lec7 anatomy

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Anatomy Dr.Israa H. Mohsen
Lecture 7
Heart
The heart could be called the engine of life. This incredibly powerful organ works
constantly, never pausing. Composed of a type of muscle found nowhere else in the
body, the heart works to pump blood throughout the body, delivering oxygen-rich
blood to organs and tissues and returning oxygen-poor blood to the lungs.
About the size of a fist, the heart lies in the thoracic cavity in the mediastinum, a
space between the lungs and beneath the sternum. The heart tilts toward the left, so
that two-thirds of it extends to the left of the body’s midline. The broadest part of the
heart, called the base, is at the upper right, while the pointed end, called the apex, is
at the lower left.
The study of the heart and the treatment of related disorders is called cardiology.
Structures of the Heart
Key structures of the heart include the pericardium, the heart wall, the chambers, and
the valves.
The Pericardium
Surrounding the heart is a double-walled sac called the pericardium. Anchored by
ligaments and tissue to surrounding structures, the pericardium has two layers: the
fibrous pericardium and serous pericardium.
The fibrous pericardium— a
loose fitting sac of strong
connective tissue— is the
outermost layer.
The serous pericardium,
which consists of two layers,
covers the heart’s surface.
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Note:
The endocardium is very smooth, an important characteristic that helps keep
blood from clotting as it fills the heart s chambers.
The Heart Chambers and Great Vessels
The heart contains four hollow chambers. The two upper chambers are called atria
(singular: atrium); the two lower chambers are called ventricles.
Attached to the heart are several large vessels that transport blood to and from the
heart. Called great vessels, they include the superior and inferior vena, pulmonary
artery (which branches into a right and left pulmonary artery), four pulmonary veins
(two for each lung), and the aorta.
Atria
The atria serve primarily as reservoirs, receiving blood from the body or lungs. The
right and left atria are separated by a common wall of myocardium called the
interatrial septum. Because the atria move blood only a short distance— from the
atria to the ventricles— they don’t have to generate much force. Consequently, the
walls of the atria are not very thick.
•parietal layer, which lines the inside
of the fibrous pericardium, and the
• visceral layer, which covers
the heart’s surface.
Between these two layers is the
pericardial cavity.
This cavity contains a small
amount of serous fluid, which
helps prevent friction as the
heart beats.
The Heart Wall
1.endocardium
lines the heart’s
chambers,
covers the
valves, and
continues into
the vessels. It
consists of a thin
layer of
squamous
epithelial cells.
2.The myocardium, composed of
cardiac muscle, forms the middle
layer. It’s the thickest of the three
layers and performs the work of the
heart.
3. The epicardium, which consists of a thin layer of
squamous epithelial cells, covers the heart’s surface.
Also known as the visceral layer of the serous
pericardium, the epicardium is closely integrated with
the myocardium.
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Ventricles
The ventricles serve as pumps, receiving blood from the atria and then pumping it either to the lungs (right ventricle) or the body (left ventricle). The right and left ventricles are separated by the interventricular septum. Because the ventricles pump rather than receive blood, they must generate more force than the atria. Therefore, the walls of the ventricles are thicker than those of the atria. Furthermore, because the left ventricle must generate enough force to push blood throughout the body, rather than just to the lungs, its walls are thicker than those of the right ventricle.
The Heart Valves
To ensure that blood moves in a forward direction through the heart, the heart contains four valves: one between each atrium and its ventricle and another at the exit of each ventricle. Each valve is formed by two or three flaps of tissue called cusps or leaflets.
The atrioventricular (AV) valves regulate flow between the atria and the ventricles.
? The right AV valve— also called the tricuspid valve (because it has three leaflets)— prevents backflow from the right ventricle to the right atrium.
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? The left AV valve— also called the bicuspid valve (because it has two leaflets), or, more commonly, the mitral valve— prevents backflow from the left ventricle to the left atrium.
The semilunar valves regulate flow between the ventricles and the great arteries. There are two semilunar valves:
? The pulmonary valve prevents backflow from the pulmonary artery to the right ventricle.
? The aortic valve prevents backflow from the aorta to the left ventricle.
Valves open and close in response to pressure changes within the heart . For example, when a ventricle relaxes, the pressure within that ventricle drops. The AV valve leaflets hang limply, allowing blood to flow through the open valve into the ventricle. As the ventricle fills, pressure in the ventricle rises. After filling, the ventricle begins to contract and the pressure rises even more. This increased pressure pushes against the cusps of the AV valve, causing it to snap closed. When pressure in the ventricle exceeds the pressure “downstream,” the semilunar valve pops open, allowing blood to flow out into the area of lower pressure.
Vascular System
When the body is at rest, only 4% of the blood is in the heart; the rest is in the blood vessels. Of that, about 13% is in circulation in the brain. The framework of this system consists of three types of blood vessels:
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1.Arteries
carry blood away from the heart.
2.Veins
return blood to the heart.
3.Capillaries
connect the smallest arteries to the smallest veins.
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Arteries
Arteries carry blood away from the heart. Every time the heart contracts, it forcefully ejects blood into the arteries. Therefore, arteries must be strong as well as resilient to withstand these high pressures. The arteries closest to the heart are the largest. As they travel farther away from the heart, the arteries branch and divide, becoming ever smaller. Finally, they become arterioles, which are the smallest arteries. Arteries can be divided into conducting arteries, distributing arteries, and arterioles.
