Summary junqueira's basic histology chapter 11 (the circulatory system)

The Circulatory System
 Circulatory System Overview:
o The circulatory system directs blood cells and substances throughout the body.
o It comprises the blood vascular system and the lymphatic vascular system.
 Blood Vascular System (Cardiovascular System):
o The heart pumps blood through the system.
o Arteries carry blood away from the heart to various organs, decreasing in size as they branch.
o Capillaries, the smallest vessels, facilitate the exchange of oxygen, carbon dioxide, nutrients, and
waste products between blood and tissues. They form microvasculature
o Veins gather blood from venules and gradually enlarge as they approach the heart, transporting
blood back to be pumped again.
o Divided into pulmonary circulation (oxygenation in the lungs) and systemic circulation (nutrient
delivery and waste removal in body tissues).
 Lymphatic Vascular System:
o Begins with lymphatic capillaries, which merge into larger vessels.
o Thin walled and closed ended vessels that is filled with lymph. They merge forming larger
lymph vessels
o Largest lymph vessels connect with the blood vascular system, draining fluid from tissue spaces
back into the bloodstream.
 Endothelium:
o The internal surface of all blood and lymphatic system components is lined with endothelium.
o Endothelial cells play crucial roles in maintaining a selectively permeable barrier, inhibiting clot
formation, regulating white blood cell migration, and secreting factors for vessel function and
growth.




HEART
o Heart Chambers and Function:
 Cardiac muscle in the heart's four chambers contracts rhythmically to pump blood
through the circulatory system.
 Right and left ventricles propel blood to the pulmonary and systemic circulation
 Right and left atria receive blood from the body and pulmonary veins
o Heart Chamber Layers:
 Endocardium:
 Thin inner layer of endothelium and connective tissue.
 Middle layer of smooth muscle fibers and connective tissue.
 Deep subendocardial layer merging with myocardium, housing branches of the
heart's impulse-conducting system.
 Myocardium:

,  Thickest layer mainly consisting of spirally arranged cardiac muscle fibers.
 Much thicker in ventricular walls, particularly the left ventricle.
 Epicardium:
 Simple squamous mesothelium supported by loose connective tissue.
 Corresponds to the visceral layer of the pericardium.
 Contains adipose tissue deposits for cushioning and produces lubricant fluid to
prevent friction within the pericardium.
o Cardiac Skeleton and Function:
 Dense fibrous connective tissue forms part of the interventricular and interatrial septa,
surrounds heart valves, and extends into valve cusps and chordae tendineae.
 Functions include anchoring and supporting heart valves, providing points of insertion for
cardiac muscle, and acting as electrical insulation between atria and ventricles.
o Impulse Conducting System:
 Consists of specialized myocardial tissue nodes: sinoatrial (SA) node, atrioventricular
(AV) node, AV bundle (of His), Purkinje fibers
 SA node initiates impulses, which propagate through atrial myocardium to stimulate
contraction.
 AV node receives impulses and stimulates depolarization before passing them to the AV
bundle.
 AV bundle bifurcates into left and right bundle branches, which further subdivide into
Purkinje fibers at the apex of the heart.
 Purkinje fibers trigger simultaneous contraction of both ventricles.
o Neural Innervation:
 Both parasympathetic and sympathetic neural components innervate the heart.
 Ganglionic nerve cells and fibers near SA and AV nodes affect heart rate and rhythm.
 Parasympathetic stimulation (vagus nerve) slows heartbeat, while sympathetic
stimulation accelerates pacemaker activity.
 Afferent free nerve endings between myocardial fibers register pain




