85 GENERAL CONCEPTS 



and molluscs is "open"; the blood vessels open to the body cavity, called 

 a hemocoel, and blood circulates partly within blood vessels and partly 

 through the cavity of the hemocoel in making a complete circuit. In 

 the typical arthropod, the heart and other organs lie free in the hemocoel 

 and are bathed in blood. In the annelid worm and vertebrates, the 

 organs lie in the coelomic cavity and are supplied by blood which 

 reaches them in closed vessels. The arthropod heart is generally a single, 

 elongate, muscular tube lying in the dorsal midline. In each segment of 

 the body there is a pair of openings, supplied with valves to prevent 

 backflow. Blood enters the heart from the pericardial sinus, which is 

 part of the hemocoel, through these openings (ostia) and is moved for- 

 ward by peristaltic waves, waves of contraction preceded by waves of 

 relaxation along the tube. Blood is carried in vessels to the head and to 

 other parts of the body, whence it returns to the heart through the 

 hemocoel. 



The hearts of most invertebrates are single muscular tubes which 

 develop only very low pressures— a few millimeters of mercury— as they 

 pump blood. In the vertebrates, with closed circulatory systems, a higher 

 pressure, as high as 100 to 200 mm. Hg, is required to drive the blood 

 through the tremendous number of narrow capillaries. This has led to 

 the evolution of powerful, thick-walled hearts. The chamber of the verte- 

 brate heart called the ventricle has quite thick walls. However, the mus- 

 cular ventricle requires a certain amount of pressure to distend it and 

 cause the blood to flow in and fill it during the relaxation phase (dias- 

 tole). The low pressure in vertebrate veins is not sufficient to do this. 

 The vertebrate heart has a second chamber, the atrium, with walls thin 

 enough to be filled by the low venous pressure yet strong enough to 

 pump blood into the ventricle and distend it. The octopus, whose heart 

 is similarly arranged with two different chambers, has the highest blood 

 pressure, 35 to 45 mm. Hg, of any of the invertebrates. 



The vertebrate heart is enclosed in a special cavity, the pericardial 

 cavity, separated from the rest of the body by a thin, strong sheet of 

 connective tissue, the pericardium. This cavity provides space for the 

 heart to change in volume as it beats. 



The circulatory systems of all vertebrates are essentially similar: a 

 closed system composed of heart, aorta, arteries, capillaries and veins 

 arranged in a basically similar plan. Arteries carry blood away from the 

 heart to the tissues, veins carry blood back to the heart from the tissues, 

 and capillaries are minute, thin-walled vessels connecting the arteries 

 to the veins and completing the circuit from heart to heart. The prin- 

 cipal changes in the vertebrate circulatory system have been associated 

 with the change from gills to lungs as respiratory organs. The changes 

 in the pattern of circulation permit the delivery of oxygen-rich blood to 

 the brain and muscles. The pattern of circulation in many lower verte- 

 brates is such that some mixing of oxygen-rich and oxygen-poor blood 

 occurs. Mammals and birds can be warm blooded because their cir- 

 culatory systems supply enough oxygen to the tissues to support a 

 metabolic rate high enough to maintain a high body temperature in cold 

 surroundings. 



