The document summarizes the structure and function of the blood vascular system. It describes the main components as the heart, arteries, capillaries and veins. It explains that the heart pumps blood through the arteries which branch and decrease in size, becoming capillaries where nutrients are exchanged with tissues. The capillaries then converge into veins which carry blood back to the heart. It provides details on the layers of blood vessels and specialized capillaries.

1. BLOOD VASCULAR SYSTEM
MD 3Y1-6
Midterm Discussion 3
bonto, mae willrose c.

4. ● Circulatory system:
1. blood vascular system
2. lymphatic vascular system
● Blood Vascular System:
1. Heart
2. Arteries
- carry blood to tissues
- become smaller as they branch
3. Capillaries
- smallest blood vessel
- interchange b/w blood & tissues
4. Veins
- carry blood to heart
- become larger as they converge

5. ● Vessels of the blood circulatory system:
1. principal organ: heart
- pumps blood throughout body
- provides for nutrients to leave capillaries
and enter tissues
2. large elastic arteries (leave heart & branch
to form muscular arteries)
3. muscular arteries (branch & enter organs)
4. arterioles
5. capillaries
6. venules
7. veins

7. ● Macrovasculature:
- vessels more than 0.1mm in diameter
- large arterioles
- muscular & elastic arteries
- muscular veins
● Microvasculature:
- visible only w/ microscope
- arterioles
- capillaries
- postcapillary venules
- site of interchanges b/w blood & surrounding
tissues under:
1. normal conditions
2. inflammatory processes

8. ● Macrovasculature:
- vessels more than 0.1mm in diameter
- large arterioles
- muscular & elastic arteries
- muscular veins
● Microvasculature:
- visible only w/ microscope
- arterioles
- capillaries
- postcapillary venules
- site of interchanges b/w blood & surrounding
tissues under:
1. normal conditions
2. inflammatory processes

9. ● General Plan of the Vascular Wall:
1. Tunica intima
- 1 layer of endothelial cells supported by
a thin subendothelial layer of loose CT
w/ occasional smooth muscle cells
- internal elastic lamina: separates the
intima from the media in arteries
- composed of elastin
- has fenestrae (holes)
- allows diffusion of substances
to cells deep in vessel wall
- slightly folded
appearance in tissues at
death

10. 2. Tunica Media:
- middle layer
- concentric layers of helically arranged
smooth muscle cells
- elastic fibers & lamellae, reticular fibers of
collagen type III, proteoglycans &
glycoproteins are interposed among &
produced by smooth muscles
- external elastic lamina: thin layer
separating the media & the adventitia in
arteries
3. Tunica Adventitia/Externa
- consists of type I collagen & elastic fibers
- gradually continuous w/ stromal CT of the
organ through w/c b.v. runs

13. – Arteries:
- thicker tunica media
- narrow lumen
– Veins:
- thicker tunica externa
- large lumen
- tunica intima often folded to form valves
– Capillaries:
- only have an endothelium
- no subendothelial later/ other tunics

17. ● Vasa vasorum:
- “vessels of the vessels”
- often present in large vessels
- arterioles, capillaries, venules in adventitia &
parts of the media
- provides metabolites to cells of respective
layers since walls of large vessels are often
too thick to be nourished solely by
diffusion
- luminal blood alone does not provide
nutrients & O2
for tunica intima
- veins have more vasa vasorum than arteries
since they carry deoxygenated blood
- large arteries supplied more sparesly w/
small sympathetic nerves for control of
vasoconstriction

18. ● Vasomotor nerves
- network of of unmyelinated nerve fibers
- neurotransmitter is norepinephrine
- discharge of norepinephrine from nerves
produce vasoconstriction
- nerves do not enter media of arteries
- neurotransmitter diffuses for several um to
affect smooth muscle cells where gap
junctions propagate response to inner
layers of muscle cells
- nerve endings found in adventitia & media of
veins but its density is less than that of arteries
- Acetylcholine released by nerves act on
endothelium producing nitric oxide w/c
diffuses into smooth muscle cells
- muscle cells relax & vessel lumen is dilated

19. ● Blood Vessels
1. Capillaries
- “exchange vessels”
- composed of a single layer of
endothelial cells in the form of a tube
- ave. diameter: 5- 10 um
- individual length: <50 um
- comprise 90% of all b.v. in body
- total length: 96,000 km
- velocity of blood flow: 0.3 mm/s
- favourable site for exchange of H2
O, CO2
,
substrates, metabolites, solutes,
macromolecules b/w blood & tissue b/c
of thin walls & slow blood flow

