HUBS192 Lecture 7 Notes: Capillaries and Lymphatics

Function: Capillaries are the site of exchange between blood and tissues. This requires:
- i) Very thin walls
- ii) Large total cross-sectional area of the capillary bed
- iii) Slow and smooth blood flow
The large total cross-sectional area of the capillary bed (compared with arterioles) results in much slower blood flow. See note: A{capillaryed} ext{ (large)} ightarrow v{blood} ext{ (slow)}.

Capillary structure varies with required exchange rate and control of exchange.
Capillary intercellular junctions exist between endothelial cells.
Common dimensions:
- Red blood cell (RBC) diameter: 8$-$10\,\mu m
Endothelium includes a nucleus and cytoplasm; capillaries contain a lumen.
Precapillary sphincters are composed of smooth muscle cells and regulate flow into capillary beds.
Associated vascular structures:
- Metarteriole
- Vascular shunt
- Thoroughfare channel
- Terminal arteriole
- Postcapillary venule
These features allow dynamic control of microvascular flow and exchange.

Continuous capillaries (skeletal and cardiac muscle): diameter $8{-}10\,\mu m$
Fenestrated capillaries (kidney glomerulus): also found in tissues requiring rapid exchange; Fenestrated capillary diameter typically $8{-}10\,\mu m$ as shown in representative images
Fenestrated capillaries in the small intestine: pictorially represented as fenestrated (pore-containing) endothelium
Sinusoidal capillaries (liver sinusoids): diameter $30{-}40\,\mu m$ (very large and highly permeable)

Capillaries exhibit distinct ultrastructures:
- Continuous: tight intercellular junctions with intact basement membrane
- Fenestrated: endothelial cells with fenestrae (pores) allowing faster exchange
- Sinusoidal: large gaps and discontinuous basal lamina, very leaky
Key components involved in exchange:
- Lumen
- Intercellular cleft
- Endothelial fenestration (pore)
- Vesicles (transcytosis)
- Basal lamina (basal lamina diffusion barrier)
Diffusion routes for exchange include:
- Diffusion through intercellular clefts
- Diffusion through pores (fenestrae)
- Direct diffusion through the basal lamina

The lymphatic system is an open-entry drainage system with multiple roles:
1) Drains excess tissue fluid and plasma proteins from tissues and returns them to the blood
2) Filters foreign material from lymph
3) Screens lymph for foreign antigens and responds by releasing antibodies and activated immune cells
4) Absorbs fat from the intestine and transports it to the blood
Major functional components:
- Lymphatic vessels (thin-walled, one-way valves)
- Lymph nodes (regional filtering stations)
- Lacteals (special lymphatic vessels in the small intestine that drain fat-containing lymph into the cisterna chyli)
- Cisterna chyli (a collecting sac that feeds the thoracic duct)
- Thoracic duct (drains into the left subclavian vein)
- Right lymphatic duct (drains into the right subclavian vein)
- Internal jugular and subclavian veins where lymph returns to blood

Lymphatic vessels are thin-walled and contain numerous valves to prevent backflow.
Lacteals drain fat-laden lymph from the intestine into the cisterna chyli, which is part of the thoracic duct system.
Collecting vessels have valves to ensure one-way flow toward the thoracic duct and venous system.
Lymphatic drainage crosses paths with the arterial system and venous system at lymph nodes and ducts.

System organization includes systemic and thoracic segments; connections to the venous system occur at:
- Thoracic duct into the left subclavian vein
- Right lymphatic duct into the right subclavian vein
Regional lymph nodes include:
- Cervical nodes
- Axillary nodes
- Inguinal nodes
A schematic comparison:
- Artery vs Lymph channel vs Lymph vessels
- Lymph channels are thin-walled and contain no RBCs; they possess valves to prevent backflow

Lymph node structure includes afferent and efferent lymphatics:
- Afferent lymphatics carry lymph into the node
- Efferent lymphatics carry lymph away from the node
Breast tissue lymphatic drainage can carry cancer cells into the blood vascular system, contributing to metastasis.
Lymphatic drainage of the breast and surrounding tissues is a clinically important pathway for metastatic spread.

After absorption, fat enters the lymphatic system via lacteals in the intestinal villi.
Fatty lymph (chyle) travels via lymphatics to the cisterna chyli and then to the thoracic duct before entering the bloodstream at the left subclavian vein.
Diagrammatic note: Lacteals drain into cisterna chyli, then thoracic duct, then left subclavian vein.

Regional lymph nodes: Cervical, Axillary, Inguinal
The lymphatic system integrates with the venous circulation at the subclavian veins via the thoracic duct (left) and right lymphatic duct (right).
Clinically relevant note: Metastasis can follow lymphatic drainage routes from the breast to regional nodes and beyond into the blood circulation.

Capillaries: Module 19.3 (pages 742–743)
Lymphatic system: Module 20.1 (page 793), Module 20.2 (pages 794–795), Module 20.5 (point 3) lymph nodes (page 801)
Readings from Martini et al., Visual Anatomy and Physiology (3rd ed)
Figures referenced include liver sinusoids, fenestrated capillaries, and lymphatic network diagrams.

The pre-lecture materials acknowledge potential differences with live lectures; use this as a supplementary guide.
The content emphasizes how structure enables function in capillaries and how the lymphatic system complements immune defense and fat absorption.
Copyright and usage reminders are noted in course materials but not essential for conceptual study.