C H A P T E R Blood Vessels 20 Types of Blood Vessels Arteries carry blood away from the heart Capillaries smallest blood vessels The site of exchange of molecules between blood and tissue fluid Veins carry blood toward the heart Structure of Blood Vessels Tunica intima Endothelium Subendothelial layer Internal elastic membrane Tunica media (smooth muscle and elastic fibers) External elastic membrane Tunica externa (collagen fibers) Valve Lumen Artery Capillary network Lumen Vein Basement membrane Endothelial cells (b) Capillary Figure 20.1b 1
Histological Structure of Blood Vessels Types of Arteries Elastic arteries the largest arteries Diameters range from 2.5 cm to 1 cm Includes the aorta and its major branches Sometimes called conducting arteries High elastin content dampens surge of blood pressure Vasa vasorum Elastin Lumen Tunica Tunica Tunica externa media intima (a) Elastic (aorta, 12 ) Figure 20.2a Types of Arteries Muscular (distributing) arteries Lie distal to elastic arteries Diameters range from 1 cm to 0.3 mm Includes most named arteries Tunica media is thick Unique feature Internal and external elastic laminae External elastic Internal elastic Lumen membrane membrane Tunica Tunica externa media (b) Muscular (40 ) Figure 20.2b 2
Types of Arteries Arterioles Smallest arteries Diameters range from 0.3 mm to 10 µm Larger arterioles possess all three tunics Diameter of arterioles controlled by Local factors in the tissues Sympathetic nervous system Lumen Endothelium Tunica media (c) Small arteriole (285 ) Figure 20.2c Capillaries Smallest blood vessels Diameter from 8 10 µm Red blood cells pass through single file Site-specific functions of capillaries In the lungs oxygen enters blood, carbon dioxide leaves In the small intestines receive digested nutrients In endocrine glands pick up hormones In the kidneys removal of nitrogenous wastes RBCs in a Capillary Figure 20.3 3
Capillary Beds Precapillary sphincters Vascular shunt Metarteriole Thoroughfare channel True capillaries Terminal arteriole Postcapillary venule (a) Sphincters open blood flows through true capillaries. Figure 20.4a Capillary Beds Terminal arteriole Postcapillary venule (b) Sphincters closed blood flows through metarteriole thoroughfare channel and bypasses true capillaries. Figure 20.4b Capillary Permeabillity Endothelial cells held together by tight junctions and desmosomes Intercellular clefts gaps of unjoined membrane Small molecules can enter and exit Two types of capillary Continuous most common Fenestrated have pores 4
Structure of Capillaries Cross Section Pericyte Basement membrane Tight junction Endothelial nucleus (a) Continuous capillary. Least permeable and most common (e.g., skin, muscle). Red blood cell in lumen Intercellular cleft Endothelial cell Pinocytotic vesicles Figure 20.5a Structure of Capillaries Cross Section Pinocytotic vesicles Red blood cell in lumen Fenestrations (pores) Endothelial nucleus Basement membrane Tight junction (b) Fenestrated capillary. Large fenestrations (pores) increase permeability. Occurs in special locations (e.g., kidney, small intestine). Intercellular cleft Endothelial cell Figure 20.5b Routes of Capillary Permeability Four routes into and out of capillaries Direct diffusion Through intercellular clefts Through cytoplasmic vesicles Through fenestrations 5
Low Permeability Capillaries Blood-brain barrier Capillaries have complete tight junctions No intercellular clefts are present Vital molecules pass through Highly selective transport mechanisms Not a barrier against Oxygen, carbon dioxide, and some anesthetics Sinusoids Wide, leaky capillaries found in some organs Usually fenestrated Intercellular clefts are wide open Occur in bone marrow and spleen Sinusoids have a large diameter and twisted course Sinusoids Endothelial cell Red blood cell in lumen Large intercellular cleft Tight junction Incomplete Nucleus of basement endothelial membrane cell (c) Sinusoidal capillary. Most permeable. Occurs in special locations (e.g., liver, bone marrow, spleen). Figure 20.5c 6
