. The biology of the amphibia. Amphibians. THE CIRCULATORY SYSTEM 193 Ven have increased the heart capacity by the development of a series of pendulent extensions of the auricles (Fig. 74) and the same maintains in the small, but very slim Pseudobranchus. Function of the Heart.—The primary function of the heart is to force blood into the arteries against the pressure caused by the tonic contraction of the smooth muscle fibers of the arteries and arterioles. The difficulty is increased by the friction within these vessels and capillaries as well as by the viscosity of the blood due chiefly to t
. The biology of the amphibia. Amphibians. THE CIRCULATORY SYSTEM 193 Ven have increased the heart capacity by the development of a series of pendulent extensions of the auricles (Fig. 74) and the same maintains in the small, but very slim Pseudobranchus. Function of the Heart.—The primary function of the heart is to force blood into the arteries against the pressure caused by the tonic contraction of the smooth muscle fibers of the arteries and arterioles. The difficulty is increased by the friction within these vessels and capillaries as well as by the viscosity of the blood due chiefly to the rela- tive amount of protein held in colloidal solution in the blood. The blood pressure maintained by the heart must be higher than that of the osmotic pressure of the plasma proteins in order to permit the filtration of urine through the glomeruli of the kidney. In the frog the sys- tolic pressure of the heart is about three times the osmotic pressure of the colloids (Bieter and Scott, 1928). The output of blood by the heart must be sufficient to in- sure an adequate supply of food and oxygen for the tis- sues of the body. The blood flow in the capillaries, because of their larger total area, is much slower than in the larger vessels, thus allowing greater opportunity for gas exchange and other functions of the blood to take place. The heart output is determined in part, by the stroke volume of blood but chiefly by the rate of pulsation. The latter is regulated principally by the vagus which inhibits the heart rate and by the sympathetic impulses which accelerate it. Kraupl (1927) has demonstrated the cardio-accelerator effects upon stimulation of the isolated sympathetic trunk, after cutting the vagus connection to the heart. The rate is affected by temperature, gradually increasing with each rise up to a certain. Fig. 74.—Heart of Siren lacertina. Numerous finger-like processes greatly increase the volume of the auricles. , left auricle; Pyl., pylangium; Syn., s
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Keywords: ., bookcentury1900, bookpublishernewyorkmcgr, booksubjectamphibians
