange; high permeability for gas diffusion
• Choroid Plexus (Blood–CSF Barrier):
  • Specialized barrier with ABC efflux transporters
  • Facilitates control of CSF composition
• Liver (Liver Sinusoidal Endothelium, LSEC):
  • Very permeable endothelium; selective loss of CD31/CD34 with maturation
  • Scavenger receptors facilitate clearance of macromolecules
• Bone Marrow:
  • Distinct endothelium with Notch signaling components (e.g., Notch ligand DI14); PDPN and Sca-1 markers in BMAEC/BMSEC
• Overall takeaway: Capillary specialization matches organ function and transport needs

Diffusion Through the Capillary Membrane

• Diffusion pathways include:
  • Lipid-soluble substances (O2, CO2, fatty acids, steroid hormones) diffuse through endothelial cytoplasm
  • Water-filled pores enable diffusion of water-soluble substances; diffusion of small solutes occurs through gaps/pores
  • Proteins generally cannot cross capillary walls by simple diffusion; some exchangeable proteins undergo transcytosis
• Endothelial cell considerations:
  • Endothelial cells help maintain plasma oncotic pressure, limiting protein loss from blood
• Transcytosis: exchangeable proteins can be transported across endothelial cells by vesicular transport
• Summary: Diffusion dominates small, nonpolar molecules; pores support water-soluble solutes; proteins largely restricted unless transcytosed

The Interstitium and Interstitial Fluid

• Components of the interstitium include:
  • Glycans (e.g., hyaluronan)
  • Fibroblasts, collagen (fiber bundles)
  • Immune cells (macrophages, dendritic cells)
  • Capillary network in close proximity for exchange with interstitial fluid
• Function: interstitial fluid acts as a reservoir for excess fluid and solutes; it is a medium for immune surveillance and cell signaling
• The interstitium interacts with lymphatics to drain excess fluid and filtered proteins back toward circulation

Interstitial Fluid and Lymphatic Drainage

• Lymphatic capillaries in tissue spaces are blind-ended and collect interstitial fluid and filtered proteins
• Lymphatic vessels transport lymph toward the venous system via a network that ultimately returns fluid to circulation
• Structural features facilitating flow:
  • Endothelial flaps act as one-way valves that prevent backflow
  • Overlapping endothelial cells function as flap-like valves
  • Anchoring filaments connect lymphatic endothelial cells to surrounding connective tissue, stabilizing vessel integrity

Lymphatic System: Anatomy and Function

• Lymphatic vessels drain interstitial fluid and proteins from tissues
• Major components include: thoracic duct (left lymphatic drainage), right lymphatic duct, cervical/axillary/inguinal lymph nodes, thymus, spleen, mammary gland lymphatics, lumbar lymph nodes
• Lymph from tissues flows through lymph capillaries → collecting lymphatics → regional lymph nodes → lymphatic trunks → ducts (thoracic and right lymphatic) → venous system near the junction of the internal jugular and subclavian veins
• Functions of the lymphatic system:
  1) Return of excess filtered fluid to the circulation
  2) Return of filtered protein to the circulation
  3) Defense against disease (immune cell transport and antigen presentation)
  4) Transport of absorbed fat from the gut (via chylomicrons) to the circulation

Rate of Lymph Flow

• Lymph flow rate is determined by two main factors:
  • Interstitial fluid pressure (drives fluid into lymphatics when elevated)
  • Activity of the lymphatic pump (intrinsic lymphatic smooth muscle contraction and extrinsic skeletal muscle action)
• In practice, higher interstitial pressure or enhanced lymphatic pumping increases lymph flow to return excess fluid and proteins to the bloodstream

Capillary Fluid Exchange: Net Filtration and Net Absorption

• Net filtration occurs when the net pressure favors filtration from capillary to interstitial space
• Net absorption occurs when the net pressure favors movement from interstitium back into capillary
• General concept equation (classical view):
  • Net pressure = hydrostatic pressure in capillary − colloid osmotic pressure of plasma
  • Positive net pressure indicates filtration; negative net pressure indicates absorption
• Example values from slides (illustrative):
  • Arterial end: $PH = 32 ext{ mmHg}$, plasma oncotic pressure $ ext{TT} = 25 ext{ mmHg}$ → net pressure $= PH - ext{TT} = 7 ext{ mmHg}$ (filtration)
  • Venous end: $P_H ext{ (venous)}
    = 15 ext{ mmHg}$, $ ext{TT} = 25 ext{ mmHg}$ → net pressure $= 15 - 25 = -10 ext{ mmHg}$ (absorption)
  • Net transcapillary flow estimated in the example: arterial end filtration contributing to a total filtered volume around several liters per day with lymphatics returning about $ ext{~3 L/day}$
• Revised Starling principle (more accurate model): net transvascular fluid movement is determined by both hydrostatic and oncotic forces with tissue-specific factors
  • Net Transvascular Fluid Movement (Jv): Jv = Kf (Pc - Pi) - c(cc - c_i)
  • Here: $Kf$ = filtration coefficient, $Pc$ = capillary hydrostatic pressure, $Pi$ = interstitial hydrostatic pressure, $c$ (or $ hos$) = reflection coefficient for proteins, $cc$ = plasma oncotic pressure, $c_i$ = interstitial oncotic pressure
• The profile across the capillary (classical vs revised):
  • Classical: filtration tends to predominate at the arterial side and absorption at the venous side, but net movement was often thought to be filtration with lymphatic return
  • Revised: filtration/absorption depends on local conditions; shedding light on why edema may occur with increased capillary hydrostatic pressure or decreased plasma oncotic pressure, or impaired lymphatic drainage
• Practical implication: the balance of these forces determines tissue edema and fluid balance in health and disease

