Chapter 22  Cardiovascular System: Vessels and Circulation
 
Histological Organization of Blood Vessels  
   Walls of arteries and veins have three distinct layers:
  1.  Tunica interna a.k.a. tunica intima
   The tunica interna is the innermost lining of the blood vessel. It is lined by endothelium, which is almost always simple squamous epithelium, and its underlying connective tissue. In arteries a layer of elastic tissue called the internal elastic membrane marks the outer boundary of the tunica interna.
  2. Tunica media
   The tunica media is the middle layer. It consists of concentric sheets of smooth muscle cells supported by varying amounts of connective tissue.
   The contraction of the smooth muscles cells is controlled by the autonomic nervous system, hormones and local chemicals. Contraction of the smooth muscles of the media is called vasoconstriction and results in a decrease in the size of the lumen and a decrease in blood flow. Relaxation of the smooth muscles is vasodilation which has the opposite effects.
   Arteries sometimes have an outer layer of elastic tissue called the external elastic membrane. When present this marks the boundary between the tunica media and tunica externa.
  3. Tunica externa a.k.a. tunica adventitia
   The tunica externa is the connective tissue that forms the outermost layer of the vessel. The fibers of this layer often blends into the surrounding tissues and stabilizes the position of the vessel.
    In thick-walled vessels blood vessels supply oxygen and nutrients to the outer layers. These vessels are called vasa vasorum. 
Distinguishing Arteries from Veins 
   Arteries and veins typically travel to and leave the tissues together. When side by side they are easily distinguished because:
  1. The walls of arteries are thicker than the veins.
  2. In cross section arteries appear smaller and rounder than veins.
  3. The endothelial lining of constricted arteries is thrown into folds and is pleated.
 
Arteries 
  Elastic arteries a.k.a. conducting arteries
   Elastic arteries are the arteries that transport large volumes of blood from the heart. They include the aorta and pulmonary trunk and their major branches.
   In elastic arteries the tunica media contains a high density of elastic fibers relative to smooth muscle cells that allow the vessels to stretch and spring back. During systole the walls stretch to absorb the pressure generated by the heart and during diastole releases the absorbed pressure. This results in an even flow of blood in spite of the intermittent pumping action of the blood. 
  Muscular arteries a.k.a. distributing arteries
   Muscular arteries distribute blood to the skeletal muscles and internal organs.
   The tunica media of the muscular arteries have a higher percentage of smooth muscles than connective tissue. Vasoconstriction and vasodilation of these vessels is controlled by the autonomic nervous system.
   Examples of these arteries include the external carotid arteries of the neck, the brachial arteries of the arms, the femoral arteries of the thigh and the mesenteric arteries of the abdomen.
  Arterioles
   Arterioles are considerably smaller than muscular arteries. The tunica externa practically disappears and the smooth muscles of the tunica media become fewer and more scattered.
   These vessels control blood flow between capillaries and arteries. Vasoconstriction and vasodilation is controlled by the autonomic nervous system and hormones but also by local factors. 
Capillaries 
     Capillaries are the smallest of blood vessels consisting of an endothelium supported by its basal lamina. The thinness of the wall, the small size of these vessels, and the slowness of blood flow permit exchange of materials between the blood and interstitial fluid.
  Types of capillaries:
  1. Continuous capillaries have an endothelial lining with the endothelial cells closely attached to one another by tight junctions and desmosomes. This type of capillary is the most common and most closely regulates exchange of materials.
  2. Fenestrated capillaries have endothelial cells that have pores that permit a more rapid exchange of materials and allow large molecules, including peptides and small proteins to pass through.
 
3. Sinusoids are capillaries that have gaps between endothelial cells that make them discontinuous. The basal lamina is thin and either partially or completely absent. Sinusoids are larger and irregular in outline with a shape that conforms to the surrounding tissue.  
   These capillaries provide for the most rapid exchange of fluids and large solutes. Even large suspended proteins and cells may pass through the walls of sinusoids.
   Sinusoids are found in the liver, bone marrow and spleen.
 
Capillary Beds  a.k.a. capillary plexus
     Capillaries work together as a network to supply a blood flow to the tissues according to need. This network is called a capillary bed or capillary network and has various mechanisms for controlling blood flow that include:
     A precapillary sphincter or ring of smooth muscles cells at the beginning of a capillary that can open or close the capillary to blood flow.
     Within capillary beds there are more direct connections between the arteriole and venules that enable blood flow to by pass capillaries beds. The arteriole end of these direct connections has smooth muscle in its wall and is called a metarteriole. As the direct connection approaches the venule it remains relatively wide while resembling a typical capillary and is called a thoroughfare channel.
     Arteriovenous anastomoses are even more direct connections between arterioles and venules that when dilated causes blood flow to completely bypass the capillary beds.
     Continual blood flow to vital organs such as the heart and the brain is insured when a region is supplied by more than one artery. These arteries are then called collaterals. These arteries fuse before giving rise to capillary beds at places called arterial anastomoses. 
Veins
   Veins collect blood from tissues and returns it to heart. The pressure of blood is veins is less than that in arterioles and as consequence the walls of veins are thinner and less elastic. Following the flow of blood and going from smallest to largest veins are classified as:
  Venules
   Venules collect blood from capillaries and resemble expanded capillaries. Venules  smaller than 50 microns lack a tunica media. When present, the tunica media contains only a few smooth muscle cells and are referred to as muscular venules.  
  Medium Veins
   Medium veins have all three layers with a tunica media that is thin and contains few smooth muscle cells. The tunica externa is the thickest layer.
  Large Veins
   Large veins include the great veins such as the superior vena cava and inferior vena cava and their tributaries. The tunica media is slender when compared to the thicker tunica externa. 
Venous valves
   Because of the low pressure in medium and large veins, blood flow must overcome the force of gravity to return to the heart. In the legs this is made possible by the presence of valves in the veins that are infoldings of the tunica intima. These valves permit flow only in one direction. When the surrounding skeletal muscles contract the blood is squeezed through these valves toward the heart. This mechanism for returning blood to the heart is called the skeletal muscle pump.