The Blood Vessels Flashcards
why blood vessels are dynamic structures
they pulsate, constrict, dilate, relax, and proliferate
what kind of system do blood vessels form?
a closed delivery system that starts and ends with the heart
- there are ~ 60,000 miles of blood vessels in the human body
3 types of blood vessels
- capillaries
- arteries
- veins
arteries
- carry blood away from the heart
- branch into arterioles which feed into the capillary beds of the body’s tissues and organs
- in systemic circulation, they carry oxygen rich blood
capillaries
- very thin-walled structures with direct tissue contact; the sites of exchange
- O2, CO2, nutrients, hormones, and waste are exchanged between blood and interstitial fluid
veins
- return blood to the heart
- from the capillary beds, blood drains into venules which then converge into larger veins until they return to the heart
- in systemic circulation, they carry oxygen-poor blood
the exceptions to the common rules of blood vessels
- pulmonary arteries
- pulmonary veins
- umbilical vessels
lymphatic vessels
the lymphatic system recovers the fluid that leaks from the blood vessels
3 walls of blood vessels
most blood vessels have a distinct, 3 layered wall surrounding the lumen (each layer = a tunic)
- tunica intima
- tunica media
- tunica externa
tunica intima
innermost layer, contacts the passing blood
- Contains endothelium – the epithelium that lines all vessels and reduces friction
- Endothelium is continuous with the endocardial lining of the heart
- In vessels larger than 1mm in diameter – a subendothelial layer supports the tunica intima
tunica media
middle layer, circularly arranged smooth muscle cells and sheets of elastin
- More robust in arteries
- Activity of the smooth muscle is regulated by sympathetic vasomotor nerve fibers and chemicals
* Vasoconstriction: smooth muscle contracts, and the lumen decreases
* Vasodilation: smooth muscle relaxes, and the lumen increases
- Small changes in vessel diameter greatly influence blood flow and blood pressure
tunica externa
loosely woven collagen fibers that protect, reinforce, and anchor the vessel
- Sometimes called “Tunica Adventitia”
- Infiltrated by nerve fibers, lymphatic vessels, and a network of elastin fibers
In larger vessels, the tunica externa contains vasa vasorum – a network of tiny blood vessels that nourish the vessel itself
elastic arteries
“conducting arteries”
- Thick-walled arteries near the heart – aorta and its major branches
- Large diameters – 1 to 2.5cm
- Elastin present in all three tunics, but tunica media contains the most
- Despite smooth muscle, relatively inactive as vasoconstrictors
- Act as pressure reservoirs – expand/recoil as the heart ejects blood
- Elastic arteries “smooth” pressure and make blood flow fairly continuously – protection for smaller arteries
muscular arteries
“distributing arteries”
- Distal to the elastic arteries
- Deliver blood to specific body tissues/organs
- Most named arteries are muscular arteries
* Ex: brachial, radial, common iliac, posterior tibial arteries
- Diameters range from the size of a pencil lead to a little finger
- Proportionate to their size, muscular arteries have the thickest tunica media
- More smooth muscle, less elastin tissue
- More active vasoconstrictors, less capable of stretch
arterioles
“resistance vessels”
- The smallest arteries, lumen size ranges 10µm to .3mm
- Larger arterioles have all three tunics - the tunica media is chiefly smooth muscle with minimal elastin
- Smaller arterioles are largely a single layer of smooth muscle around endothelial lining
- Diameter varies in response to neural, hormonal, and local chemical influences
- When arterioles constrict, the tissue is largely bypassed
- When arterioles dilate, blood flow into the local capillaries increases dramatically
capillaries
- Microscopic vessels with extremely thin walls
- Walls consist of a thin tunica intima with a basement membrane
- Pericytes: spider-shaped, contractile stem cells along the outer surface of some capillaries - can generate new vessels or scar tissue, stabilize the capillary wall, and control capillary permeability
- Average length = 1mm, average lumen diameter = 8-10µm
- Most tissues have a rich capillary supply – exceptions include tendons, ligaments, cartilage, and epithelia
