VESSELS

INTRRO:
Although the center of the cardiovascular system is the heart, it is the blood vesselse that carry blood throughout the body.
As blood flows from the heart through these vesselse, it supplies tissue with substances needed for metabolism and homeostaticregulation.
Waste products that accumulate in the cells pass in to the blood, which carries them off through other vessels for removal by the kidneys, skin, and lunge.

TYPE OF BLOOD VESSELS:
Arteries are blood vessels that carry blood away from the heart to the organs and tissue of the body.
Because different arteries contain varying amounts of elastic and smooth muscle tissue, some are called alastic, and other are called muscular.
the wall of alastic arteries axpand slightly with each heart beat.
The muscular arteries branch in to smaller arterioles.
These vessels play important role in determining the amount of blood going to the muscular arteries and arterioles can be either dilated or constricted by nervouse control.
Aarterioles branch in to smallervessels called metarterioles which carry blood either into venules (tiny veins) or in to the smallest vessels in the body, the capillaries.

CAPILLARIES:
Capillaries are microscopic vessels with walls mostly one cell thick.
The thin capillary wall is porouse, and allows the passage of water and small particles of dissolved materials.
Capillaries are distributed throughout the special places as the lenses of the eyes.
Capillaries converge in to larger vessels called venules, which merge to form larger vessels called veins.

VEINS:
Carry blood toward the heart.
They are generally more flexible than arteries, and they collapse if blood pressure is not maintained.

HOW BLOOD VESSELS ARE NAMED:
The majority of blood vessels named according to the major organ or anatomical site supplie (by arteries) or drained (by veins), or according to their location in the body.
For example: The renal artery supplies the kidneys, and the vessel is also used.
For example: The internal jugular vein lies deeper in the neck than the external jugular vein does.

In some instance, the name of a blood vessel change as it passes in to a different part of the body.
For example: The subclavian artery runs "underneath" the clavicle in the neck region.
When it enters the region of the armpit (axilla), its name change to the axillary artery, and it becomes the brachial artery when it enters the arm(brachium).
In general, veins run parallel to most arteries and are given the same name, although there are exception.

ARTERIES:
Most arteries are efferent vessels that carry blood away from the heart to the capillary beds throghout the body.
In the adult, all arteries except the pulmonary trunk and pulmonary arteries carry oxygenated blood.
The left and right pulmonary thus, the most reliable way to classify blood vessels is by the heart or away from it.
The great arteries that emerge from the heart are often called trunks.
The major arterial trunks are the aorta(ascending aorta)from the right ventricle.
The central canal of all blood vessels, including arteries, is called the lumen.
Surrounding the lumen of an artery is a thick wall composed of three layers, or tunicae (TEW-nih-see;pl. of L, tunica,covering)
The tunica intima ("innermost covering") has alining of endothelial cells(simple squamous epithelium), a thin sub endothelial layer of fine areolar tissue an internal elastic called tha internal elastic lamina.
Small arteries donot contain vasa vasorum, but do have the three layer shown.


The tunica media (middle coreing) is the thickest layer of arterial wall in the large arteries.
It is composed mainly of connective tissue, smooth muscle cells, and elastic fibers.
The wall of the largest arteries (elastic arteries) have alastic tissue rather than smooth muscle.]
In the smaller arteries (muscular arteries), the elastic fibers in the tunica media are reolaced by smooth muscle cells.
The tunica adventitia (outermost covering) is composed mainly of collagen fibers and elastic fibers.
Occasional smooth muscle fibers run longutudinally next to the outer border of the tunica media.
Nerves and lymphatic vessels are also found with this layer.
The walls of the large arteries (greate than 20 mm) are nourished by smallblood vessels called the vasa vasorum (vessels of the vessels) which form capillary networks with tha tinica adventitia and outer part of the tunica media.
The strong elastic walls of the largest arteries allow these vessels to adjust to the great pressure created by the contraction of the ventricles during systole.
Blood is ejected in to the aorta at a speed of 30 to 40 cm/sec (almost a mile an houre).

PULSE:
When an artery lies close to the surfase of skin, a pulse can be felt that corrosponds to the beating of the heart and the arterial wall.
The pulse is produced when the left ventericle forces blood against the wall of the branches of the aorta and the rest of the elastic arterial walls.
A venous pulse occurs only in the largest veins.
It is priduced by the chanfes in pressure that accompany arterial contractions.
All arteries have a plus [but it canbe felt most easily at the points shown].

The most common site for checking the pulse rate is the radial artery on the undersite of the wrist.
The three middle finger are usually used to check the pulse (but not the thumb, which may be close enough to the radial artery to reflect it own pulse beat).
The pulse is checked for several reason.
For example: A physician can detect the number of heartbeating per minute (heart rate)
The streng of the beat and several other diagnostic factors.
The average resting pulse can range between 70 to 90 beats per min in adult and form 80 to 140 in children.
When the pulse rate exceed 100 beats per minute, the condition is called TACHY CARDIA (Gr.takhus, swift) when the rate is below 60 beats per minute, the condition is called BRADY CARDIA (GR.bradus,slow).
The pulse rate normally decreases during sleep and increases after eating or exercising.
During a fever it may increase about five beat per minute for every degree fahrenheit above the normal body temprature. pulse rates tend to increase significantly after severe blood loss, and are usually high in cases of seriouse anemia.

