VESSELS OF THE MICROCIRCULATORY BED

BY VISITH DANTANARAYANA OF GROUP 101

UNDER THE SUPERVISION OF PROFESSOR ZADNIPRYANY

VESSELS

MICROCIRCULATORY BED COMPRISES SEVEN STRUCTURES

ARTERIOLES

PRECAPILLARY ARTERIOLES

CAPILLARIES

POSTCAPILLARY VENULES

VENULES

PRECAPILLARY SPHINCTERS

AV SHUNT

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.

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

ARTERIOLES

Arterioles have

thin muscular

walls (usually

only one to two

layers

of smooth

muscle)

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.

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.

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.

The precapillary sphincter

The entire capillary bed may be bypassed by blood flow

through arteriovenous anastomoses.

VASCULAR SHUNTS

CAPILLARY

CAPILLARY BED

TYPES OF CAPILLARIES

•Continuous•Fenestrated•Fenestrated open capillaries •Discontinuous

TYPES OF CAPILLARIES

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 cellsthat can take up and destroy foreign material such as bacteria entering the sinusoids.

SINUSOIDAL CAPILLARIES

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

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.

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 thanalbumin)

• 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.

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)

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

IMPORTANT MICROCIRCULATORY BARRIERS IN THE BODY

•Blood brain Barrier

•Air blood Barrier

•Placental blood Barrier

•Blood testicular Barrier

Blood brain barrier

BLOOD TESTICULAR BARRIER

BLOOD PLACENTAL BARRIER

AIR BLOOD BARRIER

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

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.

Venule