Circulatory System

• Circulatory system


  Network of blood vessels that provide the body a means of transporting various substances to and from the different tissues
• Components of the circulatory system

  • Heart
  • Arteries
  • Veins
  • Capillaries
  • Lymphatics

  
• Heart


  Muscular pump that propels blood into the blood vessels
• Chambers of the heart

  • Two atria
  • Two ventricles
• Atria


  • Primer pumps
  • Left and right
• Ventricles


  • Supply the main pumping force that propels the blood into the blood vessels
  • Left ventricle is thicker than the right ventricle
• Layers of the heart wall

  • Inner endocardium
  • Middle myocardium
  • Outer epicardium

  
• Endocardium


  Consists of a very thin inner layer of (1) endothelium and supporting connective tissue, (2) middle myoelastic layer of smooth muscle fibers and connective tissue, and a deep layer of connective tissue called the (3) subendocardial layer that contains the cardiac conduction system, which merges with the myocardium

  
• Myocardium


  The thickest layer, consists of cardiac muscle cells with fibers arranged spirally around each heart chamber. Provides the pumping force for the blood to flow into the blood vessels

  
• Epicardium


  Simple squamous mesothelium supported by a layer of loose connective tissue. Forms the outer covering of the heart. Contains the main branches of the coronary blood vessels and nerves. Corresponds to the visceral layer of the pericardium - the membrane surrounding the heart

  
• Cardiac conduction system


  Modified cardiac muscle cells within the subendocardial layer and the adjacent myocardium that generate and propagate waves of depolarization that spread through the myocardium to stimulate rhythmic contractions
• Components of the cardiac conduction system

  • Sinoatrial (SA) node
  • Atrioventricular (AV) node
  • AV bundle (of His)
  • Purkinje fibers
• Cardiac skeleton


  Dense irregular connective tissue that forms part of interventricular and interatrial septa, surrounds the valves of the heart and extends into the valve cusps and the chordae tendinae. Anchors and supports heart valves, provides firm points of insertion for cardiac muscle, and helps coordinate the heartbeat by acting as electrical insulation between atria and ventricles
• Arteries


  Carry blood going out of the heart to the tissues. Transport oxygenated blood EXCEPT in the pulmonary arteries
• Veins


  Drain blood from the tissues and return it back to the heart. Transport deoxygenated blood EXCEPT in the pulmonary veins. More distensible than the arteries, hence they are called "capacitance" vessels

  
• Capillaries


  Smallest blood vessel where exchange of nutrients, oxygen and metabolites occur
• Lymphatic vessels


  Drain extravasated fluid from tissues and return them back to the central circulation
• Layers of the vasculature

  • Tunica intima
  • Tunica media
  • Tunica adventitia
• Tunica intima


  Innermost layer consisting of the endothelium and a thin subendothelial layer of loose connective tissue. In arteries and large veins, includes an internal elastic lamina

  
• Tunica media


  Middle layer consisting of chiefly concentric layers of smooth muscle cells, with interposed elastic fibers, elastic lamellae, reticular fibers, and proteoglycans
• Tunica adventitia


  Outermost layer consisting of type I collagen and elastic fibers, continuous with and bound to the stromal connective tissue of organ through which the blood vessel runs. Contains vasa vasorum that supply nutrients and oxygen to areas of larger blood vessels that are far from the lumen

  
• Endothelium


  Specialized epithelium that acts as a semi-permeable barrier between the blood plasma and the interstitial tissue fluid where metabolic exchanges occur.
• Smooth muscle


  Occurs in the walls of all vessels larger than capillaries, permitting vasoconstriction and vasodilation
• Connective tissue


  Present in vascular walls in varying amounts, with collagen fibers in the subendothelial layer and elastic fibers in large arteries providing the ability to expand under high pressure and recoil during diastole

  
• Elastic arteries


  • Includes the aorta, the pulmonary arteries and their largest branches. Have a thick tunica media with alternating layers of smooth muscle and elastic lamellae, allowing for uniform blood flow

  
• Muscular arteries


  • Distribute blood to the organs, with a very thin subendothelial layer, a prominent internal elastic lamina, and a tunica media containing up to 40 layers of large smooth muscle interspersed with a variable number of elastic lamellae

  
• Arterioles


  • Resist and control blood flow to the capillaries, with a very thin subendothelial layer devoid of elastic lamellae and a media consisting of circularly arranged smooth muscle cells

  
• Capillaries


  Permit and regulate metabolic exchange between blood and surrounding tissues, always functioning in groups called capillary beds. Histologically, composed of a single layer of endothelial cells rolled up as a tube, with an average diameter of 4-10μm and a length usually not more than 50μm

  
• Types of capillaries


  • Continuous capillaries
  • Fenestrated capillaries
  • Discontinuous capillaries (sinusoids)
• Continuous capillaries


  Have many tight, well-developed occluding junctions (tight junctions) between endothelial cells, providing for continuity and a well-regulated exchange across cells
• Fenestrated capillaries


  Have a sieve-like structure that allows more extensive molecular exchange across endothelium
• Discontinuous capillaries (sinusoids)


  Have large perforations between individual endothelial cells, allowing maximal exchange of macromolecules and easier movement of cells between tissues and blood

  
• Pericytes


  Mesenchymal cells with long cytoplasmic processes partly surrounding the endothelial layer, with contractile function to facilitate the flow of blood along the capillaries. Proliferate and differentiate into smooth muscle and other types of cell in case of tissue injuries

  
• Veins


  Carry blood from the tissues back to the heart, with postcapillary venules draining blood from the capillaries and converging to form larger collecting venules and small veins. Have valves that prevent backflow of blood towards the tissues
• Large veins


  • Have a relatively thin tunica media compared to the thicker adventitia, contributing to their distensibility and ability to act as "capacitance vessels" allowing blood to pool
• Functions Of Endothelium


  Provides a nonthrombogenic surface for blood,

  regulates local vascular tone and blood flow,

  plays a role in inflammation and local immune responses,

  and secretes growth factors