The Cardiovascular System: Blood Vessels

1
The Cardiovascular SystemBlood Vessels

• Mr. Tsigaridis

2
Introduction

• The blood vessels of the body form a closed
  delivery system that begins and ends at the heart
• Often compared to a plumbing system, it is a far
  more dynamic system of structures that pulse,
  constrict and relax and even proliferate to meet
  changing body needs

3
Blood Vessel Structure Function

• The major types of blood vessels are
• Arteries
• The large distributing vessels that bring blood
  to the body
• Capillaries
• The tiny vessels that distribute blood to the
  cells
• Veins
• The large collecting vessels that bring blood
  back to the heart
• Intermediate vessels connect
• Arterioles bring blood to the capillaries
• Venules drain blood from the capillaries

4
Blood Vessel Structure Function

• The pattern of distribution starts with arteries
  to arterioles to capillaries to venules to veins
• The blood vessels in the adult human body carry
  blood in a distribution network that is
  approximately 60,000 miles in length
• Only capillaries come into intimate contact with
  tissue cells and serve cellular needs

5
Structure of Blood Vessel Walls
6
Blood Vessel Walls

• The walls of blood vessels are composed of three
  distinct layers or tunics
• The tunics surround a central opening called a
  lumen

7
Arteries

• Arteries are vessels that carry blood away from
  the heart
• All arteries carry oxygen rich blood with the
  exception of those in the pulmonary circuit
• Blood proceeds to the tissues through
• Elastic arteries
• Muscular arteries
• Arterioles

8
Elastic (Conducting) Arteries

• Elastic arteries are thick walled arteries near
  the heart - the aorta and its major branches
• These arteries are the largest in diameter and
  the most elastic
• A large lumen allows them to serve as low
  resistance pathways that conduct blood from the
  heart to medium-sized arteries and thus are
  called conducting arteries

9
Elastic (Conducting) Arteries

• The elastic arteries contain more elastin than
  any other type of vessel
• While present in all three layers, the tunica
  media contains the most
• The abundant elastin enables these arteries to
  withstand and smooth out large pressure
  fluctuations by expanding when the heart forces
  blood into them and then recoiling to propel
  blood onward into the circulation when the heart
  relaxes

10
Muscular (Distributing) Arteries

• The muscular distributing arteries deliver blood
  to specific body organs and account for most of
  the named arteries
• Proportionately, they have the thickest media of
  all vessels
• Their tunica media contains relatively more
  smooth muscle and less elastic tissue than that
  of elastic arteries
• They are more active in vasoconstriction and are
  less distensible

11
Muscular (Distributing) Arteries

• As in all vessels, concentric sheets of elastin
  occur within the tunica media of muscular
  arteries although these sheets are not as thick
  or abundant as those of elastic arteries

12
Muscular (Distributing) Arteries

• A feature unique to muscular arteries, especially
  thick sheets of elastin lie on each side of the
  tunica media
• An external elastic lamina lies between the
  tunica media and tunica externa

13
Muscular (Distributing) Arteries

• The elastin in muscular arteries, like that in
  elastic arteries, helps dampen the pulsatile
  pressure produced by the heartbeat

14
Arterioles

• Arterioles have a lumen diameter from 0.3 mm to
  10 ?m, and are the smallest of the arteries
• Larger arterioles exhibit all three tunics, but
  their tunica media is chiefly smooth muscle with
  a few scattered muscle fibers
• The smaller arterioles that lead into capillary
  beds, are little more than a single layer of
  smooth muscle cells spiraling around the
  endothelial lining

15
Arterioles

• The diameter of each arteriole is regulated in
  two ways
• Local factors in the tissues signal the smooth
  musculature to contract or relax, thus regulating
  the amount of blood sent downstream to each
  capillary bed
• Sympathetic nervous system adjusts the diameter
  of arterioles throughout the body to regulate
  systemic blood pressure

16
Capillaries

• The microscopic capillaries are the smallest
  blood vessels
• In some cases, one endothelial cell forms the
  entire circum- ference of the capillary wall
• The average length of a capillary is 1 mm and the
  average diameter is 8-10 ?m