Conducting Arteries
? The body’s largest arteries, these arteries expand as blood surges into them and recoil when the ventricles relax.
? Because of the large number of elastic fibers embedded in the tunica media, they are also called elastic arteries.
? Examples: Aorta, common carotid artery, subclavian artery
Distributing Arteries
? These arteries carry blood farther away from the heart to specific organs and areas of the body.
? Also called muscular arteries, these arteries are smaller in diameter than elastic arteries.
? Examples: Brachial, femoral, and renal arteries
Arterioles
? These are the smallest arteries.
? They’re also called resistance vessels because, through the contraction of smooth muscle in their walls, they can resist the flow of blood, thus helping regulate blood pressure as well as control how much blood enters an organ.
? They are too numerous to be named.
? Arterioles are connected to capillaries by short connecting vessels called metarterioles
Veins
Blood returns to the heart through veins. In contrast to arteries that branch and divide, forming progressively smaller vessels as they lead away from the heart—veins converge, forming progressively larger and fewer vessels as they lead back to the heart. Either way, the vessels closest to the heart are the largest. Veins are distinct from arteries in other ways:
? Because they aren’t subjected to the same high pressures as arteries, the walls of veins are thinner.
? Veins have a great ability to stretch, which allows them to carry varying amounts of blood with almost no change in pressure. Because of this great capacity for storing blood, they’re sometimes called capacitance vessels.
? Veins can constrict extensively. This helps the body maintain blood pressure when blood volume drops, such as from a hemorrhage.
Large Veins
? Formed as medium-sized veins converge, these veins have a thick tunica externa.
? Examples: Vena cavae, pulmonary veins, internal jugular veins
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Medium-Sized Veins
? Formed by the convergence of venules on their route toward the heart, medium sized veins have thicker, more elastic walls.
? These veins contain one-way valves. Formed from the thin endothelium lining,
valves keep blood moving toward the heart and prevent backflow. Veins in the legs,
which must fight the forces of gravity as they transport blood to the heart, contain
the most valves.
? Examples: Radial and ulnar veins of the forearm, saphenous veins in the legs
Venules
? These are the smallest veins and collect blood from capillaries.
? The endothelium consists of squamous epithelial cells and acts as a membrane; the tunica media is poorly developed, giving venules thinner walls.
? They are porous and can exchange fluid with surrounding tissues.
Capillaries
Capillaries are microscopic vessels that link arterioles to venules. More importantly, it’s within capillaries that nutrients, wastes, and hormones are transferred between blood and tissues. These are the exchange vessels of the circulatory system. Properly functioning capillaries are as vital to survival as a properly beating heart. For this reason, no cell in the body is more than four or six cell-widths from a capillary.
Capillaries aren’t evenly distributed, however. Tissues with high metabolic rates—such as the liver, kidneys, and myocardium—contain large numbers of capillaries. Fibrous connective tissues, such as tendons, have lower metabolic rates and contain fewer capillaries. Still other tissues—such as the epidermis, cartilage, and the lens and cornea of the eye—don’t have any capillaries.
Sinusoid
Some organs—such as the liver, bone marrow, and spleen—contain a unique capillary called a sinusoid. These irregular, blood-filled spaces are more permeable, allowing for the passage of large substances such as proteins and blood cells. This is how blood cells formed in bone marrow as well as clotting factors and other proteins synthesized in the liver enter the bloodstream.
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Branching off the aortic arch is the:
1. Subclavian artery, which
supplies blood to the arm
2.Axillary artery, which
is the continuation of
the subclavian artery in
the axillary region
Radial artery, which
is often palpated to
measure a pulse
The thoracic aorta and its
branches supply the chest
wall and the organs within
the thoracic cavity.
3.Brachial artery, which is
the continuation of the
axillary artery and the
artery most often used
for routine blood
pressure measurement
The abdominal aorta gives rise
to the
1.Celiac trunk, which divides
into the gastric artery (which
supplies the stomach), the
splenic artery (which supplies
the spleen), and the hepatic
artery (which supplies the liver):
2.Renal arteries, which
supply the kidneys
3.Superior mesenteric
artery, which supplies most
of the small intestine
and part of the large intestine
4.Inferior mesenteric
artery, which supplies the
other part of the large
intestine
iliac arteries, which
supply the pelvic
organs, thigh, and
lower extremities.
Major arteries
branching off the iliac
arteries include the:
1.Internal iliac artery
2.External iliac
artery
3.Femoral artery
4.Popliteal artery
5.Anterior tibial artery
6.Posterior tibial artery
7.Dorsalis pedis artery
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Brachiocephalic vein
Subclavian vein
Superior vena cava
Axillary vein
Inferior vena cava
The hepatic veins drain the liver.
Because of its proximity to the
heart, right-sided heart failure can
cause congestion in the liver.
Common iliac vein
Internal iliac vein
External iliac vein
The great saphenous vein is the
longest vein in the body; it’s
frequently harvested for use as
grafts in coronary artery bypass
surgery.
The internal jugular vein
drains most of the blood from
the brain. In right-sided heart
failure, blood backs up from
the heart and causes jugular
vein distension.
The cephalic vein, at
its distal end, is a
frequent site for the
administration of
intravenous fluids.
The median
cubital vein is the
most common site
for drawing blood.
The popliteal vein
runs behind the knee.
Fibular (peroneal) vein
Anterior tibial vein
Posterior tibial vein