Veins Conduct blood from capillaries toward the heart Blood pressure is much lower than in arteries Smallest veins called venules Diameters from 8 100 µm Smallest venules called postcapillary venules Venules join to form veins Tunica externa is the thickest tunic in veins Mechanisms to Counteract Low Venous Pressure Valves in some veins Particularly in limbs Skeletal muscle pump Muscles press against thin-walled veins Valve (open) Contracted skeletal muscle Valve (closed) Vein Direction of blood flow Figure 20.6 Vascular Anastomoses Vessels interconnect to form vascular anastomoses Organs receive blood from more than one arterial source Neighboring arteries form arterial anastomoses Provide collateral channels Veins anastomose more frequently than arteries 7
Vasa Vasorum Tunica externa of large vessels have: Tiny arteries, capillaries, and veins Vasa vasorum (vessels of vessels) Nourish outer region of large vessels Inner half of large vessels receive nutrients from luminal blood Figure 20.2a The Aorta Ascending aorta arises from the left ventricle Branches coronary arteries Aortic arch lies posterior to the manubrium Branches Brachiocephalic trunk Left common carotid Left subclavian arteries The Aorta Descending aorta continues from the aortic arch Thoracic aorta in the region of T 5 -T 12 Abdominal aorta ends at L 4 Divides into right and left common iliac arteries 8
The Aorta Right common carotid Right subclavian Right internal jugular vein Right subclavian vein Right brachiocephalic vein Brachiocephalic trunk Right pulmonary Superior vena cava Ascending aorta Right atrium Right ventricle Left internal jugular vein Left subclavian Left subclavian vein Left brachiocephalic vein Left common carotid Aortic arch Left pulmonary Ligamentum arteriosum Thoracic aorta Pulmonary trunk Left atrium Left ventricle Figure 20.9 Pulmonary Circulation Pulmonary trunk leaves the right ventricle Divides into right and left pulmonary arteries Superior and inferior pulmonary veins Carry oxygenated blood into the left atrium Pulmonary Circulation Left pulmonary Aortic arch Pulmonary trunk Right pulmonary Three lobar arteries to right lung Pulmonary veins Right atrium Right ventricle Air-filled alveolus of lung O 2 CO 2 Pulmonary Gas exchange capillary Two lobar arteries to left lung Pulmonary veins Left atrium Left ventricle Figure 20.7 9
The Hepatic Portal System A specialized part of the vascular circuit Picks up digested nutrients Delivers nutrients to the liver for processing The Basic Scheme of the Hepatic Portal System Veins of the Hepatic Portal System Hepatic veins Liver Hepatic portal vein (not part of hepatic portal system) Gastric veins Spleen Splenic vein Right gastroepiploic i vein Inferior mesenteric vein Superior mesenteric vein Small intestine Large intestine Rectum (b) The veins of the hepatic portal system Figure 20.21b 10
Fetal and Newborn Circulation Compared Fetus Aortic arch Superior vena cava Ductus arteriosus Ligamentum arteriosum Pulmonary Pulmonary veins Heart Lung Foramen ovale Fossa ovalis Liver Ductus venosus Ligamentum venosum Hepatic portal vein Umbilical vein Ligamentum teres Umbilicus Abdominal aorta Common iliac Umbilical arteries Medial umbilical ligaments (a) Urinary bladder Umbilical cord Placenta High oxygenation Moderate oxygenation Low oxygenation Very low oxygenation Figure 20.24a The Aorta Right common carotid Right subclavian Right internal jugular vein Right subclavian vein Right brachiocephalic vein Brachiocephalic trunk Right pulmonary Superior vena cava Ascending aorta Right atrium Right ventricle Left internal jugular vein Left subclavian Left subclavian vein Left brachiocephalic vein Left common carotid Aortic arch Left pulmonary Ligamentum arteriosum Thoracic aorta Pulmonary trunk Left atrium Left ventricle Figure 20.9 Fetal and Newborn Circulation Compared Newborn Aortic arch Superior vena cava Ductus arteriosus Ligamentum arteriosum Pulmonary Pulmonary veins Heart Lung Foramen ovale Fossa ovalis Liver Ductus venosus Ligamentum venosum Hepatic portal vein Umbilical vein Ligamentum teres High oxygenation Moderate oxygenation Low oxygenation Very low oxygenation Umbilicus Abdominal aorta Common iliac Umbilical arteries Medial umbilical ligaments Urinary bladder (b) Figure 20.24b 11