Download The Circulatory System – The Heart

Lectures 15/16 for Fall 2012
The Circulatory System – The Heart
Make sure you know the pathway of blood flow through the chambers and valves of the
heart, as shown in figure 20.10 in your book, and as explained on page 580
The Pulmonary and Systemic Circuits:
 The pulmonary circuit carries blood to the lungs for gas exchange and returns it to
the heart
 The systemic circuit carries blood to every organ of the body, including other
parts of the lungs and the wall of the heart itself
The Position, Size, and Shape of the Heart
 The heart is located in the thoracic cavity in the mediastinum, between the lungs
and deep to the sternum
o The mediastinum is the median portion of the thoracic cavity that
separates the lungs and contains the heart, great blood vessels, and thymus
 The broad superior portion is called the body, and the blunt point at the inferior
end is called the apex
 The heart is roughly the size of one’s fist, and it weighs about 300g
The Pericardium
 The heart is enclosed in a double-walled sac called the pericardium
 Between the layers is about 5 to 30 ml of pericardial fluid that lubricates the
membranes, so that the heart can beat almost without friction
The Heart Wall
 The epicardium is a serous membrane on the surface of the heart
 The myocardium is the thickest layer of the heart and consists of cardiac muscle
 The endocardium is the innermost layer of the heart, and lines the interior
chambers
More about the Chambers
 The atria are separated from one another by a wall called the interatrial septum
 The ventricles are separated from one another by an interventricular septum
 The atrioventricular valves (tricuspid and bicuspid valves) allow blood to flow in
only one direction
o They have chordae tendinae (tendinous cords) that look like the lines of a
parachute, extending down into the ventricles
o The chordae tendinae attach to dome-shaped, or conical, papillary muscles
 Both ventricles exhibit internal ridges called trabeculae carneae.
The Conduction System
 The cardiac conduction system insures that the four chambers of the heart are
coordinated with each other
o The SA node is in the right atrium, just under the epicardium
 It initiates each heartbeat and determines the heart rate
 It sends signals through the atria, causing them to contract
o The AV node is near the tricuspid valve in the interatrial septum
 It acts as an electrical gateway to the ventricles
o The AV bundle forks to the right and left sending signals through the
interventricular septum toward the apex
o Purkinje fibers distribute signals from the apex through the myocardium of
the ventricles
The Circulatory System – Blood Vessels
General Anatomy of Blood Vessels
 The Vessel Wall consists of multiple layers
o The tunica interna lines the inside of the vessel and is exposed to blood
 It consists of simple squamous epithelium called endothelium
overlying a basement membrane
 The endothelium acts as a selectively permeable barrier to
materials entering or leaving the bloodstream
 It secretes chemicals that stimulate the muscle of the vessel
wall to contract or relax
o The tunica media is the middle layer, and it’s usually the thickest
 It consists of smooth muscle, collagen, and sometimes elastic
tissue
 It strengthens the vessel and provides for changes in vessel
diameter
o The tunica externa is the outermost layer
 It consists of loose connective tissue
 It anchors the vessel and provides passages for small
nerves, lymphatic vessels, and smaller blood vessels
 Arteries
o These are blood vessels that carry blood away from the heart
o They have relatively strong, resilient tissue to withstand high blood
pressure surges
o Categories
 Conducting arteries
 These are the largest arteries
 The tunica media consists of layers of elastic sheets
alternating with layers of smooth muscle, collagen, and
elastic fibers
 These arteries expand during ventricular systole to receive
blood and recoil during diastole
o This expansion takes pressure off the blood, so that
there is less stress on arteries downstream
 Distributing arteries
 These are branches that distribute blood to specific organs
 The smooth muscle is more conspicuous than the elastic
tissue
 Resistance arteries
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These are small arteries that are too variable in number and
location to have individual names
The smallest of these are called arterioles
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Capillaries
o For materials such as nutrients, wastes, and hormones to pass between
blood and tissue fluids, they must pass through the walls of capillaries (or
venules, which are less common and less permeable)
o They are composed of only an endothelium and basement membrane
o Most cells of the body are within a few cell widths of a capillary
o Types of capillaries
 Continuous capillaries occur in most tissues
 The endothelial cells are held together by tight junctions to
form an uninterrupted tube
 Endothelial cells are separated by narrow intercellular
clefts that allow small solutes to pass through, while
holding back plasma proteins and formed elements of the
blood
 Some continuous capillaries have pericytes that contain
contractile proteins which contract to regulate blood flow
 Fenestrated capillaries are found in organs where rapid absorption
or filtration occurs, such as the kidneys or small intestine
 The contain filtration pores that allow for rapid passage of
small molecules but still retain larger proteins and larger
particles in the bloodstream
 Sinusoids are irregular blood-filled spaces in the liver, bone
marrow, spleen, and some other organs.
 The endothelial cells are separated by wide gaps, and the
cells also have large fenestrations through them
 Even proteins and blood cells can pass through these pores.