Clinical Relevance: Edema and Cardiac Function

• In left ventricular failure, pulmonary venous pressure rises, which can raise pulmonary capillary pressure, potentially exceeding plasma oncotic pressure and causing pulmonary edema
• Pathophysiology example: when capillary hydrostatic pressure rises or plasma oncotic pressure falls, net filtration increases and fluid accumulates in tissues

Lymphatic Flow and Edema: Mechanisms and Implications

• Edema arises when there is an imbalance that favors filtration or a failure to remove interstitial fluid efficiently via lymphatics
• Mechanisms of edema development (summary from slides):
  • Increased capillary hydrostatic pressure (e.g., venous obstruction, heart failure, hepatic cirrhosis, constrictive pericarditis, restrictive cardiomyopathy, renal failure, pregnancy)
  • Decreased plasma oncotic pressure (e.g., malabsorption, nephrotic syndrome, liver failure, malnutrition)
  • Increased capillary permeability (not explicitly listed with separate bullet, but commonly discussed clinically)
  • Lymphatic obstruction leading to lymphedema
• Lymphedema results from impaired lymphatic drainage and accumulation of interstitial fluid in tissues

Quick Connections and Practical Takeaways

• The microcirculation finely tunes tissue perfusion and exchange via arterioles, capillaries, and venules; precapillary sphincters control flow into capillary beds
• Capillary wall structure (continuous, fenestrated, discontinuous) matches organ-specific exchange needs (brain vs liver vs kidney vs bone marrow)
• The Starling forces (classical or revised) govern fluid exchange; local changes can shift toward filtration or absorption and influence edema formation
• The lymphatic system is essential for returning filtered fluid and plasma proteins to the circulation and for fat absorption from the gut; its dysfunction leads to edema and fluid imbalance
• Clinically, edema is a sign of altered hydrostatic pressures, oncotic pressures, or lymphatic drainage and must be interpreted in the context of cardiac, hepatic, renal, or nutritional status

Ethical/Philosophical/Practical Implications

• Understanding fluid balance is critical for managing patients with heart failure, liver disease, malnutrition, or renal disease; decisions about diuretics, plasma expanders, and nutritional support hinge on this balance
• The lymphatic system’s role in immune surveillance and fat absorption underscores the interconnectedness of fluid homeostasis with immunity and metabolism
• Accurate interpretation of edema requires considering the site-specific capillary dynamics and the contribution of lymphatic drainage, not just gross swelling

Key Equations and Notation (LaTeX)

• Classical net filtration pressure (illustrative):
  ext{NFP} = (Pc - Pi) - ( c -  i)
• Revised Starling principle (net transvascular fluid movement):
  Jv = Kf (Pc - Pi) - ch (c - i)
• Notes:
  • $P_c$ = capillary hydrostatic pressure
  • $P_i$ = interstitial hydrostatic pressure
  • $_c$ = plasma oncotic pressure
  • $_i$ = interstitial oncotic pressure
  • $K_f$ = filtration coefficient; $ch$ = reflection coefficient for proteins
• Units mentioned in slides (provided as-is): mmHg for pressures; μm for vessel diameters; L/day for filtration/absorption estimates

Summary of Structure and Function (Condensed)

• The microcirculation comprises arterioles, capillaries, and venules with regulatory control for tissue perfusion
• Capillary wall types determine permeability and route of exchange for gases, solutes, and proteins
• The interstitium and lymphatics form a system to balance fluid, proteins, and immune function; lymphatics also enable fat transport
• Fluid exchange is governed by Starling forces; the revised model provides a more accurate description of transcapillary fluid movement
• Edema results from disruptions in hydrostatic pressure, oncotic pressure, capillary permeability, or lymphatic drainage, and has direct clinical relevance to heart, liver, kidney, and nutritional disorders