ARTERIOLES:
The arterioles nearest the heart are the largest.
As their distance from the heart increases, the arteries branch in to amaller and amaller arteries, then in to arteriols shortly befor reaching the capillary networks.
Arteriols are covered by the three tunica.
Because their walls constrict thus controlling the flow of blood from arteries in to capillaries by as much as 400 percent.
Terminal arterioles (those closest to a capillary) are muscular and well supplied with nerves, but they donot have an internal elastic layer or their own blood vessels (vasa vasorum), as larger arteries do.
The tunica media is particulary well supplied with sympathetic nerves, which cause the smooth muscle cells to contract and the lumen to contrict.

CAPILLARIES:
Capillaries terminal arteriols branch out to form capillaries (L-capillaries, hair) which connect the arterial and venous system.
Capillaries are generally composed of only a single layer (tunica intima) of endothelial cell on a thin basement membrane of glycoprotein.
Capillaries are the smallest and most numerous blood vessels in the body.
If all the capillaries in an adult body were connected, they would stretch about (96,000 mi).
This abundance of capillaries makes an enormous surface area available for the exchange of gases, fluid, nutrients, and wastes between the blood and nearby cells.
Capillaries are about (600) tomes narrower than a medium-size vein, and about (500) times narrower than a medium-sized artery.
The diameter of capillaries varies with the function of tissue.

TYPE OF CAPILLARIES:
At least three different type of capillaries are recognized, and each one performs a specific function. (FIGURE4)

CONTINUOUS CAPILLARIES:
Are found in skeletal muscle tissue.
Their walls are made up of one continuouse endothelial cell, with the ends overlapping in a tight endothelial cell junction. (FIGURE5)
(Pinocytic) vesicle help to move liquid across the membrane of the cell by exocytosis and endocytosis.
The wall of fenestrated capillaries (L.fenestra window) consist of two or more adjacent endothelial cell connected by thin endothelial membranes called fenestrations, or pores. (FIGURE6) Fenestrated capillaries are usually found in the kidneys, endocrine glands, and intestines.
Discontinuous capillaries, also known as a sinusoid or vascular sinuses, have fenestrations and a much wider lumen than the other types. (FIGURE6)
Such an open, irregular structure is highly permeable.
This type of capillaries system is found in the liver and spleen.
Liver sinusoids also contain active phagocytes called stellate reticuloendothelial cells (Kupffer cells;koop-fur) which are part of the reticuloendothelial system.

CAPILLARY BLOOD FLOW:
Blood leavers the heart trareling about 30 to 40 cm/sec(12 in./sec) but it is slowed to only 2.5 cm/sec(1 in./sec) in the capillaries.
Blood remains in the capillaries for only a second or two, but given the short lengh of capillaries (about 1 mm) that is long enough for the crucial exchanges of nutrient and wastes.
Because capillaries walls are usually only cell thick, certain material pass through rather easily.
Small cules, includinggases, such as oxygen and carbodioxide, pass through freely, but the large molecules of plasma proteins pass through only with difficulty it at all.
Red blood cells (R.B.C) cannot pass through the capillary walls.

MICRO CIRCULATION OF BLOOD:
The capillaries and their associated structures (including terminal arterioles, metarteriols,and venules) constitute the micro-circulation of the blood,
this name reflects the extremly narrow diameters of the vessels.
A metarteliole is a vessel that emerge from an arteriole,traverse the capillary network, and empties in to the precapillaries sphincter, a ring-like smooth muscle cell that regulates the flow of blood in to the capillaries.
Some metarterioles connect directly to venules by way of THOROGHFARE channels.
Similar to THOROUGHFARE channels are COLLATERAL channels (capillary shunts or arteriovenuouse anastomases), which bypass the capillary beds and act as direct links between arterioles and venules.
Collateral channels have thick walls, they are not exchange vessels in the way that capillaries are.
They are heavily innervated, and muscular at the arteriole end elastic at the venule end.

VENULES:
Blood drain form the capillaries in to venules (VEHN-YOOLZ) tiny veins that unite to form large venules occurs gradually.


THE IMMEDIATE POST-CAPILLARY ADVENTITIA:

Post-capillary venules play an important role in the exchange between blood and interstitial fluid.
Unlike capillaries, these venules are easily affected by inflammation and allergic reactions.
They respond to these conditions by opening their pores, allowing water, solutes, and white blood cells (W.B.C) to move out in to the extera cellular space.

VEINS:
venules join to form veins.
Superficial veins are found in areas where blood is collected near the surface of the body, and are specially abundant in the limbs.
Veins become larger and less branched as they move away from the capillaries and toward the heart.
Most vein are relatively large.

They carry deoxygenated blood from the body tissue to the heart, however, there are three exceptions:

1) The four pulmonary veins carry oxygenated blood from the lungs to the left artrium of heart.
2) The hepatic portal system of vein carries blood from the capillaries of the intestines of the capillaries and sinusoids of the liver.
3) In the hypothalamus unite to form veins that divide in to a second set of sinusoidal capillaries in the "anterior pituritary glang".

The wall of vein contain the same three layers (tunicae) as arterial walls, but the tunica media is much thinner
also venous wall contain less elastic tissue, collagenous tissue and smooth muscle.
As a result, veins are very distensible and compressible.
The smooth muscle cells that are found in veins are arranged in either a circular or longitudinal pattern.
Like arteries veins are nourished by small vasa vasorum.
Veins usually contain paired semilunar bicuspid valves that permit blood to flow in only one direction restricting any back flow.
Blood is helped along by the skeletal muscle pump ( the compression of the venous walls by the contration of surrounding skeletal muscle).
The venous valves, which are derived from folds of the tunica intima, are especially abundant in the lower limbs, where gravity opposes the return of blood to the heart.
There are no valves in veins narrow than (1 mm) or in regions of greatmuscular pressure, such as the thoracic and abdominal cavities.