 Sinusoids may contain macrophages or other specialized
cells
o Capillary beds
 Capillaries are organized into groups called capillary beds
 When precapillary sphincters are open, the capillaries are filled
with blood and engage in exchanges with tissue fluid
 When the sphincters are closed, the blood bypasses the capillaries
 In skeletal muscles, 90% of the capillaries have little to no flow
during periods of rest
 During exercise, blood flow to the skin and intestines is reduced
Veins
o Veins are relatively thin walled
 They are subjected to relatively little pressure
 An arteries, pressure may surge to 120 millimeters of
mercury
 In veins, it averages about 10 mmHg
o Many medium sized veins have venous valves
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Venous valves are infoldings of the tunica interna that meet in the
middle of the lumen
 They allow blood to flow in only one direction
 The pressure of blood in the veins is not high enough to push up
towards the heart in a standing or sitting person, so the upward
flow of blood depends on the actions of skeletal muscles
 When the skeletal muscles that surround a vein contract,
they force blood through the valves
Circulatory Routes
o The simplest and most common route of blood flow is:
 Heart  arteries  capillaries  veins  heart
o In a portal system, blood flows through two consecutive capillary
networks before returning to the kidneys
 These systems occur in the kidneys, between hypothalamus and
pituitary, and between the intestines and the liver
o The Hepatic Portal System
 The hepatic portal system connects capillaries of digestive organs
to the hepatic lobules of the liver
 The hepatic portal vein carries blood to the liver to provide it with
nutrients
 The system also allows the blood to be cleansed of bacteria and
toxins within the toxin prior to sending it to the rest of the body
Anatomy of the Pulmonary Circuit
 The system begins with the pulmonary trunk, which ascends from the right
ventricle
 The pulmonary trunk branches into right and left pulmonary arteries
 The pulmonary arteries branch and connect to the lungs, where blood unloads
CO2 and loads O2
 Blood then leaves the lungs to return to the heart through pulmonary veins
Anatomy of the Systemic Arteries
 The Aorta and its Major Branches
o The ascending aorta rises from the left ventricle
o The aortic arch curves like an inverted U
 There are three branches from the aortic arch
 The brachiocephalic trunk
 The left common carotid artery
 The left subclavian artery
o The descending aorta proceeds down behind the heart in two parts
 The thoracic aorta is above the diaphragm
 The abdominal aorta is below the diaphragm
 Arterial Supply to the Head and Neck
o Origins of head-neck arteries
 The common carotid arteries branch from the brachiocephalic
trunk on the right side and from the aortic arch on the left side
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The vertebral arteries arise from the right and left subclavian
arteries and run through the transverse foramina of the cervical
vertebrae
o Continuation of the head-neck arteries
 The external carotid arteries ascend to the side of the head
external to the cranium
 The internal carotid arteries enter through the carotid foramina and
supplies 80% of the blood to the crebrum
Arterial Supply to the Upper Limb
o The subclavian artery travels under the clavicle and above the first rib
o The axillary artery is a continuation of the subclavian artery
o The brachial artery is a continuation of the axillary artery and continues
down the brachium
 Two arteries branch from the brachial artery
 The radial artery follows the radius
 The ulnar artery follows the ulna
Arterial Supply to the Thorax
o Bronchial arteries extend from the thoracic aorta to the lungs (systemic
circulation only)
o Esophageal arteries extend from the thoracic aorta to the esophagus
o (Posterior) intercostals arteries run from the thoracic aorta to the posterior
portion of the thoracic cavity between the ribs
o The superior phrenic arteries run from the thoracic aorta to the superior
aspect of the diaphragm
Arterial Supply to the Abdomen
o Inferior phrenic arteries supply blood to the inferior surface of the
diaphragm
o Celiac trunk branches to upper abdominal viscera
 Splenic artery goes to the spleen
 Left gastric artery goes to the stomach
 Hepatic artery goes to the liver
o Superior mesenteric artery supplies the intestines
o Suprarenal arteries supply the adrenal glands
o Renal arteries supply the kidneys
o Gonadal arteries supply the gonads
o Inferior mesenteric artery supplies the distal end of the large intestine
o Common iliac arteries supply the urinary and reproductive organs and
lower limbs
Arterial Supply to the Pelvic Region and Lower Limb
o The internal iliac artery supplies the pelvic wall and viscera
o The external iliac artery supplies mainly the lower limb
o The femoral artery passes through the femoral triangle to through the thigh
Fetal Circulation and changes after birth
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The fetus has certain shunts by which most blood bypasses the nonfunctional
lungs
o The foramen ovale is an opening between the two atria
o The ductus arteriosus is a short vessel from the left pulmonary artery to
the aorta
 Most of the blood that the right ventricle pumps into the
pulmonary trunk takes this bypass directly to into the aorta instead
of following the usual path to the lungs
o After birth, when the lungs are functional, these shunts close, leaving a
fossa ovalis in the interatrial septum and ligamenumt artriosum between
the aortic arch and left pulmonary artery
In the fetus, a shunt called the ductus venosus bypasses the liver which is not very
functional before birth
o This vein caries blood returning from the placenta through the umbilical
vein
o After birth, the ductus venosus constricts and blood is forced to flow
through the liver