20. Endothelial Cells:
- polygonal & elongated in direction of blood flow
- nucleus causes bulge into capillary lumen
- cytoplasm: small Golgi body, mitochondria, free
ribosomes, sparse cisternae of RER
1. small molecules diffuse/actively transported to
plasmalemma of capillary endothelial cells
2. substances diffuse through endothelial
cytoplasm to opposite cell surface
3. discharged into extracellular space
- water & some hydrophobic molecules w/
<1.5 nm diameter & <10 kDain molec.mass diffuse
through intercellular junctions (paracellular pathway)
- pathways for large molecules: pores of
fenestrated capillaries, spaces b/w endothelial cells
of sinusoidal capillaries & pinocytotic vessels

21. Pericytes/Adventitial cells
- cells of capillaries & venules of
mesenchymal origin
- long cytoplasmic processes partly
surrounding the endothelial layer
- enclosed in their own basal lamina
- well-developed networks of myosin,
actin & tropomyosin: contractile function
- proliferate & differentiate to form tunica
media of new b.v. & cells w/ functions
in re-establishing the microvasculature
& its extra-cellular matrix

22. Types of Capillaries
1. Continuous/tight capillary
- most common
- allows regulated exchange of material
- characterized by continuity of cells
- tight occluding junctions seal intercellular
clefts b/w all endothelial cells for minimal
fluid leakage
- found in muscles, CT, exocrine glands &
nervous tissues
- in some places (except NS) numerous
pinocytotic vesicles present on
endothelial cell surfaces
- vesicles appear isolated in cytoplasm
- resp. for transcytosis of macromolecules
across endothelial cytoplasm

23. 2. Fenestrated capillary
- allows more extensive exchange
- characterized by presence of small
circular fenestrae through endothelial cells
- each fenestra usually covered w/ very thin
diaphragm containing heparan
proteoglycans w/o lipid bilayer
- continuous basal lamina covers fenestrae
- macromolecules can cross through
fenestrae
- found in tissues where rapid interchange of
substances occurs b/w tissues & blood
- ex. Kidneys, intestine, choroid plexus
& endocrine glands

24. 3. Sinusoid/ Discontinuous capillary
- allows maximal exchange of
macromolecules & cells b/w tissues &
blood; easy movement of blood cells
- greater diameter than most capillaries
- found in liver, speen, bone marrow &
some endocrine glands
- characterized by:
1. endothelial cells w/ large fenestrae
w/o diaphragm
2. discontinuous layer separated by wide
spaces
3. discontinuous basal lamina

26. ● Tissues w/ high metabolic rates
- abundant capillary network:
kidney
liver
cardiac muscles
skeletal muscles
● Tissues w/ low metabolic rates
- less capillary network:
dense CT
smooth muscle

29. ● Arterioles
- repeated branches of muscular arteries
- 1-2 muscle layers
- smallest arteries
- indicate organ's microvasculature where
exchanges of blood & tissue fluid occur
- diameter: <0.5 mm
- lumens as wide as wall is thick
Metarterioles
- smaller branches of arterioles
- layer of smooth muscle disperse as bands of cells
acting as precapillary sphincters
- allow blood to enter bed of capillaries in a pulsatile
manner
Thorough Channel
- lacks smooth muscle

30. ● Post capillary Venules
- convergence of capillaries & metartioles
● Arteriovenous shuts/ anastomoses
- regulates blood flow by allowing direct
communication b/w arterioles & venules
- richly innervated by sympathetic &
parasympathetic nervous system
- abundant in skeletal muscles, skin of
hands & feet
- when vessels contract, all blood must pass
through capillary network
- when vessels relax, some blood flows to
venules instead of circulating in capillaries

31. ● Anastomoses (cont.,)
- changes in diameter of vessels regulate:
blood pressure
blood flow
temperature
heat conservation
- arterioles in shunts:
- thick, capsule-like adventitia
- thick smooth muscle layer
● Venules
- diameter of lumen in all venules are large
compared w/ thickness of its wall

32. ● Venules (cont.,)
1. post capillary venules
- diameter: 15-20 um
- w/ pericytes
- participate in exchanges b/w bood &
tissues
- primary site where WBC leave circulation
at sites of infection/tissue damage
2. collecting venules
- convergence of postcapillary venules
- more contractile cells
3. muscular venules
- surrounded by tunica media
- 2-3 smooth muscle layers

33. ● Blood Vessels
2. Arteries
Elastic arteries help stabilize blood flow
- incl. aorta & its larger branches
- b/w elastic laminae:
smooth muscles, reticular fibers,
proteoglycans, glycoproteins
Media:
- elastin produces yellowish colour
- consists of: elastic fibers + series of
concentrically arranged, perforated
elastic laminae whose no. increase
w/ age ≈40: newborn
≈70: adults
Intima: thicker than those of muscular artery
Adventitia: underdeveloped

