Unit Six. Animal Life
In chapter 22, we described one of the greatest evolutionary achievements of animals—locomotion. As the body size and physiological abilities of animals increased, so did the need for more efficient mechanisms to both deliver nutrients and oxygen and remove wastes and carbon dioxide from the growing mass of tissues. Vertebrates have evolved a remarkable set of adaptations to meet this challenge.
The chordates ancestral to the vertebrates are thought to have had simple tubular hearts, similar to those now seen in lancelets (see chapter 19). The heart was little more than a specialized zone of the ventral artery, more heavily muscled than the rest of the arteries, which contracted in simple peristaltic waves.
The development of gills by fishes required a more efficient pump, and in fishes we see the evolution of a true chamber-pump heart. The fish heart, shown in figure 23.11a, is in essence, a tube with four chambers arrayed one after the other. The first two chambers—the sinus venosus (SV) and atrium (A)—are collection chambers, while the second two—the ventricle (V) and conus arteriosus (CA)—are pumping chambers. The SV and the CA chambers are greatly reduced in higher vertebrates.
As might be expected from the early chordate hearts from which the fish heart evolved, the sequence of the heartbeat in fishes is a peristaltic sequence, starting at the rear (the SV) and moving to the front (to the CA). The first of the four chambers to contract is the sinus venosus, followed by the atrium, the ventricle, and finally the conus arteriosus. Despite shifts in the relative positions of the chambers in the vertebrates that evolved later, this heartbeat sequence is maintained in all vertebrates. In fish, the electrical impulse that produces the contraction is initiated in the sinus venosus; in other vertebrates, the electrical impulse is initiated by their equivalent of the sinus venosus.
Figure 23.11. The heart and circulation of a fish.
(a) Diagram of a fish heart, showing the chambers in series with each other. (b) Diagram of the fish circulation, showing that blood is pumped to the gills, and then blood flows directly to the body. Blood rich in oxygen (oxygenated) is shown in red; blood low in oxygen (deoxygenated) is shown in blue.
The fish heart is remarkably well suited to the gill respiratory apparatus and represents one of the major evolutionary innovations in the vertebrates. Perhaps its greatest advantage is that the blood it delivers to the tissues of the body is fully oxygenated because it is pumped through the gills first, as shown in the circulation cycle in figure 23.11b. Blood is pumped first through the gills, toward the right side of the cycle, where it becomes oxygenated; from the gills, it flows through a network of arteries and capillaries to the rest of the body; then it returns to the heart through the veins. This arrangement has one great limitation, however. Recall from the discussion in section 23.2 that blood loses pressure when it passes through capillaries, and so in fish, the blood loses much of the pressure developed by the contraction of the heart as it passes through the capillaries in the gills. Because of this, the circulation from the gills through the rest of the body is sluggish. This feature limits the rate of oxygen delivery to the rest of the body.
Key Learning Outcome 23.5. The fish heart is a modified tube, consisting of a series of four chambers. Blood first enters the heart at the sinus venosus, where the wavelike contraction of the heart begins.