Chapter 11: The Circulatory System
The internal surface of all components of the blood and lymphatic systems is lined by a single layer of a
squamous epithelium, called endothelium
Th e walls of all four heart chambers consist of three major layers: the internal endocardium; the
middle myocardium; and the external epicardium.
Endocardium:
1. Inner: Very thin layer of endothelium and supporting connective tissue
2. Middle: myoelastic layer of smooth muscle fibers and connective tissue
3. Deep: layer of connective tissue called subendocardial layer that merges with the myocardium
Myocardium:
1. Thickest layer
2. Consists mainly of cardiac muscle with its fibers arranged spirally around each heart chamber
Epicardium:
1. Simple squamous mesothelium supported by a layer of loose connective tissue containing -+blood
vessels and nerves.
2. Corresponds to the visceral layer of the pericardium
3. Friction is prevented by lubricant fluid produced by both layers of serous mesothelial cells in
pericardium
Cardiac skeleton: Fibrous connective tissue that forms part of the interventricular and interatrial septa,
and surrounds all valves of the heart, extending into the valve cusps and chordae tendinae to which
they are attached.
Function:
1. Anchoring and supporting heart valves
2. Providing firm points of insertion for cardiac muscle
3. Helping coordinate the heartbeat by acting as electrical insulation between atria and ventricles.
Conducting system of the heart is found within the subendocardial layer and adjacent myocardium.
It generates and propagates waves of depolarization that spread through the myocardium to stimulate
rhythmic contraction. It consists of 2 nodes:
1. Sinoatrial node (SA)
2. Atrioventricular node (AV)
, Followed by the AV bundle (of His) and the subendocardial conducting network.
SA node is found in the right atrial wall near the superior vena cava. It has fewer myofibrils and fewer
intercalated discs than neighboring muscle fibers.
Impulses from SA node move to both atria through myocardial fibers, stimulating their contraction.
AV node is slightly smaller and located in the floor of the right atrium near the AV valve and composed
of cells similar to those of the SA node.
Conducting muscle fibers from the AV node form the AV bundle, and pass through an opening in the
cardiac skeleton into the interventricular septum, then bifurcate into the wall of each ventricle.
Purkinje Fibers: also known as the subendocardial conducting network of myofibers that are found at
the apex of the heart.
Pale-staining fibers, larger than the adjacent contractile muscle fibers, with sparse peripheral myofibrils
and much glycogen.
Trigger waves of contraction through both ventricles simultaneously.
Parasympathetic and sympathetic components innervate the heart.
Ganglionic nerve cells and nerve fibers are present in the regions close to the SA and AV nodes, where
they affect heart rhythm and rate, such as during physical exercise and emotional stress.
Stimulation of parasympathetic nerves (Vagus): slows heartbeat
Stimulation of sympathetic nerves: accelerate activity of pacemaker
Afferent nerve endings: found between the fibers of myocardium that register pain such as the
discomfort called angina pectoris that occurs when partially occluded coronary arteries cause local
oxygen deprivation.
Walls of blood vessels (EXCEPT CAPILLARIES) contain smooth muscle and connective tissue, in addition
to the endothelial lining.
Endothelium:
Specialize epithelium that acts as a semipermeable barrier between two internal compartments: blood
plasma and interstitial tissue fluid.
Vascular endothelium are squamous, polygonal and elongated in the direction of blood flow.
Other functions of endothelium:
1. It presents a non-thrombogenic surface on which blood will not clot. It releases agents that control
local clot formation: heparin, tissue plasminogen activator, and Von Willebrand factor.
2. Cells regulate local vascular tone and blood flow by secreting factors that stimulate smooth muscle
contraction: endothelin 1, angiotensin converting enzyme (ACE), or NO and prostacyclin for relaxation.
The internal surface of all components of the blood and lymphatic systems is lined by a single layer of a
squamous epithelium, called endothelium
Th e walls of all four heart chambers consist of three major layers: the internal endocardium; the
middle myocardium; and the external epicardium.
Endocardium:
1. Inner: Very thin layer of endothelium and supporting connective tissue
2. Middle: myoelastic layer of smooth muscle fibers and connective tissue
3. Deep: layer of connective tissue called subendocardial layer that merges with the myocardium
Myocardium:
1. Thickest layer
2. Consists mainly of cardiac muscle with its fibers arranged spirally around each heart chamber
Epicardium:
1. Simple squamous mesothelium supported by a layer of loose connective tissue containing -+blood
vessels and nerves.
2. Corresponds to the visceral layer of the pericardium
3. Friction is prevented by lubricant fluid produced by both layers of serous mesothelial cells in
pericardium
Cardiac skeleton: Fibrous connective tissue that forms part of the interventricular and interatrial septa,
and surrounds all valves of the heart, extending into the valve cusps and chordae tendinae to which
they are attached.
Function:
1. Anchoring and supporting heart valves
2. Providing firm points of insertion for cardiac muscle
3. Helping coordinate the heartbeat by acting as electrical insulation between atria and ventricles.
Conducting system of the heart is found within the subendocardial layer and adjacent myocardium.
It generates and propagates waves of depolarization that spread through the myocardium to stimulate
rhythmic contraction. It consists of 2 nodes:
1. Sinoatrial node (SA)
2. Atrioventricular node (AV)
, Followed by the AV bundle (of His) and the subendocardial conducting network.
SA node is found in the right atrial wall near the superior vena cava. It has fewer myofibrils and fewer
intercalated discs than neighboring muscle fibers.
Impulses from SA node move to both atria through myocardial fibers, stimulating their contraction.
AV node is slightly smaller and located in the floor of the right atrium near the AV valve and composed
of cells similar to those of the SA node.
Conducting muscle fibers from the AV node form the AV bundle, and pass through an opening in the
cardiac skeleton into the interventricular septum, then bifurcate into the wall of each ventricle.
Purkinje Fibers: also known as the subendocardial conducting network of myofibers that are found at
the apex of the heart.
Pale-staining fibers, larger than the adjacent contractile muscle fibers, with sparse peripheral myofibrils
and much glycogen.
Trigger waves of contraction through both ventricles simultaneously.
Parasympathetic and sympathetic components innervate the heart.
Ganglionic nerve cells and nerve fibers are present in the regions close to the SA and AV nodes, where
they affect heart rhythm and rate, such as during physical exercise and emotional stress.
Stimulation of parasympathetic nerves (Vagus): slows heartbeat
Stimulation of sympathetic nerves: accelerate activity of pacemaker
Afferent nerve endings: found between the fibers of myocardium that register pain such as the
discomfort called angina pectoris that occurs when partially occluded coronary arteries cause local
oxygen deprivation.
Walls of blood vessels (EXCEPT CAPILLARIES) contain smooth muscle and connective tissue, in addition
to the endothelial lining.
Endothelium:
Specialize epithelium that acts as a semipermeable barrier between two internal compartments: blood
plasma and interstitial tissue fluid.
Vascular endothelium are squamous, polygonal and elongated in the direction of blood flow.
Other functions of endothelium:
1. It presents a non-thrombogenic surface on which blood will not clot. It releases agents that control
local clot formation: heparin, tissue plasminogen activator, and Von Willebrand factor.
2. Cells regulate local vascular tone and blood flow by secreting factors that stimulate smooth muscle
contraction: endothelin 1, angiotensin converting enzyme (ACE), or NO and prostacyclin for relaxation.