17
Capillaries

• Capillaries have a lumen just large enough for
  blood cells to slip through in single file

18
Capillaries

• Capillaries are the bodys most important blood
  vessels because they renew and refresh the
  surrounding tissue fluid (interstitial fluid)
  with which all cells in the body are in contract
• Capillaries deliver to interstitial fluid the
  oxygen and nutrients that cells need while
  removing carbon dioxide and nitrogenous wastes
  that cells deposit in the fluid

19
Capillaries

• Given their location and the thinness of their
  walls capillaries are ideally suited for their
  role of providing access to nearly every cell
• Along with the universal functions just described
  some capillaries also perform site-specific
  functions
• Lungs gas exchanges
• Endocrine glands pick up hormones
• Small intestine nutrients
• Kidneys removal of nitrogenous wastes

20
Capillary Beds

• A capillary bed is a network of the bodys
  smallest vessels that run throughout almost all
  tissues, especially the loose connective tissue
• This flow is also called a microcirculation

21
Capillary Beds

• In most body regions, a capillary bed consists of
  two types of vessel a vascular shunt (meta-
  arteriole) and true capillaries

22
Capillary Beds

• The terminal arteriole leads into a metarteriole
  which is directly continuous with the thorough-
  fare channel

23
Capillary Beds

• The thoroughfare channel joins the post-
  capillary venule that drains the capillary bed

24
Capillary Beds

• The true capillaries number 10 to 100 per
  capillary bed, depending on the organ served
• Branch from metarteriole to thoroughfare channel

25
Capillary Beds

• A cuff of smooth muscle fibers, called a pre-
  capillary sphincter surrounds the root of each
  capillary at the metarteriole and acts as a valve
  to regulate the flow of blood into the capillary

26
Capillary Beds

• When the precapillary sphincters are relaxed,
  blood flows through the true capillaries and
  takes part in exchanges with tissue cells

27
Capillary Beds

• When the precapillary sphincters are contracted,
  blood flows through the shunts and bypasses the
  tissue cells

28
Capillary Beds

• Most tissues have a rich supply, but there are a
  few exceptions
• Tendons and ligaments / poorly vascularized
• Cartilage / from adjacent connective tissue
• Epithelia / from adjacent connective tissue
• Cornea / nourished by aqueous humor

29
Capillary Beds

• The relative amount of blood entering a capillary
  bed is regulated by vasomotor nerve fibers and
  local chemical conditions
• A capillary bed may be flooded with blood or
  almost completely bypassed, depending on
  conditions in the body or in that specific organ
• Example of shunting blood from digestive organs
  to skeletal muscles

30
Capillary Permeability

• The structure of capillaries is well suited for
  their function in the exchange of nutrients and
  wastes between the blood and the tissues through
  the tissue fluid
• A capillary is a tube consisting of thin
  endothelial cells surrounded by a basal lamina
• The endothelial cells are held together by tight
  junctions and occasional desmosomes

31
Capillary Permeability

• Tight junctions block the passage of small
  molecules, but such junctions do not surround the
  whole perimeter of the endothelial cells
• Instead, gaps of unjoined membrane called
  intercellular clefts occur through which small
  molecules exit and enter the capillary

32
Routes of Capillary Permeability

• Molecules pass into and out of capillaries via
  four routes
• Direct diffusion through endothelial cell
  membranes
• Through the intercellular clefts
• Through cytoplasmic vesicles
• Through fenestrations in fenestrated capillaries

33
Routes of Capillary Permeability

• Most exchange of small molecules is thought to
  occur through intercellular clefts
• Carbon dioxide and oxygen seem to be the only
  important molecules that diffuse directly through
  endothelial cells because these uncharged
  molecules easily diffuse through lipid containing
  membranes of cells

34
Low Permeability Capillaries

• The blood-brain barrier prevents all but the most
  vital molecules(even leukocytes) from leaving the
  blood and entering brain tissue
• The blood-brain barrier derives its structure
  from the capillaries of the brain
• Brain capillaries have complete tight junctions,
  so intercellular clefts are absent

