Muscular arteries: (medium size arteries)
Distal to the elastic arteries.Most of the named arteries are muscular arteries.
The elastic arteries will gradually branch & become a muscular arteries by loosing most of their elastic laminae in the media leaving only two layers of elastic fibers which are the internal elastic lamina between T. intima & media & the external elastic lamina between the media & adventitia.
Muscular artery has the following features:
T. intima thinner then that of the elastic artery, consist of endothelial cells & very thin subendothelial layer. The internal elastic lamina is prominent.
T. media consist mainly of smooth muscle cells which make these vessels highly contractile (the diameter of the lumen is controlled by the autonomic nervous system). Few elastic fibers can be seen between the smooth muscle cells. The external elastic lamina is not well defined and often incomplete.
T. adventitia consist of loose connective tissue with adipose cells and contain lymphatic capillaries & vasa vasorum.
Muscular arteries varies in diameter from about 1 cm. close to their origin to about 0.5 mm., in the large muscular artery there might be 30 or more layers of smooth muscle cells whereas in the smallest muscular artery there are only 3 muscle layers.
Arterioles:
Less then 0.5 mm in diameter with narrow lumen
T. intima consist of endothelial cells with very thin subendothelial layer
internal elastic lamina only found in the larger arterioles.
T. media is muscular & consist of one or 2 layers of smooth muscle cells. As the arteriole get smaller the smooth muscle cells become discontinuous (called met-arteriole) which will branch later on to form a network of capillaries.
No external elastic lamina.
T. adventitia is thin and insignificant.
The endothelial cells of the arterioles have a granules that contain a protein called Von Willebrands factor, which is regarded as part of the coagulation mechanism. Deficiency of this protein lead to impaired adhesion of platelets to the injured endothelium, a condition known as Hemophilia.
Capillaries:
Smallest blood vessel, about 5-10 m. in diameter.Receive blood from met-arteriole
Have a very thin wall & it is the major site of gaseous exchange from & to the tissues.
Capillary wall is formed of single layer of endothelial cells that are rolled in the form of a tube, these cells rest on a basement membrane. Occasional scattered contractile cells called pericytes are found external to the endothelial cells.
When cut transversely the wall of the capillary appear to be formed of a portion of one or more endothelial cells.
The nucleus of the endothelial cells cause the cell to bulge into the capillary lumen.
Pericytes:
Are mesenchymal cells with long cytoplasmic processes that partly surround the endothelial cells at various locations along the capillaries & small venules. Function of these cells:
The presence of actine & myocine filaments in their cytoplasm suggest that they have a contractile function.
After tissue injury these cells will proliferate & differentiate to form new blood vessels & connective tissue cells.
Types of capillaries:
Depending on the structure of endothelial cell & the presence or absence of basal lamina, capillaries can be grouped into 4 groups:
Continuous (somatic) capillary: characterized by the absence of fenestrations in its wall, it is found in all types of muscle tissues, connective tissues, exocrine glands & nervous tissues. The cytoplasm of the endothelial cells of the continuous capillary contain many large pinocytotic vesicles, these vesicles are important for the transport of macromolecules in both directions across the endothelial cell, however very few or even no pinocytotic vesicles are seen in the endothelial cells of capillaries supplying the nervous system. This feature is important for the isolation of the nervous system from the rest of the body by the blood-brain barrier.
fenestrated (visceral) capillary: characterized by the presence of large fenestrae in the wall of the endothelial cells, these are closed by a diaphragm that is thinner then the cell membrane, this type of capillaries have a continuous basal lamina, it is mainly found in tissues where rapid exchange of substances occur as in kidney, intestine & endocrine glands. Even macromolecules can cross the capillary wall through these fenestrae to enter the tissue.
fenestrated capillaries with no diaphragm which is characteristic type of capillaries found in the renal glumeriolus but it has a very thick basal lamina.
discontinuous sinusoidal capillary: it has a:
tortuous path
greatly enlarged diameter up to 30-40 m. which is important to slow the circulation of blood through these capillaries
the endothelial cells form a discontinuous layer and are separated from each others by wide spaces.
The cytoplasm of the e3ndothelial cell shoe s multiple fenestrations without diaphragm
Basal lamina is often discontinuous or even abscent to facilitate the exchange of materials between blood and tissues.
This type of capillaries are mainly found in the liver & the hematopoietic organs as bone marrow and spleen
macrophages are found between the endothelial cells.
Capillary circulation:
the met-arterioles that are surrounded by a discontinuous layer of smooth muscle branch profusely to form a network of capillaries with a large surface area to facilitate exchange of materials between blood & tissues.Contraction of smooth muscles of the met-arterioles help to regulate but dosenot completely stop the circulation in the capillaries so it maintain the pressure difference between the arterial & venous sides.
There is a simple ring of smooth muscle (sphincter) at the point where capillaries originate from met-arterioles called pre-capillary sphincter, contraction of this sphincter can completely stop blood flow within the capillaries.
The richest capillary network is found in tissues with high metabolic activities such as kidney, liver, cardiac muscle and skeletal muscles.
Arteriovenous anastomosis:
These are direct communications between arteriolar and venous circulations.At its arteriolar end, the arteriovenous anastomosis has a thick wall mainly due to abundant smooth muscle coat in the wall of the arteriole. Contraction of this smooth muscle coat lead to closure of the lumen of the anastomosis so the blood will be diverted more to the capillary network.
Relaxation of these smooth muscles will open the lumen allowing the blood to flow directly to the venule bypassing the capillary network.
Changes in the diameter of these anastomosis regulate blood pressure, blood flow & temperature. It has also an important role in the skins thermoregulatory function.
Those AV anastomosis are wide spread in the body, but are most common in the skin of fingertips, lips, nose, ears & toes.
The Circulatory System Dr. Luma Al-Allaf
Lecture:2