The microcirculatory bed comprises seven main structures: arterioles, precapillary arterioles, capillaries, postcapillary venules, venules, precapillary sphincters, and arteriovenous shunts. Arterioles regulate blood flow and diameter in response to neural and hormonal signals. Precapillary arterioles are the smallest arteries and feed into capillaries, which are the primary sites of nutrient exchange, or may bypass them via shunts. Capillaries come in continuous, fenestrated, and discontinuous types and have selective permeability barriers. Venules then collect blood from the capillaries and gradually increase in size. Key microcirculatory barriers

1. VESSELS OF THE
MICROCIRCULATORY BED
BY VISITH DANTANARAYANA OF GROUP
101
UNDER THE SUPERVISION OF
PROFESSOR ZADNIPRYANY

2. VESSELS

3. MICROCIRCULATORY BED COMPRISES
SEVEN STRUCTURES
ARTERIOLES
PRECAPILLARY ARTERIOLES
CAPILLARIES
POSTCAPILLARY VENULES
VENULES
PRECAPILLARY SPHINCTERS
AV SHUNT

4. ARTERIOLES
An
arteriole is
a small
diameter
(<20 μm,
up to 5-9
μm) blood
vessel that
extends and
branches out
from an
artery and
leads
to capillaries.

5. • Arterioles receive autonomic
nervous system innervation
and respond to various
circulating hormones in order
to regulate their diameter.

6. ARTERIOLES
Arterioles have
thin muscular
walls (usually
only one to two
layers
of smooth
muscle)

7. PRECAPILLARY ARTERIOLES
They are much smaller than the arterioles in
diameter. In fact the precapillary arteriole is the
smallest variety of arteries. Like arterioles they
too have the same wall with the exception that
in the case of precapillary arterioles the walls
are thinner. Apart from this they have smooth
muscle rings and at the end of a precapillary
arteriole is a precapillary sphincter.

8. VON WILLIEBRANDS’ FACTOR
The Weibel Palade granules located on the
luminal side of the endothelium comprise the
Von Willibrand’s factor, which is a specific
glycoprotein.

9. Total peripheral resistance
• Total peripheral resistance refers to the
cumulative resistance of the thousands of
arterioles in the body, or the lungs, respectively.
• It is approximately equal to the resistance of the
arterioles, since the arterioles are the chief
resistance vessels in the body.
• Total Peripheral Resistance = Mean Arterial
Pressure / Cardiac Output.
• The total peripheral resistance of healthy lung
arterioles is typically about 0.15 to 0.20 that of
the body, so pulmonary artery mean blood
pressures are typically about 0.15 to 0.20 of
aortic mean blood pressures.

10. The precapillary sphincter

11. The entire capillary bed may be
bypassed by blood flow
through arteriovenous anastomoses.

12. VASCULAR SHUNTS

13. CAPILLARY

14. CAPILLARY BED

15. TYPES OF CAPILLARIES
•Continuous
•Fenestrated
•Fenestrated open
capillaries
•Discontinuous

16. TYPES OF CAPILLARIES

17. Sinusoidal capillaries
• A sinusoid is a type of a capillary with a
fenestrated endothelium.
• Located in: liver, lymphoid tissue, endocrine
organs, and hematopoietic organs (bone marrow,
spleen).
• Their highly permeable nature, which is due to
larger inter-cellular clefts allows small and
medium-sized proteins such as albumin to enter
and leave the blood stream.
• Some spaces are large enough for blood cells to
pass.
• Liver sinusoids are equipped with Kupffer cells
that can take up and destroy foreign material
such as bacteria entering the sinusoids.

18. SINUSOIDAL CAPILLARIES

19. FUNCTIONS OF CAPILLARIES
• They form a selectively permeable barrier
between the circulatory system and the
tissues supplied.
• Play a metabolic role – Produce PgI2, growth
factors for blood cells, fibroblast GF, platelet
GF, and in the lungs; angiotensin converting
enzyme
• Inactivation of intercellular messengers
• Antithrombotic function

20. DID YOU KNOW?
The total length of capillaries in an average
adult human is approximately 42 000 km
(25,000 miles), this is approx. equator of Earth.

21. Endothelium (1)
• The endothelium (0.5 μm) is the layer of thin
specialized epithelium, comprised of a single
layer of flat cells that line the interior surface of
blood vessels, forming an interface between
circulating blood in the lumen and the rest of the
vessel wall.
• Space between cells 6-7 nm (little bit less than
albumin)
• Endothelial cells line the entire circulatory
system, from the heart (endocardium) to the
smallest capillary.
• Both blood and lymphatic capillaries are
composed of a single layer of endothelial cells.

22. Endothelium (2)
• Function
– vasoconstriction and vasodilation, and hence the control
of blood pressure
– blood clotting (thrombosis & fibrinolysis)
– formation of new blood vessels (angiogenesis)
– inflammation and swelling (oedema)
– transit of white blood cells
• Pathology
– Atherosclerosis (patients with diabetes mellitus,
hypertension and hyperlipidemia)

23. Capillary pressures
• Middle pressure 25 mm Hg
– 30-40 mm Hg by arterioles
– 10-15 mm by venules
• Oncotic pressure 28 mm Hg
– 19 mm Hg because of proteins
– 9 mm Hg because of some cations
• Because of differences in capillary pressures by arterioles and venules
– Venous end has lower pressure, but there is higher permeability -
therefore 90 % of liquid that goes out at arterial end comes back at
venous end.
• Balance disorder
– Increase of capillary pressure of 20 mmHg increases filtration
pressure cca 68x
– Lymphatic system is not able to accomodate the increase of IC liquid
= results in oedemas
– The oposite – when capillary pressure is lower, the IC liquid decrease

25. IMPORTANT MICROCIRCULATORY
BARRIERS IN THE BODY
•Blood brain Barrier
•Air blood Barrier
•Placental blood Barrier
•Blood testicular Barrier

26. Blood brain barrier

27. BLOOD TESTICULAR BARRIER

28. BLOOD PLACENTAL BARRIER

29. AIR BLOOD BARRIER

30. AIR BLOOD CARRIER
• Surface active protein layer
• Respiratory epithelium
• Basement membrane; which by the
way is common for both the
endothelium of the capillary and the
respiratory alveoli
• Endothelium of blood capillary

32. VENULES
• Venules have three layers:
– An inner endothelium composed of squamous
epithelial cells that act as a membrane
– a middle layer of muscle and elastic tissue (poorly
developed so that venules have thinner walls than
arterioles)
– an outer layer of fibrous connective tissue.

33. Venule