TISSUES OF THE VASCULAR WALL
 Composition of Blood Vessel Walls:
 Except for capillaries, all blood vessels contain smooth muscle and connective
tissue along with the endothelial lining.
 The arrangement and amount of these tissues are influenced by mechanical
factors such as blood pressure and metabolic factors reflecting local tissue needs.
 Endothelium Structure:
 Specialized epithelium acting as a semipermeable barrier between blood and
interstitial tissue fluid.
 Squamous, polygonal, and elongated endothelial cells with the long axis aligned
in the direction of blood flow.
 Highly differentiated to mediate bidirectional exchange of molecules through
various mechanisms including diffusion, endocytosis, and transcytosis.
 Functions of Endothelial Cells:

,  Present a nonthrombogenic surface and secrete agents controlling clot formation.
 Regulate local vascular tone and blood flow through secretion of vasoconstrictors
and vasodilators.
 Play roles in inflammation and local immune responses by inducing white blood
cell migration and secreting factors affecting white blood cell activity.
 Secrete growth factors promoting proliferation of white blood cell lineages and
cells comprising the vascular wall.
 selectively permeable barrier, inhibiting clot formation, regulating white blood
cell migration, and secreting factors for vessel function and growth.
 Smooth Muscle and Connective Tissue Components:
 Smooth muscle fibers arranged helically in layers in vessels larger than
capillaries.
 Arterioles and small arteries permit vasoconstriction and vasodilation due to more
gap junctions between smooth muscle cells.
 Connective tissue components include collagen fibers, elastic fibers, and ground
substance components, varying based on local functional requirements.
 Structure of Blood Vessel Walls:
 Larger vessels have three concentric layers or tunics: intima, media, and
adventitia.
 Intima consists of endothelium and a subendothelial layer.
 Media comprises concentric layers of smooth muscle cells, elastic fibers, and
proteoglycans.
 Adventitia is primarily composed of collagen and elastic fibers, continuous with
the organ stroma.
 Vasa Vasorum and Vasomotor Nerves:
 Larger vessels have vasa vasorum (arterioles, capillaries, and venules) in the
adventitia and outer part of the media to supply nutrients to thicker vessel walls.
 Adventitia contains a network of unmyelinated autonomic nerve fibers
(vasomotor nerves) releasing norepinephrine, more densely innervating arteries
than veins.




VASCULATURE
 Elastic Arteries:
 Includes aorta, pulmonary artery
 Also known as conducting arteries.
 Well-developed tunica intima and internal elastic lamina. Has endothelium
and connective tissue with smooth muscle
 Thick tunica media with elastic lamellae alternating with smooth muscle
layers.
 Thin adventitia with vasa vasorum has Connective tissue

,  Arterial Sensory Structures:
 Carotid sinuses act as baroreceptors monitoring arterial blood pressure.
 Carotid bodies and aortic bodies contain chemoreceptors monitoring blood
CO2 and O2 levels. Chemoreceptors contribute to autonomic nervous
system regulation.
 Muscular Arteries:
 Distribute blood to organs and regulate blood pressure by vaso
constriction and dialation
 Intima has endothelium and connective tissue with smooth muslce and
interal elastic lamina. Thin subendothelial layer
 Thinner tunica media, with up to 40 layers of smooth muscle cells and
elastic lamellae.
 Adventita has connective tissue with vasa varorum sometimes
 Presence of an external elastic lamina in larger muscular arteries.




 Arterioles:
 Branch from muscular arteries and form microvasculature. Smallest
arteries. Resist and control blood flow to capillaries. Imortant in systemic
blood pressure
 Intima has endothelium. Has no connective tissue or smooth muslce. Has
Thin subendothelial layer, absent elastic laminae
 Media has 1-3 layers of smooth muscle
 Adventita: inconspicuous adventitia, very thin connective tissue layer
 Act as major determinants of systemic blood pressure.
 Can have specialized functions such as thermoregulation or participation
in venous portal systems.




 Specialized Microvascular Pathways:
 Arteriovenous shunts allow direct connection between arterioles and
venules, important for thermoregulation.
 Venous portal systems involve blood flowing through two successive
capillary beds separated by a portal vein, efficient for delivering hormones
or nutrients to specific tissues before returning to the heart.



 Capillary