Blood vessels are composed of three layers called tunics. Arteries carry oxygenated blood away from the heart to tissues and have thick muscular walls to withstand high blood pressures. Veins carry deoxygenated blood back to the heart and have thinner walls. Capillaries are the smallest blood vessels that connect arterioles and venules, allowing for exchange of water, oxygen and nutrients between blood and tissues. They come in three types - continuous, fenestrated and sinusoidal - with different permeability properties to meet the needs of different tissues. Capillary exchange involves diffusion of substances and ultrafiltration of fluid across the capillary walls. Both short-term nervous system responses and long-term hormonal mechanisms help regulate blood

1. ARTER
Y

2. General Structure of Blood Vessels
Arteries and veins are composed of three tunicas:
1. Tunica interna (intima):
 Endothelial layer that lines the lumen of all vessels.
 In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present
2. Tunica media:
 Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system
 Controls vasoconstriction/vasodilation of vessels
3. Tunica externa (adventitia):
 Collagen fibers that protect and reinforce vessels
 Larger vessels contain vasa vasorum

3. COMPARISON ARTERY VS VENAE

5. ARTERY
Elastic Arteries:
Thick-walled arteries near the heart; the aorta and its major branches.
Large lumen allows low-resistance conduction of blood.
Contain lots of elastin in all three tunics.
walls stretch and recoil to propel blood
Withstand and regulate large blood pressure fluctuations.
Allow blood to flow fairly continuously through the body
Muscular arteries
 Distal to elastic arteries deliver blood to body organs
(distributing arteries)
 Have thick tunica media with more smooth muscle and
less elastic tissue
 Active in vasoconstriction
Arterioles
 Smallest arteries  lead to capillary beds
 Control flow into capillary beds via vasodilation and
constriction

9. Capillary walls consist of endothelium, which is a
layer of simple squamous epithelium surrounded
by a delicate loose connective tissue. Each
capillary is 0.5 to 1 millimeter (mm) long.
Capillaries branch without changing their diameter,
which is approximately the same as the diameter of
a red blood cell (7.5 m).

10. Continuous capillaries are abundant in the skin
and muscles, and have:
 Endothelial cells that provide an uninterrupted lining
 Adjacent cells that are held together with tight
junctions
 Intercellular clefts of unjoined membranes that allow
the passage of fluids
Continuous capillaries of the brain:
 Have tight junctions completely around the
endothelium
 Constitute the blood-brain barrier
CONTINOUS CAPILLARIES

11. Found wherever active capillary absorption or
filtrate formation occurs (e.g., small intestines,
endocrine glands, and kidneys)
Characterized by:
• An endothelium riddled with pores (fenestrations)
• Greater permeability to solutes and fluids than other
capillaries
FENESTRATED CAPILLARIES

13. Highly modified, leaky, fenestrated capillaries
with large lumens
Found in the liver, bone marrow, lymphoid tissue,
and in some endocrine organs
Allow large molecules (proteins and blood cells)
to pass between the blood and surrounding
tissues
Blood flows sluggishly, allowing for modification
in various ways
SINUSOIDAL CAPILLARIES

14. A microcirculation of interwoven networks of
capillaries, consisting of:
 Vascular shunts – metarteriole–thoroughfare
channel connecting an arteriole directly with a
postcapillary venule
 True capillaries – 10 to 100 per capillary bed,
capillaries branch off the metarteriole and return
to the thoroughfare channel at the distal end of the
bed
CAPILLARY BEDS

15.  Nutrients diffuse across the capillary walls into the
tissue spaces, and waste products diffuse in the
opposite direction.
 In addition, a small amount of fluid is forced out of
the capillaries into the tissue spaces at their
arteriolar ends. Most of that fluid, but not all,
reenters the capillaries at their venous ends.
 Two major forces are responsible for the
movement of fluid through the capillary wall.
 Blood pressure forces fluid out of the
capillary,
 Osmosis moves fluid into it.
 The greater osmotic pressure of blood is caused
by the large concentration of blood proteins
(unable to cross the capillary wall).
 The capillary wall acts as a selectively permeable
membrane, which prevents proteins from moving
from the capillary into the interstitial space but
allows fluid to move across the wall of the
capillary.
CAPILLARY EXCHANGE

22. Short-Term and Long-Term Regulation
 The baroreceptor mechanisms are most
important in shortterm regulation of blood
pressure.
 Hormonal mechanisms, such as the renin-
angiotensinaldosterone system and atrial
natriuretic hormone, are more important in long-
term regulation of blood pressure