types of capillaries
- Structurally, there are 3 types of capillaries
- All 3 types have tight junctions between their epithelial cells
- Gaps of un-joined membrane are intercellular clefts
- Intercellular Clefts allow limited passage of fluids and small solutes
3 types of capillaries
continuous
fenestrated
sinusoid
continuous capillaries
most common, least permeable
- Abundant in skin, muscles, lungs, and the CNS
- The structural basis of the blood-brain barrier
fenestrated capillaries
large fenestrations or pores increase permeability
- abundant in kidneys, small intestine, and areas of hormone secretion
sinusoid capillaries
occur in limited locations, the most permeable
- Found in the liver, bone marrow, spleen, and adrenal medulla
- Have large intercellular clefts and irregular shapes
- Contain macrophages to catch prey
capillary beds
- interweaving networks formed by capillaries because they do not function independently
microcirculation
how blood flows from an arteriole to a venule through a capillary bed
- gases, nutrients, hormones, and waste are exchanged during microcirculation
terminal arteriole
arteriole that (in most body regions) branches into 10-20 capillaries - the capillary bed
- blood flow through the capillary bed is controlled by the diameter of the terminal arteriole - this is altered by chemical conditions and vasomotor nerve fibers
- secondary to changes in arteriole diameter, a capillary bed may be flooded with blood or it may be bypassed - important rerouting
vascular shunt
a vessel that directly connects the terminal arteriole to the post capillary venule - allows blood to bypass the true capillaries
- consists of a metarteriole and throughfare channel
precapillary sphincter
a cuff of smooth muscle that surrounds true capillaries as they branch from the metarteriole
- act as valves to regulate blood flow into the capillaries - they are controlled by chemical conditions
veins: blood reservoirs
- carry blood from the capillary beds back to the heart
- as you move towards the heart, veins grow larger in diameter - and their walls thicken
venules
- capillaries unite to form venules
- 8-100 µm in diameter
- Postcapillary venules are the smallest venules – they are made entirely of endothelium meaning they are porous and “leaky”
- Larger venules will have both tunicas media and externa
veins
- Venules merge to form veins
- Veins have all three tunics, but thinner walls and a larger lumen than arteries
- Veins have little elastin or smooth muscle in the tunica media
- The tunica externa is thick with longitudinal bundles of collagen
- Secondary to larger lumens, veins can act as blood reservoirs – they can hold up to 65% of the body’s blood supply at any given time
- Blood pressure in veins is much lower than in arteries – allowing for the veins’ thinner walls
Structural Adaptations of Veins
Secondary to low pressure, veins have structural adaptations to help move blood back to the heart
- Large diameters lower resistance to flow
- Venous valves prevent backflow
* Formed from folds of tunica intima, resemble the semilunar valves
* Most abundant in the limbs - where gravity opposes the upward flow of blood
Venous Sinus
a highly specialized, flat vein with extremely thin walls
- Walls are made only of endothelium
- Sinuses are supported by the structures around them – not additional tunics
* Ex: coronary sinus, the dural venous sinuses of the brain
Varicose Veins
veins that are torturous and dilated secondary to incompetent/leaky valves
- 15% of adults experience varicose veins – typically, in superficial veins of the lower limbs
- Risk Factors: prolonged standing, obesity, pregnancy
Anastomosis
special interconnection between blood vessels
- Most organs receive their arterial blood supply from > 1 arterial branch
Arterial Anastomoses
arteries supplying the same regions often merge and provide alternate pathways/collateral channels
- If one arterial branch is lost, a collateral channel can provide sufficient blood to the region
- Arterial anastomoses can be found around joints, abdominal organs, the heart, and the brain
- Regions without redundancy: retina, kidneys, and spleen
Venous Anastomoses
very common, an occluded vein rarely leads to tissue death
