Cardiovascular System Blood Vessels
Structure of Blood Vessels
Structure of Arteries, Veins, and Capillaries
• The three layers (tunics)
– Tunica intima – composed of simple squamous epithelium
– Tunica media – sheets of smooth muscle
• Contraction – vasoconstriction
• Relaxation – vasodilation – Tunica externa (adventitia) – composed of connective tissue
• Large vessels contain a vasa vasorum
• Lumen
– central blood‐filled space of a vessel
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Types of Blood Vessels
• Arteries – carry blood away from the heart
– Types: elastic (conducting), muscular (distributing), and arterioles
• Capillaries – smallest blood vessels
– The site of exchange of molecules between blood and tissue fluid
• Veins – carry blood toward the heart
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
– Thicker tunica intima
• Due to thicker subendothelial layer
Types of Arteries
• Muscular (distributing) arteries
– Lie distal to elastic arteries
– Diameters range from 1 cm to 0.3 mm
– Includes most of the named arteries
– Tunica media is thick
– Unique features • Internal and external elastic laminae 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 tone
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
• In the liver – removal of toxins, nutrients for metabolic events…
Capillary Beds
• Network of capillaries running through tissues
• Control of blood in capillary beds
– Precapillary sphincters – regulate the flow of blood to tissues
• Tissues & Structures with little or no blood flow
– Tendons and ligaments – poorly vascularized
– Epithelia and cartilage – avascular
• Receive nutrients from nearby connective tissues
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Capillary Beds
Capillary Permeabillity
Cross Section of Continuous Capillaries
• Endothelial cells – held together by tight junctions and desmosomes • Intercellular clefts – gaps of unjoined membrane – Small molecules can enter and exit • Three types of capillaries
– Continuous – most common
– Fenestrated – have pores
– Sinusoids – wide porous capillary found in some organs
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Cross Section of Fenestrated Capillaries
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
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
– Other blood barrier?
Mechanisms to Counteract Low Venous Pressure
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
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Valves in some veins
– Particularly in limbs
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Skeletal muscle pump
– Muscles press against thin‐walled veins
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Respiratory pump
– Causes changes in thoracic vs. abdominal pressure
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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
Circulation Routes
Pulmonary Circulation
• Pulmonary
• Systemic
– Arteries
– Capillaries
– Veins
• Special Venous Routes
• Veins anastomose more frequently than arteries
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Systemic Circulation
Flow Chart – Main Systemic Arteries
Flow Chart – Main Veins of Systemic Circulation
• Systemic Arteries
– Carry oxygenated blood away from the heart
– Aorta – largest artery in the body
• Capillaries – point of exchange
• Systemic Veins
– Carry deoxygenated blood towards the heart
– Venae Cavae – the largest of the arteries that enter into the right atrium
Figure 19.25
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The Basic Scheme of the Hepatic Portal System
Veins of the Hepatic Portal System
Blood Vessels Throughout Life
• Fetal Circulation
– All major vessels in place by month 3 of development
– Differences between fetal and postnatal circulation
• Fetus must supply blood to the placenta
• Very little blood is sent through the pulmonary circuit
Figure 19.22
Figure 19.23
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Vessels to and from the Placenta
• Umbilical vessels run in the umbilical cord
– Paired umbilical arteries
– Unpaired umbilical vein
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Shunts Away from the Pulmonary Circuit
Fetal and Newborn Circulation Compared
• Foramen ovale
• Ductus arteriosus Ductus venosus Ligamentum teres
Ligamentum venosum Medial umbilical ligaments
Figure 19.26b
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