34. ● Blood Vessels
2. Arteries
Elastic arteries help stabilize blood flow
- incl. aorta & its larger branches
- b/w elastic laminae:
smooth muscles, reticular fibers,
proteoglycans, glycoproteins
Media:
- elastin produces yellowish colour
- consists of: elastic fibers + series of
concentrically arranged, perforated
elastic laminae whose no. increase
w/ age ≈40: newborn
≈70: adults
Intima: thicker than those of muscular artery
Adventitia: underdeveloped

35. - largest arteries are elastic & expand when
heart contracts
- several elastic laminae function to make
blood flow more uniform:
Systole: elastic laminae of large arteries
stretch, reducing the force of pressure
Diastole: elastic rebound of large arteries
help maintain arterial pressure when
ventricular pressure drops
= arterial pressure & blood velocity
decrease & become less
variable as distance from heart
increases
- w/ increasing distance from the heart,
arteries gradually have less elastin and
more smooth muscle

37. Muscular arteries
Media: contraction/relaxation of smooth
muscle cells control blood flow to organs
- contains up to 40 layers of prominent
smooth muscle cells w/c intermingle w/ a
no. of elastic laminae, reticular fibers &
proteoglycans
- external elastic lamina present only in
larger muscular arteries
Intima:
- thin subendothelial layer
- prominent internal elastic lamina
Adventitia:
- consists of CT
- lymphatic capillaries, vasa vasorum, nerves
may penetrate to outer part of media

39. ● Blood Vessels
3. Veins
- blood entering veins under low pressure
- move towards heart by contraction of
tunica media & external compressions from
surrounding muscles & other organs
-valves from tunica intima prevent back-flow
of blood
- most are small or medium veins <1cm
- usually travel parallel w/ corresponding
arteries
Intima usually has a thin subendothelial layer
Media consists of small bundlesof smooth
muscle cells intermixed with reticular fibers
& a delicate network of elastic fibers
Adventitia is collagenous & well-developed

40. Small veins:
- very thin
- 2-3 layers of smooth muscle
Large veins:
- big venous trunks, paired w/ elastic
arteries close to heart
Media:
- thin w/ few thin layers of smooth
muscles
- abundant CT (elastic fibers
Intima: well developed
Adventitia:
- thick & often contains longtitudinal
bundles of smooth muscle
- elastic fibers

43. Valves
- most prominent in large veins
- consists of paired semi-lunar folds of
tunica intima projecting across parts of
the lumen
- rich in elastic fibers
- lined both sides by endothelium
- esp. numerous in veins of legs
- help keep flow of venous blood
directed towards the heart
● HEART
- muscular organ contracting rhythmically,
pumping blood throughout circulatory system
- ventricles pump blood to lungs & body
- atria receives blood from body

44. - walls of all 4 heart chambers consist of 3 tunics:
1. internal endocardium
2. middle myocardium
3. external epicardium
Endocardium
- single layer of squamous endothelial cells on
a thin layer of loose CT (elastic + collagen
fibers + smooth muscle cells)
- subendocardial connects subendothelial
layer to mycardium
- veins, nerves, branches of impulse-
conducting system of heart

45. Myocardium
- thickest tunic
- consists of cardiac muscle cells arranged in
layers surrounding heart chambers in a
complex spiral
- thicker in ventricles than in atria
Epicardium
- covers the heart
- simple squamous epithelium (mesothelium)
supported by a thin layer of CT
- subepicardial layer of loose CT: veins,
nerves, adipocytes
- corresponds to visceral layer of pericardium
(serous membrane of heart)
- small amount of lubricant fluid facilitates
heart's movements in space b/w epicardium
& parietal layer

46. Sinoatrial (SA) node
- located at right atrium
- “pace maker”
- small mass of modified cardiac muscle cells
- in fusiform smaller, w/ fewer myofibrils than
neighbouring muscle cells
Atrioventricular (AV) node
- located at right atrium
- cytoplasmic projections branch in
various directions, forming a network
Atrioventricular bundle
- originates from AV node
- passes interventricular septum & splits into
L & R bundles then branch further to
ventricles

47. - cells/fibers of impulse-conducting system are
modified cardiac muscles functionally integrated
by gap junctions
Purkinje fibers (conducting myofibers)
- 1 or 2 central nuclei
- cytoplasm rich in mitochondria & glycogen
- myofibrils are sparse & restricted to periphery
of cytoplasm
- fibers penetrate myocardial layer of
ventricles, allowing stimulus for contraction
to reach innermost layers of ventricular
musculature

48. - parasympathetic & sympathetic neural
components innervates heart
- ganglionic nerve cells & nerve fibers present
in areas close to SA & AV nodes
- affects heart rate & rhythm
ex. physical excersize
emotional stress
- stimulation of parasympathetic division
(vagus nerve) slows heart beat
- stimulation of sympathetic nerve accelerated
rhythm of pacemaker