Hearts and Circulatory Systems summary
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Hearts and Circulatory Systems
The Mammalian Heart
The mammalian (and avian) heart consists of two ventricles (left and right) and two atria (left
and right). The left ventricle and left atria function as the “left heart” pumping oxygenated blood
to the systemic tissues (systemic circulation). The right ventricle and right atria function as the
“right heart” pumping blood to the lungs (pulmonary circulation).
Venous or deoxygenated blood enters the heart from the systemic tissues via the superior vena
cava (upper regions of the body) and inferior vena cava (lower regions of the body).
Note, although the term deoxygenated blood is often used to refer to venous blood that has left
the systemic tissues, venous blood still has a fair amount of oxygen in it (partial pressure of 40
mmHg; 1 mmHg = 1 Torr).
Blood flows from the two vena cava into the right atria. From there, blood passes through the
tricuspid valve and enters the right ventricle. Blood is pumped from the right ventricle, through
the pulmonary artery to the lungs.
In the lungs the blood is oxygenated. Blood leaves the lungs with a partial pressure of
approximately 100 mmHg. Blood from the lungs flows back to the heart via the pulmonary vein.
The pulmonary vein enters into the left atria.
From the left atria blood flows through the bicuspid valve into the left ventricle. Blood is then
pumped from the left ventricle, through the aorta, to the systemic tissues. The blood delivers
oxygen (and other nutrients to the tissues) while picking up carbon dioxide and other metabolic
waste products. The blood leaving the systemic tissues(now called deoxygenated blood) travels
through veins and ultimately ends up in the vena cava and the cycle starts over.
Note, in almost all circumstances, arteries carry oxygenated blood and veins carry deoxygenated
blood. However, the pulmonary artery carries deoxygenated blood from the heart to the lungs
while the pulmonary vein carries oxygenated blood from the lungs to the heart. This terminology
is used because arteries take blood away from the heart and veins return blood to the heart.
The mammalian heart essentially functions as two separate hearts that function in parallel. The
left heart pumps oxygenated blood to the tissues while the right heart pumps deoxygenated blood
to the lungs.
Teleost (Bony) Fish Heart and Circulation
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Teleost fish are bony fish. This distinguishes them from the other major group of fish, the
elasmobranches (cartilaginous fish who have no bone; the skeleton is made entirely of cartilage).
The teleost heart consists of four chambers in series, the sinus venosus (which can also be
considered part of the venous system), the atrium (atria), the ventricle and the bulbous arteriosus
(which can also be considered part of the arterial system).
Blood enters the sinus venosus through the hepatic vein and common cardinal vein. It then flows
through the sinoatrial valve and enters the atrium. From the atrium blood flows through the
atrioventricular valve and enters the ventricle. Blood flows from the ventricle, through the bulbal
valve and into the bulbous arteriosus. From the bulbous arteriosus blood leaves the heart through
the ventral aorta. Blood therefore flows through the chambers of the heart in series (i.e., through
one chamber then the next, etc.). The ventricle provides the vast majority of the contractile force.
The bulbous arteriosus plays a “windkessel” function. A windkessel absorbs pressure or energy
and then releases it. When the ventricle is contracting blood is forced into the bulbous arteriosus.
Most of this blood flows out of the ventricle through the ventral aorta. However, some of the
blood stays in the bulbous arteriosus causing it to expand. When the ventricle stops contracting
(i.e., during the relaxation phase of the heart; called diastole) the bulbous arteriosus contracts
(collapses back inward) forcing the blood in the bulbous arteriosus through the ventral aorta.
This allows for a relative continual and smooth flow of blood out of the heart even when the
ventricle is not contracting.
Blood leaves the heart through the ventral aorta and flows into the gills. Most teleost fish have
four gill arches so blood enters each gill arch through an afferent filament artery (Af). The term
filament is used here because the gills are composed of gill filaments. Blood leaves the gills
through the efferent filament artery (Ef) and enters the dorsal aorta. From the dorsal aorta blood
is carried to the systemic tissues both in front of (head region) and behind the gills.
In the arrangement of this circulatory system, blood leaving the heart must be pumped across two
capillary beds – the capillaries in the gills and those in the systemic circulation. When blood
flows across a capillary bed there is a drop in blood pressure. In other words, blood pressure is
always greater in the blood vessel entering a capillary bed than in the blood vessel leaving the
capillary bed. The teleost heart must therefore generate enough pressure to force blood across
two capillary beds.
The mammalian heart only has to pump blood across one capillary bed because it is divided into
left and right sides. The left heart pumps blood across the capillary bed that is the systemic
tissues while the right heart pumps blood across the capillary bed that is the lungs. This allows
for two different levels of blood pressure in the two circuits; a high pressure in the systemic (left)
circuit and a low pressure in the pulmonary (lung) circuit.
Circulation in Air Breathing Fish
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