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Summary BLOOD VESSEL BIOS 255 Anatomy And Physiology III With Lab
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BIOS255 – Anatomy & Physiology IIIProf Haneen Salhieh, M.S.
Week 2 - The Cardiovascular System: Heart
Overview of the Heart
The heart is a muscular organ that pumps blood through vessels that deliver blood to organs and bring
blood back to the heart. The vessels that carry oxygen-rich blood from the left side of the heart to all
organs of the body are part of a cardiovascular subdivision known as the systemic circuit. The vessels
that carry blood to the lungs for gas exchange and return blood to the right side of the heart are part of the
pulmonary circuit.
The heart sits between the lungs in the central region of the thoracic cavity called the mediastinum. It is
orientated in the center of the chest with a pointed inferior end called the apex, turned downward and to
the left. The heart is encased in a double-layered membrane that cushions and protects it called the
pericardium.
Gross Anatomy of the Heart
As mentioned, the heart is surrounded by a double-layered membranous sac called the pericardium. The
outer layer of this sac is a tough fibrous wall called the fibrous pericardium. This helps to provide structure
support and anchor the heart in the chest cavity. Beneath this is a thin membrane called the serous
pericardium that consists of two layers: a parietal layer and a visceral layer.
• The parietal layer lines the inside of the fibrous pericardium, or in other words, lines the inner wall of
the sac containing the heart.
• The visceral layer closely adheres to the surface of the heart, forming the outermost layer of the heart
called the epicardium.
Between the parietal and visceral layers exists a potential space known as the pericardial cavity.
Normally, this cavity contains a small amount of fluid referred to as pericardial fluid, which acts as a
lubricant. This fluid plays a crucial role in reducing friction, allowing the two layers to glide smoothly
against each other during the heart's contractions without causing harm to the heart. Furthermore, it
affords the heart the flexibility to expand while preventing excessive expansion.
To visualize the serous pericardium, think of it as an air-filled balloon and your fist represents the heart. If
you were to gently press your fist into the balloon, the outer layer of the balloon would correspond to the
parietal layer, while the part of the balloon that makes direct contact with your fist represents the visceral
layer. The air space between these layers in the balloon corresponds to the pericardial cavity.
Clinical Correlations
Pericarditis: inflammation of the pericardium. The membranes may become roughened and can
cause painful friction rub each time the heart beats. Common symptoms include chest pain,
difficulty breathing, fever, and fatigue.
, BIOS255 – Anatomy & Physiology III
Prof Haneen Salhieh, M.S.
Cardiac Tamponade: a condition in which there is an accumulation of fluid in the pericardial
cavity. This accumulation of fluid puts pressure on the heart, preventing it from pumping blood
effectively. Common Symptoms include low blood pressure, chest pain, difficulty breathing, and
vein distension.
Layers of the Heart Wall
The heart walls consist of 3 distinct layers: the epicardium, myocardium, and endocardium.
Epicardium
The epicardium, or visceral pericardium, is the outermost layer of the heart wall. It is composed of
connective tissues such as areolar and adipose tissue. This layer functions in the production of pericardial
fluid and contains blood vessels, lymphatics, and nerve fibers.
Myocardium
The myocardium is the middle and thickest layer of the heart wall, primarily composed of cardiac muscle
tissue. The thickness of this layer is proportional to the workload. This layer functions in pumping
blood throughout the body to deliver nutrients and oxygen. Myocardium produces wringing motion during
contraction.
Endocardium
The endocardium is the innermost layer of the heart. It consists of connective tissue and a thin simple
squamous epithelial layer called endothelium. This layer lines the heart chambers, covers the heart
valves, and is continuous with the inner lining of blood vessels attached to the heart. It provides a smooth
surface blood to easily flow through.
Internal Anatomy of the Heart
Heart Chambers
Looking at the internal anatomy of the heart, it consists of four chambers.
The two superior chambers are atria. Left and right atria are separated by a wall called the interatrial
septum. These chambers receive blood that is returning to the heart: the right atria receives
deoxygenated blood from tissues, and the left atria receives oxygenated blood from the lungs. Each
atrium has an earlike flap called an auricle that helps to increase its volume slightly. The right atrium
contains muscular ridges called pectinate muscles.
The two inferior chambers are ventricles. Right and left ventricles are separated by a thick muscular wall
called the interventricular septum. These chambers contract and eject blood into arteries that deliver
blood: the right ventricle pumps blood only to the lungs, and the left ventricle pumps blood out to all
organs of the body. Both ventricles contain muscular ridges called trabeculae carneae that help keep the
ventricular walls from clinging to each other when the heart contracts. The left ventricle is the thickest of
all chambers as it bears the greatest workload.
Comparing the chambers, atrial walls are thin as they function to collect blood. Ventricular walls are
significantly thicker with myocardium which allows them to generate enough force (pressure) to pump
blood through the pulmonary and systemic circuits.
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