Veins, Capillaries, and Lymphatics

Lily Marsden, MD

Key Facts

Embryology and Macroscopic Anatomy

Paired vessels form basis of early venous system that returns blood to sinus venosus of primitive heart
- Umbilical veins carry oxygenated blood from chorion (primitive placenta)
- Vitelline veins drain poorly oxygenated blood from yolk sac
- Cardinal veins drain poorly oxygenated blood from body of embryo
Umbilical veins (UVs)
- Right and proximal left UVs regress at ˜ 7 weeks gestation
- Distal left persists as single UV that drains into liver
Vitelline veins (VVs)
- Hepatic veins form from right and left proximal VVs
- Portal vein forms from anastomosis of remaining distal right and left VVs
Cardinal veins (CVs)
- Common CVs form paired anterior, posterior, subcardinal, and supracardinal veins
- Anterior CVs form superior vena cava and internal jugular veins
- Posterior, subcardinal, and supracardinal veins all contribute to the inferior vena cava
- Posterior CVs form the common iliac veins
- Subcardinal veins form renal and gonadal veins
- Supracardinal veins form intercostal, hemiazygos, and azygos veins
Lymphatic system develops between 5 and 6 weeks gestation, ˜ 2 weeks after arteriovenous system
- 6 primary lymph sacs are initially formed
- 2 jugular lymph sacs at site of future internal jugular veins
- 2 iliac lymph sacs at site of future common iliac veins
- 1 retroperitoneal lymph sac at mesenteric root along abdominal inferior vena cava
- 1 cisterna chyli dorsal to retroperitoneal lymph sac at the level of the adrenal glands
- Lymphatic vessels extend from the lymph sacs, following pathways of major veins
- Cisterna chyli connects to jugular lymph sacs through right and left thoracic ducts
- By 9 weeks gestation, a single thoracic duct is formed from caudal portion of right thoracic duct and cranial portion of left thoracic duct
- Cranial portion of right thoracic duct becomes right lymphatic duct

Microscopic Anatomy

Veins are composed of 3 tissue layers: Tunica intima, tunica media, and tunica adventitia
Large veins include vena cavae and portal vein
- Intima consists of flat endothelial cells overlying scant subendothelial connective tissue and smooth muscle
- Media is composed of a thin layer of smooth muscle and collagen
- Adventitia is thick and consists of collagenous connective tissue, elastic fibers, and fibroblasts
Medium-sized veins include majority of named veins
- Intima has endothelial cells, thin subendothelium, and smooth muscle
- Media consists of concentric smooth muscle and collagen
- Adventitia is composed of collagen that blends into surrounding connective tissue
Small veins and venules vary significantly in their histologic appearance
- Intima consists of only an endothelial cell lining
- Media varies from 1-3 smooth muscle layers in thickness but can be absent in post capillary venules
- Adventitia blends into surrounding connective tissue
Capillaries are small in diameter, only allowing single red blood cells to pass at a time
- A single layer of endothelial cells and basement membrane attach to each other through tight junctions
Lymphatics vessels are lined by a single layer of flattened endothelial cells