Circulatory System

The circulatory system of a reptile, like those of all terrestrial vertebrates, is composed of two loops. The pulmonary loop carries deoxygenated blood from the heart to the lungs and returns oxygenated blood to the heart. The systemic loop transports oxygenated blood to the tissues of the body, where oxygen and nutrients are unloaded and where carbon dioxide and wastes are picked up, and returns deoxygenated blood to the heart.

Heart Structure and Function

In lizards, snakes, tuataras, and turtles, the heart has two atria and a single ventricle partially divided by a wall of tissue called a septum. In crocodiles, there are two atria and two separate ventricles. The sinus venosus and the conus arteriosus, which are major accessory structures to the heart of a fish, are much smaller in reptiles. The sinus venosus is absent in some species. When it is present, it collects blood from the body and channels it into the right atrium. The conus arteriosus forms the base of the three large arteries exiting from the reptilian heart.

Because the ventricle is not completely divided (except in crocodiles), it might seem that deoxygenated and oxygenated blood would mix. However, very little blood mixing occurs when a reptile is active. Deoxygenated and oxygenated blood are kept separate during contraction of the heart by the actions of the heart valves and the movement of the septum and ventricular walls.

Pumping blood through lungs requires energy. Under some conditions, it is advantageous for a reptile to divert blood away from the lungs to conserve energy. For example, an inactive reptile needs so little oxygen that it may go a long time without breathing. Similarly, aquatic reptiles do not breathe while they are underwater. Under these conditions, the heart pumps blood to the body while reducing circulation through the lungs.

objectives

• Identify advantages associated with the structure of a reptile's heart.

• Describe the respiratory system of reptiles.

• Describe four methods reptiles use to sense their environment.

• Explain how reptiles regulate their body temperature.

• Compare oviparity, ovoviviparity, and viviparity as reproductive strategies.

vocabulary septum alveolus

Jacobson's organ thermoregulation ectotherm endotherm oviparity ovoviviparity viviparity placenta

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figure 41-8

Aortic arches

* Left pulmonary A vein

Left atrium

Left pulmonary artery

Ventricle

Aortic arches

The turtle's heart, shown in cross section, has a partially divided ventricle, unlike an amphibian's three-chambered heart or a crocodile's four-chambered heart. Because the flow of blood through a turtle's heart is asynchronous, deoxygenated blood and oxygenated blood pass through the upper part of the ventricle at different times and so mix very little.

* Left pulmonary A vein

Left atrium

Left pulmonary artery

Ventricle t \

By constricting the pulmonary arteries, a reptile's blood flow through the heart can be redirected to send some deoxygenated blood back to the body instead of to the lungs. Bypassing the lungs may help a reptile raise its body temperature quickly—warm blood from the skin can be directed to the organs deep within the body. During periods of activity, almost all deoxygenated blood is directed to the lungs to meet the muscles' demand for oxygen. The reptilian heart has a degree of circulatory flexibility that the hearts of birds and mammals do not. Instead of being a handicap, this flexibility is actually well suited to reptilian physiology and activity patterns. Figure 41-8 shows a schematic diagram of the heart of a turtle.

The lungs of reptiles are large, and they are often divided internally into several chambers. The lining of the lungs may be folded into numerous small sacs called alveoli. Alveoli greatly increase the internal surface area of the lungs, thus increasing the amount of oxygen that can be absorbed. In most snakes, only the right lung actively functions. It is elongated and may be half as long as the body. The left lung is either reduced to a small nonfunctional sac or absent entirely.

A reptile fills its lungs by expanding its rib cage. This expansion reduces the pressure within the thorax and draws air into the lungs. When the ribs return to their resting position, pressure within the thorax increases, and air is forced out of the lungs. Similar movements help humans to breathe.

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