THE PROCESS OF SCIENCE 12. Mammals have a four-chambered heart, with two ventricles and two atria. So do birds, alligators, and crocodiles, but other living reptiles have a three-chambered heart, with just one ventricle. Paleontologists debate whether dinosaurs had a three-chambered or a four-chambered heart. Long-necked sauropod dinosaurs could have had unusual circulatory demands because their head may have been raised far above the heart. The farther the head is above the heart, the greater the systolic pressure needs to be for blood to reach the brain. For example, the long-necked dinosaur Brachiosaurus may have carried its head as much as 6 m (20 feet) above its heart. It is estimated that such an anatomy demanded a systolic blood pressure of 500 mm of mercury for blood to reach the brain! Some paleontologists consider this evidence that dinosaurs had a four-chambered heart that supported a dual circulatory system similar to that of birds and mammals, rather than the three-chambered heart of most nonbird reptiles. Can you explain why?
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Physiologists speculate about cardiovascular adaptations in dinosaurs-some of which had necks almost $10 \mathrm{m}$ (33 feet) long. Such animals would have required a systolic pressure of nearly 760 mm Hg to pump blood to the brain when the head was fully raised. Some analyses suggest that dinosaurs' hearts were not powerful enough to generate such pressures, leading to the speculation that long-necked dinosaurs fed close to the ground rather than raising their heads to feed on high foliage. Scientists also debate whether dinosaurs had a "reptile-like" or "bird-like" heart. Most modern reptiles have a three-chambered heart with just one ventricle. Birds, which evolved from a lineage of dinosaurs, have a four-chambered heart. Some scientists believe that the circulatory needs of these long-necked dinosaurs provide evidence that dinosaurs must have had a four-chambered heart. Why might they conclude this?
SECTION 6: Ventricles of the Brain A. Inside the cerebral hemispheres and brainstem is a series of interconnected cavities called ventricles. These spaces are continuous with the central canal of the spinal cord and contain cerebrospinal fluid. The lateral ventricles (first and second ventricles) are the largest, and lie within the right and left cerebral hemispheres. The third ventricle is a narrow cavity along the midline of the brain in the area of the diencephalon. The fourth ventricle lies between the cerebellum and the brainstem. The cerebral aqueduct (aqueduct of the midbrain) is a narrow canal that connects the third and fourth ventricles. Label Figure 13.8 with the terms provided. Refer to your textbook as a guide. Lateral ventricles Third ventricle Fourth ventricle Cerebral aqueduct (aqueduct of the midbrain) Central canal
Madhur L.
Observation: Internal Anatomy of the Heart Remove the ventral half of the human heart model. Using a sharp scalpel, cut through the apex of the heart. Be sure the cut is deep enough to reach the septum of the heart. Then, cut through the right atrium, being careful not to cut the left ventricle. Cut the heart in longitudinal section to observe the internal anatomy: right atrium, right ventricle, left atrium, and left ventricle. 2. Which ventricle is more muscular? Why is this appropriate? 3. Find the left ventricle. 4. Find the pulmonary semilunar valve, located in the base of the pulmonary trunk. 5. Find the aortic semilunar valve, located in the base of the aorta. What is the function of the atrioventricular valves? What is the function of the semilunar valves? 6. Note the chordae tendineae ("heartstrings") that hold the atrioventricular valves in place while the heart contracts. These extend from the papillary muscles. The chordae tendineae prevent the atrioventricular valves from inverting into the atria when the ventricles contract.
Adi S.
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