Blood Vessels Part 1: Exam 2 Flashcards
Blood Vessels
they are the delivery system of dynamic structures that begins and ends at heart
- work with lymphatic system to circulate fluid
Arteries
carry blood away from heart; oxygenated except for pulmonary circulation and umbilical vessels of fetus
Capillaries
direct contact with tissue cells; directly serve cellular needs
Veins
carry blood toward heart; deoxygenated except for pulmonary circulation and umbilical vessels of fetus
Structure of Blood Vessel Wall
- all vessels consist of a lumen: central blood-containing space, surrounded by a wall
- capillaries (endothelium with sparse basal lamina)
- walls of all vessels, except capillaries, have 3 layers or tunics:
1) Tunica Intima- innermost layer that is in “intimate” contact with blood
- endothelium: simple squamous epithelium that lines lumen of all vessels (continuous with endocardium &slick surface reduces friction)
- subendothelial layer: connective tissue basement membrane (found only in vessels larger than 1 mm)
2) Tunica Media - middle layer composed mostly of smooth muscle and sheets of elastin
- sympathetic vasomotor nerve fibers that supply this layer, controlling:
- vasoconstriction: decreased lumen diameter
- vasodilation: increased lumen diameter
- bulkiest layer responsible for maintaining blood flow and blood pressure
3) Tunica Externa - outermost layer of wall
- also called tunica adventitia
- composed mostly of loose collagen fibers that protect and reinforce wall and anchor it to surrounding structure
- infiltrated with nerve fibers, lymphatic vessels
- large veins also contain elastic fibers in this layer - vasa vasorum: system of tiny blood vessels found in larger vessels
- function to nourish outermost external layer
Arteries divided into three groups, based on size and function
- elastic arteries
- muscular arteries
- arterioles
Elastic Arteries
they are thick-walled with large, low resistance lumen
- aorta and its major branches, also called conducting arteries because they conduct blood from heart to medium-sized vessels
- elastin found in all three tunics, mostly tunica media
- contain substantial smooth muscle, inactive in vasoconstriction
- act as pressure reservoirs that expand and recoil as blood is ejected from heart
- allows for continuous blood flow downstream even between heartbeats
Muscular Arteries
- elastic arteries give rise to muscular arteries
- also called distibuting arteries because they deliver blood to body organs
- diameters range from pinky finger size to pencil lead
- account for most named arteries
- have thickest tunica media with more smooth muscle, less elastic tissue
- tunica media sandwhiched between elastic membranes
- active in vasoconstriction
Arterioles
they are the smallest of all arteries
- larger arteries contain all 3 tunics
- smaller arterioles are most single layer of smooth muscle surrounding endothelial cells
- control flow into capillary beds by vasodilation and vasoconstriction of smooth muscle
- also called resistance arteries because changing diameters change resistance to blood flow
lead to capillary beds
Capillaries
- microscopic vessels; diameters small only single RBC can pass through at a time
- walls just thin tunica intima, in smallest vessels, one cell forms entire circumference
- pericytes: spider-shaped stem cells help stabilize capillary walls, control permeability, and play role in vessel repair
- supply almost every cell, except cartilage, epithelia, cornea and lens of eye
- functions: exchange gases, nutrients, wastes, hormones, etc between blood and interstitial fluid
Types of Capillaries
- all capillary endothelial cells are joined by tight junctions with gaps called intercellular clefts
- allow passage of fluids and small solutes
3 types of capillaries
1) Continuous Capillaries - abundant in skin, muscles, lungs and CNS
- continuous capillaries of brain are unique (form blood-brain barrier, enclosed with tight junctions and no intercellular cleft)
2) Fenestrated Capillary - found in areas involved in active filtration (kidneys), absorption (intestines), or endocrine hormone secretion
- fenestrations: endothelial cells contain Swiss cheese-like pores
- allow for increased permeability
- fenestrations covered with thin glycoprotein diaphragm
3) Sinusoidal Capillaries
- allow passage of fluids and small solutes
- fewer gap junctions, usually fenestrated with larger intercellular clefts; incomplete basement membranes
- larger lumens - found only in liver, bone marrow, spleen and adrenal medulla
- blood flow is sluggish-allows time for modification of large molecules and blood cells that pass between blood and tissue
- contain macrophages in lining to capture and destroy foreign invaders
Capillary Beds
- capillary bed: interwoven network of capillaries between arterioles and venules
- microcirculation: flow of blood through bed from arteriole to venule
- terminal arteriole: branch off arteriole that further branches into 10 to 20 capillaries (exchange vessels) that form capillary bed
- exchange of gases, nutrients, and wastes from surrounding tissue takes place in capillaries
- capillaries then drain into postcapillary venule
- flow through bed, controlled by diameter of terminal arteriole and upstream arterioles
- local chemical conditions and arteriolar vasomotor nerve fibers regulate amount of blood entering capillary bed
- arteriole and terminal arteriole dilated when blood needed; constricted to shunt blood away from bed when not needed
Capillaries found in serous membranes of intestinal mesenteries have 2 additional feature that form a special arrangement of capilaries:
1) vascular shunt: channel that directly connects arteriole with venule (bypasses true capillaries)
- consist of metarteriole and throughfare channel
2) precapillary sphincter: cut off smooth muscle surrounding each true capillary that branches off metarteriole, acts as valve regulating blood flow into capillary bed
- controlled by local chemical conditions (not innervated)
Veins
veins carry blood toward the heart
- formation begins when capillary beds unite in postcapillary venules and merge into larger and larger veins
- venules
- capillaries unite to form postcapillary venules
- consist of endotheiulm and a few pericytes
- very porous; allow fluids and WBC into tissues
- larger venules, have one or two layers of smooth muscle cells
- formed when venules converge
- have all tunics, but thinner walls with large lumens compared with corresponding arteries
- tunica media is thin, but tunica externa is thick
- contain collagen fibers and elastic networks
- large lumen and thin walls make veins good storage vessels
- called capacitance vessels (blood reservoirs) because they contain up to 65% of blood supply
- blood pressure lower than in arteries, so adaptions ensure return of blood to heart
- large-diameter lumens offer little resistance
- other adaptions
- venous valves
- prevent backflow of blood
- most abundant in veins of limbs
- venous sinuses
- flattened veins with extremely thin walls
- composed only of endothelium
- examples: coronary sinus of heart and dural sinuses of brain
(CLINICAL) Varicose Veins
varicose veins are dilated and painful veins due to incompetent (leaky) valves
- factors that contribute include heredity and conditions that hinder venous return
- example: prolonged standing in one position, obesity, or pregnancy; blood pools in lower limbs, weakening valves, affect more than 15% of adults
- elevated venous pressure can cause varicose veins
- example: straining to deliver a baby or have a bowl movement raises intrabdominal pressure, resulting in varicosities in anal veins called hemorrhoids
