The Blood Vessels

Question 1

why blood vessels are dynamic structures

Answer 1

they pulsate, constrict, dilate, relax, and proliferate

Question 2

what kind of system do blood vessels form?

Answer 2

a closed delivery system that starts and ends with the heart
- there are ~ 60,000 miles of blood vessels in the human body

Question 3

3 types of blood vessels

Answer 3

• capillaries
• arteries
• veins

Question 4

arteries

Answer 4

• 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

Question 5

capillaries

Answer 5

• 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

Question 6

veins

Answer 6

• 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

Question 7

the exceptions to the common rules of blood vessels

Answer 7

• pulmonary arteries
• pulmonary veins
• umbilical vessels

Question 8

lymphatic vessels

Answer 8

the lymphatic system recovers the fluid that leaks from the blood vessels

Question 9

3 walls of blood vessels

Answer 9

most blood vessels have a distinct, 3 layered wall surrounding the lumen (each layer = a tunic)
- tunica intima
- tunica media
- tunica externa

Question 10

tunica intima

Answer 10

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

Question 11

tunica media

Answer 11

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

Question 12

tunica externa

Answer 12

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

Question 13

elastic arteries

Answer 13

“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

Question 14

muscular arteries

Answer 14

“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

Question 15

arterioles

Answer 15

“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

Question 16

capillaries

Answer 16

• 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

Question 17

types of capillaries

Answer 17

• 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

Question 18

3 types of capillaries

Answer 18

continuous
fenestrated
sinusoid

Question 19

continuous capillaries

Answer 19

most common, least permeable
- Abundant in skin, muscles, lungs, and the CNS
- The structural basis of the blood-brain barrier

Question 20

fenestrated capillaries

Answer 20

large fenestrations or pores increase permeability
- abundant in kidneys, small intestine, and areas of hormone secretion

Question 21

sinusoid capillaries

Answer 21

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

Question 22

capillary beds

Answer 22

• interweaving networks formed by capillaries because they do not function independently

Question 23

microcirculation

Answer 23

how blood flows from an arteriole to a venule through a capillary bed
- gases, nutrients, hormones, and waste are exchanged during microcirculation

Question 24

terminal arteriole

Answer 24

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

Question 25

vascular shunt

Answer 25

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

Question 26

precapillary sphincter

Answer 26

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

Question 27

veins: blood reservoirs

Answer 27

• 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

Question 28

venules

Answer 28

• 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

Question 29

veins

Answer 29

• 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

Question 30

Structural Adaptations of Veins

Answer 30

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

Question 31

Venous Sinus

Answer 31

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

Question 32

Varicose Veins

Answer 32

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

Question 33

Anastomosis

Answer 33

special interconnection between blood vessels
- Most organs receive their arterial blood supply from > 1 arterial branch

Question 34

Arterial Anastomoses

Answer 34

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

Question 35

Venous Anastomoses

Answer 35

very common, an occluded vein rarely leads to tissue death