Blood Vessels
Histological Organization of Blood Vessels
* Capillaries
* Smallest of all vessels
* Most delicate of all vessels
* Walls are thin enough to permit exchange of gases between the blood and the interstitial fluid
* The diameter is about 8 microns
* A red blood cell diameter is also about 8 microns
* Types of Capillaries
* Continuous
* Endothelial lining is complete
* Fenestrated
* Endothelial lining is NOT complete
* These capillaries have pores in their lining
* Capillaries (continued)
* There are 4 mechanisms regarding the passage of material across the walls of capillaries
* Material can diffuse across the endothelial lining
* Material can diffuse through the gaps between adjacent cells of the lining
* Material can diffuse through pores
* Material can move via endocytosis
* Capillary Beds
* Capillaries do not function as individual units
* Capillaries form an interconnected network of capillaries (capillary beds)
* The capillary bed consists of vessels connecting arterioles with venules
* There are precapillary sphincters involved in regulating blood flow through the capillaries
* Capillary Beds (continued)
* In areas such as the brain, heart, and stomach, a continuous, rich flow of blood is required
* In these areas, more than one artery supplies a specific area
* These arteries (collateral arteries) typically fuse forming an arterial anastomosis
* If one arteriole is blocked, the other one will supply blood to the capillary bed
* Capillary Beds (continued)
* In areas such as the joints or visceral organs, blood flow through some vessels may be hindered due to body movement
* In order to accommodate this, there must be a direct connection between arterioles and venules
* This direct connection is called an arteriovenous anastomosis
Cardiovascular Changes at Birth
* The fetal cardiovascular system differs from the adult cardiovascular system
* The fetal lungs are nonfunctional
* The fetal nutrition and respiratory needs are provided by diffusion across the placenta
* Blood in the fetal internal iliacs enters the umbilical arteries
* Enters the umbilical cord
* Enters the placenta
* All fetal nutritional and respiratory needs are provided by diffusion across the placenta
* Blood leaves the placenta
* Enters the umbilical vein
* Enters the ductus venosus
* Enters the fetal liver
* Enters the inferior vena cava
* Enters the fetal right atrium
* Fetal heart circulation uses two “short circuits” to the lungs
* Blood in the right atrium can enter into the left atrium via the foramen ovale
* Blood in the pulmonary trunk can enter into the aortic arch via the ductus arteriosus
* Upon birth:
* Smooth muscles of the ductus arteriosus contract forming the ligamentum arteriosum found in the adult heart
* Pressure in the left atrium increases, thus closing the valvular flap of the foramen ovale, forming the fossa ovalis found in the adult heart