Histology - Circulatory System

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circulatory flow
elastic arteries
muscular arteries
postcapillary venules
medium-sized vein
large vein
blood vessel composition (3 layers)
1 - tuncia intima
2 - tunica media
3 - tunica adventitia
tunica intima
*delimits the vessel wall towards the lumen of the vessel and comprised its endothelial lining (typically simple squamous with many tight junctions) and connective tissue (basal lamina and loose connective tissue)
*beneath the connective tissue there is the internal elsatic lamina

The tunica intima (or just intima) is the innermost layer of an artery or vein. It is made up of one layer of endothelial cells and is supported by an internal elastic lamina. The endothelial cells are in direct contact with the blood flow.
The inner coat (tunica intima) can be separated from the middle (tunica media) by a little maceration, or it may be stripped off in small pieces; but, on account of its friability, it cannot be separated as a complete membrane. It is a fine, transparent, colorless structure which is highly elastic, and, after death, is commonly corrugated into longitudinal wrinkles.
The inner coat consists of: -A layer of pavement endothelium, the cells of which are polygonal, oval, or fusiform, and have very distinct round or oval nuclei. This endothelium is brought into view most distinctly by staining with silver nitrate.
  1. -A subendothelial layer, consisting of delicate connective tissue with branched cells lying in the interspaces of the tissue; in arteries of less than 2 mm. in diameter the subendothelial layer consists of a single stratum of stellate cells, and the connective tissue is only largely developed in vessels of a considerable size.
  2. -An elastic or fenestrated layer, which consists of a membrane containing a net-work of elastic fibers, having principally a longitudinal direction, and in which, under the microscope, small elongated apertures or perforations may be seen, giving it a fenestrated appearance. It was therefore called by Henle the fenestrated membrane. This membrane forms the chief thickness of the inner coat, and can be separated into several layers, some of which present the appearance of a network of longitudinal elastic fibers, and others a more membranous character, marked by pale lines having a longitudinal direction. In minute arteries the fenestrated membrane is a very thin layer; but in the larger arteries, and especially in the aorta, it has a very considerable thickness.
tunica media
*formed by a layer of circumferential smooth muscle and variable amounts of connective tissue
*a second layer of elastic fibers - the external elastic lamina - is located beneath the smooth muscle away from the tunica intima
*ite of histological specialization in arterial walls
tunica adventitia / externa
* outter most layer; surrounds tunica media
*consists mainly of connective tissue fibers (COLLAGEN which serves to anchor blood vessle to nearby organs - stability)
*blends in with surrounding connective tissue surrounding the vessel
*hard to indicate precisely where the tunica adventitia ends
*3 types (large elastic, medium muscular, small arteries/arterioles)
*specialization of arterial walls depend on: (i) pressure pulses generated during contractions of heart (systole), and (ii) regulation of blood supply to target tissues of arteries
*tunica media = site of histological specialization in arterial walls
elastic arteries
*tunica intima of elastic arteries thicker than in other arteries
*layer of subendothelial connective tissue allows the tunica intima to move independently from other layers as the elastic arteries distend with the increase in systolic blood pressure
*in adult humans, 50 elastic lamellae found in tunica media of aorta
*energy stored in elastic fibers of the tunica media allows elastic arteries to function as a PRESSURE RESEVOIR which forwards blood along during ventricular relaxation (diastole)
*smooth muscle cells and collagen fibers are present between layers of elastic fibers
*both fiber types produced by smooth muscle cells
external elastic lamina different to discern from other layers of elastic fibers in tunica media
*tunica adventitia appears thinner than tunica media and contains collagen fibers and cell types typically seen in connective tissue
*vasa vasorum (blood vessels in tunica adventitia that nourish cells of tunica media and adventitia)
vasa vasorum
blood vessels in tunica adventitia that nourish cells of tunica media and tunica adventitia
muscular arteries
*tunica media thinner than in elastic arteries
*subendothelial connective tissue other than the internal elastic lamina often hard to discern
*internal elastic lamina appears as well defined layer
*tunica media dominated by numerous concentric layers of smooth muscle cells. Fine elastic fibers and a few collagen fibers are also present.
*external elstic lamina clearly distinguished - though may be incomplete
*thickness and appearance of tunica adventitia is variable
*diameter less than 0.1 to 0.5mm
*endothelium still rests on internal elastic lamina - which may be incomplete and which is not always well-defined in histological sections
*tunica media consists of 1-3 concentric layers of smooth muscle cells that regulate the blood flow to target tissues
*hard to identify external elastic lamina or to distinguish tunica adventita from the connective tissue that surrounds the vessel
*final branching of the arterioles finally gives rise to the capillary network (microcirculation)
*sum of diameters of all capillaries is significantly larger than that of the aorta - which decreases the blood pressure and flow rate
*capillary diameter ranges from 4 to 15 micrometers

*functions of capillaries:
-provides nutrients and oxygen to surrounding tissue
-absorption of nutrients, waste products and carbon dioxide
*excretion of waste products from the body

*only tunica intima is present! which typically only consists of the endothelium and its basal lamina
*an incomplete layer of cells surrounding the capillary - pericytes
*incomplete layer of cells surrounding capilaries
*contractile properties
*regulate blood flow in capillaries by surrounding the tunica intima
*during vascular remodeling and repair they differentiate into endothelial and smooth muscle cells
*walls of veins are thinner than walls of arteries
*diameter is larger than that of arteries
*layering in the wall of veins is NOT distinct
*tunica intima is very thin; only the largests veins contain subendothelial connective tissue
*internal and external laminae absent or very thin
*tunica media appears thinner than tunica adventitia and these two layers blend into eachother
*venous vessels originate from capillary network which lead into smallest venous vessels
vein valves
*loose, pocket-shaped folds of tunica intima
*extends into the lumen of the vein
*opening of the pocket will point in the direction of blood flows towards the heart
lymphatic vessels
parts of the blood vessels with exude from the blood vessels into the surrounding tissues because of transport across the endothelium or due to blood pressure
*role of lymph vessels is to return surplus liquid (lymph) to blood circulation
lymph capillaries
*larger than blood capillaries
*very irregularly shaped
*begin as blind-ending tubes in connective tissue
*basal lamina almost completely absent
*endothelial cells do not form tight junctions - thereby facilitates flow of liquids into lymph capillary
*temporary openings in endothelial lining of the lymph capillaries allow entry of larger particles into lymph capillaries (lipid droplets)
* merge to form lymph collecting vessels
lymph vessel
*formed from merging lymph capillaries
*larger than lymph capillaries - though otherwise very similar
*have valves
*lymph moved via compression of lymph vessels by surrounding tissues - little lymh moves without this muscular activity
*empty into lymph nodes - travel though other lymph vessels and eventually merge to form lymph ducts
*endothelial cell nuclei appear to be flattened
*vessels have irregular shapes
lymph ducts
*one or two layer of smooth muscle cells in wall
*form vales
*movement of lymph toward the heart enhanced by (i) peristaltic contractions of smooth muscle, and (ii) compression of ducts by surrounding tissues
thorasic duct
largest lymph duct, 5mm in diameter, drians lymph from the lower half and upper left quadrant of the body, empties lymph into circulation by merging with vascular system close to junction of left internal jugular and subclavian veins

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