Blood Vessels Flashcards
Blood vesels
Blood Vessel
The Vascular System •Transport blood to the tissues and back •3 major types of blood vessels are •Arteries •Capillaries •Veins •Carry blood away from the heart •Arteries •Arterioles – the smallest arteries •Exchange between tissues and blood •Capillaries •Return blood toward the heart •Venules – the smallest veins •Veins
Structure of blood vessel
Composed of three layers (tunics) 1. Tunica intima •Endothelium 2. Tunica media •Smooth muscle •Contraction - vasoconstriction •Relaxation - vasodilation 3. Tunica externa (adventitia) •Mostly fibrous connective tissue •Lumen •Central blood-filled space of a vessel
Structure of
blood vessel: Tunica intima
The endothelium forms a smooth surface that minimize the friction of blood moving across them.
•In vessels larger than 1 mm in diameter, a thin layer of loose CT. called the subendothelial layer,
lies just external to the endothelium.
Structure of blood vessels: tunica media
Tunica media: the middle tunic consists of circularly arranged sheets of smooth muscle fibers that sandwich sheets of elastin and collagen fibrils.
•Contraction of the smooth muscle cells decreases the diameter of the vessel … a process called vasoconstriction, whereas their relaxation increases the vessel’s diameter … a process called vasodilation.
Structure of blood vessel Tunica Externa
Tunica externa: the outermost layer of the vessel wall; composed of connective tissue (elastic and collagen fibers)
•Protect the vessel, further strengthen its wall, and anchors the vessel to surrounding structures.
Types of arteries
Elastic arteries – recoil when stretched, which prevents blood pressure from
falling rapidly.
•Large amount of elastic tissue; smaller amount of smooth muscle
•Muscular arteries: regulate blood flow to different regions of the body
•Thicker layer of smooth muscle in the tunica media
Elastic Arteries
Elastic arteries – the greatest arteries •Diameters range : 2.5 cm – 1 cm •Includes the aorta and its major branches •Sometimes called conducting arteries •Greater proportion of the wall is elastic tissue. •Smaller proportion is smooth muscle, compared to other arteries. •High elastin content reduces surge of blood pressure.
Muscular arteries
Muscular arteries – distributing arteries •Lie distal to elastic arteries •Diameters range : 1 cm – 0.3 cm •Includes most named arteries •Tunica media is thick •Unique features •Internal and external elastic laminae •Vasoconstriction •Vasodilation
Arterioles
Arterioles •Smallest arteries •Diameters range : 0.3 mm – 10 𝜇m •Transport blood from small arteries to capillaries. •Three tunics are identified. •Diameter of arterioles controlled by •Local factors in the tissues •Sympathetic nervous system
Capillaries
Smallest blood vessels
•Endothelium surrounded by a loose connective tissue
•Diameters range : 8 – 10 𝜇m
•Red blood cells pass through single file
•Site-specific functions of capillaries
•Lungs – O2, CO2exchange
•Small intestines – receive digested nutrients
•Endocrine glands – deliver hormone
•Kidneys – get rid of nitrogenous wastes
capillaries
Capillaries are composed of only a single layer of endothelial cells surrounded by a basement
membrane.
•Pericytes: spider-shaped cells who form a network around the periphery of the capillary and serve
to strengthen and stabilize the capillary.
•Function: Renew and refresh interstitial fluid with O2nutrients cells need and removes CO2and
nitrogenous wastes that cells deposit into the fluid.
Types of Capillaries
. Continuous Capillaries
- Fenestrated Capillaries
- Sinusoids
continous capillaries
Most common type of capillary •Occur in most organs •Tight junctions and desmosomes join epithelial cells •Intercellular clefts •Gaps of unjoined membranes •Allow small molecules across capillaries •Pericytes •Spider-shaped contractile stem cells •Generate new vessels
low permeability capillaries
Blood Brain Barrier (BBB) •Capillaries have complete tight junctions •No intercellular clefts are present •Vital molecules pass through •Highly selective transport mechanisms •Not a barrier against: •O2,CO2,some anesthetic
Fenestrated Capillaries
Joined by tight junctions and intercellular clefts
•Have poresin their endothelium
•Occur where high rates of exchange occur
•Intestines
•Glomeruli of kidneys
•Endocrine glands
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
Capillary beds
Capillary bed: blood flow from arterioles into a network of capillaries
•Arteriole connects to thoroughfare
channels or capillaries via metarteriole.
•Thoroughfare channels extend from
arterioles to venules continuously.
•Several capillaries branch from the
thoroughfare channels.
•Precapillary sphincters:
•Located at the origin of capillaries from
thoroughfare channels and arterioles.
•Regulate blood flow into capillaries and
tissues.
Venous Vessels
Venous aids for the return of blood to the heart.
•Veins:
•Walls are thinner than those of comparable arteries
•Have a thinner tunic media
•Less elastin in walls
•Operate under low pressure
•Have a larger lumen than arteries
•65% of blood in veins at any given time
•Venules:
•The smallest veins; venules join to form veins.
•Diameters from 8-100 𝜇m
•Smallest venules are called postcapillary venules.
Structural differences among blood vessels
Veins have a thinner tunica media than arteries; Lumen of veins are larger than arteries.
•Venules (8-100 μm): the simplest veins that are formed when capillary beds unite.
•Postcapillary venules: smallest venules, composed of endothelium and a few pericytes; function
like capillaries that leak fluid and leukocytes during an inflammatory response.
•Large venules have one or two layers of smooth muscle (tunica media)
Mechanism to conteract low venous pressure
Valves in some veins
•Larger veins have valves to prevent backflow
•Each valves consists of folds in the tunica intima
that forms 2 flaps.
•This prevents the flow of blood toward the leg and
feet in response to the pull of gravity.
•Particularly in limbs (lower limbs > upper limbs)
•Not located in veins of thoracic and abdominal
cavities.
Mechanisms to counteract low venous pressure
Varicose Veins: •Permanently dilated veins in which the valves in the veins do not prevent the backflow of blood. •Standing in place for prolonged periods and pregnancy results in increased venous pressure in the veins that drain the lower limbs.
Mechanism to counteract low venous pressure
Skeletal muscle pump •Muscles press against thin-walled veins •Pump blood in veins toward the heart •Veins use the milking action of muscles to help move blood. •Exercise !!!
Vascular Anastomoses
Vessels interconnected to form vascular anastomoses
•Organs receive blood from more than
one arterial source
•Neighboring arteries from arterial anastomoses
•Provide collateral channels
•Veins anastomose more frequently than arteries.
•As the venules converge, they become larger
in diameter.
Vasa Vasorum
The walls of blood vessels contain living cells and therefore require a blood
supply of their own.
•Vasa vasorum (“vessels of the vessels”)
•Tiny arteries, capillaries, and veins that supply and
drain the walls of larger blood vessels.
•Located in the tunica externa
•Arise either as tiny branches from the same vessels
or as small branches from other, nearby vessels
and nourish the outer half of the wall of
the larger vessel.
Blood Circulation
Pulmonary Circulation •Systemic Circulation •Aorta •Ascending aorta •Aortic arch •Branches •Brachiocephalic trunk •Left common carotid •Left subclavian arteries
