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