35
Low Permeability Capillaries

• Brain capillaries are continuous, not fenestrated
  and they also lack caveolae
• Vital capillaries that must cross brain
  capillaries are ushered through by highly
  selective transport mechanisms in the plasma
  membranes of the endothelial cells

36
Veins

• Veins are the blood vessels that conduct blood
  from the capillaries back to the heart
• Because blood pressure declines substantially
  while passing through the high-resistance
  arterioles and capillary beds, blood pressure in
  the venous part of the circulation is much lower
  than in the arterial part

37
Veins

• Because they need not withstand as much pressure,
  the walls of veins are thinner than those of
  comparable arteries
• The venous vessels increase in diameter, and
  their walls gradually thicken as they progress
  from venules to the larger and larger veins
  leading to the heart

38
Venules

• Venules, ranging from 8 to 100 ?m in diameter are
  formed when capillaries unite
• The smallest venules, the postcapillary venules,
  consist of endothelium on which lie pericytes

39
Venules

• Venules join to form veins
• With their large lumens and thin walls, veins can
  accommodate a fairly large blood volume
• Up to 65of the bodys total blood supply is
  found in the veins at any one time although the
  veins are normally only partially filled with
  blood

40
Veins
externa

• Veins have three distinct tunics, but their walls
  are always thinner and their lumens larger than
  those of corresponding arteries
• There is little smooth muscle even in the largest
  veins

41
Veins

• The tunica externa is the heaviest wall layer and
  is often several times thicker than the tunica
  media
• In the venae cavae, the largest veins, which
  return blood directly to the heart the tunica
  externa is further thickened by longitudinal
  bands of smooth muscle

Tunica externa
42
Veins

• Veins have less elastin in their walls than do
  arteries, because veins do not dampen any
  pulsations (these have been smoothed out by the
  arteries)
• Because blood pressure within veins is low, they
  can be much thinner walled than arterioles
  without danger of bursting

43
Veins

• Low-pressure conditions demand some special
  adaptations to help return blood to the heart at
  the same rate as it was pumped into circulation
• One structural feature that prevents the backflow
  of blood away from the heart is the presence of
  valves within veins

44
Veins

• Venous valves are formed from folds of the tunica
  intima and they resemble the semilunar valves of
  the heart in structure and function
• Venous valves are most abundant in the veins of
  the limbs, where the upward flow of blood is
  opposed by gravity

45
Veins

• A few valves occur in the veins of the head and
  neck, but none are located in veins of the
  thoracic and abdominal cavities
• A functional mechanism that aids the return of
  venous blood to the heart is the normal movement
  of our body and limbs

46
Veins

• Another mechanism of venous return is called the
  skeletal muscular pump
• Here contracting muscles press against the
  thin-walled veins forcing valves proximal to the
  contraction to open and propelling the blood
  toward the heart

47
Vascular Anastomoses

• Where vessels unite or interconnect, they form
  vascular anastomoses
• Most organ receive blood from more than one
  arterial branch and arteries supplying the same
  area often merge, forming arterial anastomoses
• Arterial anastomoses provide alternative pathways
  called collateral channels for blood to reach a
  given body region

48
Vascular Anastomoses

• If one arterial branch is blocked arterial
  anastomoses provide the region with an adequate
  blood supply
• Arterial anastomoses are abundant in abdominal
  organs and around joints, where active movement
  may hinder blood flow through one channel

49
Vascular Anastomoses

• Anastomoses are also prevalent in the abdominal
  organs, brain, and heart
• Because of the many anastomoses among the smaller
  branches of the coronary artery in the heart
  wall, a coronary artery can be 90 occluded by
  atherosclerosis (plaque) before a myocardial
  infarction (heart attack) occurs

50
Vasa Vasorum

• The wall of the blood vessels contain living
  cells and therefore require a blood supply of
  their own
• For this reason the larger arteries and veins
  have tiny arteries, capillaries and veins in
  their tunica externa
• These tiny vessels the vasa vasorum nourish the
  outer half of the wall of a large vessel with the
  inner half being nourished by the blood in the
  lumen

51
End of Material