Blood Vessels

Histological Organization of Blood Vessels

Capillaries

• Smallest and most delicate vessels
• Walls permit exchange of gases between blood and interstitial fluid
• Diameter: about 8 microns (similar to red blood cells)

Types of Capillaries

• Continuous Capillaries: Endothelial lining is complete.
• Fenestrated Capillaries:
  • Endothelial lining is NOT complete
  • Have pores in their lining

Mechanisms of Material Passage in Capillaries

• Diffusion across the endothelial lining
• Diffusion through gaps between adjacent cells
• Diffusion through pores
• Movement via endocytosis

Capillary Beds

• Capillaries form interconnected networks called capillary beds
• Connect arterioles with venules
• Regulated by precapillary sphincters

Blood Flow Regulation

• Continuous, rich blood flow required in areas like the brain, heart, and stomach
  • Multiple arteries (collateral arteries) supply these areas, often forming an arterial anastomosis
  • If one arteriole is blocked, another supplies the capillary bed
• Direct connections in joints/visceral organs
  • Movement may hinder blood flow; direct connections between arterioles and venules are called arteriovenous anastomosis

Cardiovascular Changes at Birth

Fetal Cardiovascular System

• Differences from adult system:
  • Nonfunctional fetal lungs
  • Nutritional and respiratory needs met via placenta diffusion
• Blood flow sequence:
  • Internal iliacs → Umbilical arteries → Umbilical cord → Placenta
  • Placenta → Umbilical vein → Ductus venosus → Fetal liver → Inferior vena cava → Fetal right atrium

Fetal Heart Circulation

• Short circuits to lungs:
  • Right atrium to left atrium via foramen ovale
  • Pulmonary trunk to aortic arch via ductus arteriosus

Changes Upon Birth

• Ductus arteriosus muscles contract, forming ligamentum arteriosum
• Increased pressure in left atrium closes foramen ovale, forming fossa ovalis