Section V-Cardiovascular System
Remove the musculature ventral to the pericardium to expose the vessels anterior to the heart, as shown in Figure 6.14. Make a longitudinal, midventral slit through the pericardium to open the pericardial cavity and expose the heart.
Several of the heart’s components are plainly visible in ventral view. Its most prominent structure is the single ventricle, which lies in the posterior half of the pericardial cavity. Lift the ventricle to see the sinus venosus (Figure 6.15). The right and left atria (sing., atrium) are the conspicuous structures anterior to the ventricle (Figure 6.14). Between them, the bulbus cordis extends from the ventricle anteriorly and slightly to the left. This anterior part of the heart may be covered by fat and connective tissue. Carefully pick away at and remove it. In an injected specimen the structures are clearly identifiable and easy to expose. In uninjected specimens the vessels are harder to identify and the thin walled atria readily torn, so proceed cautiously.The bulbus cordis leads into the truncus arteriosus, which continues a short distance anteriorly before bifurcating into right and left branches, each of which extends anterolaterally and gives rise to three large
FIGURE 6.15 Heart of the frog in ventral view, with the ventricle reflected.
arteries (Figures 6.14 and 6.16). Follow one of the branches, and identify these arteries and their branching patterns. The most anterior branch is the carotid arch. It divides into a small branch, the external carotid artery, that extends anteriorly to supply the tongue and lower jaw, and a bulbous carotid body that continues as the internal carotid artery to supply the eye, brain, and upper jaw.
The most posterior branch is the pul- mocutaneous arch. It curves sharply posteriorly, giving off a cutaneous artery extending laterally to supply the skin, and then continues to the lung as the pulmonary artery. The middle branch is the systemic arch. It supplies most of the rest of the body with blood, and so is the largest of the three branches. Initially the systemic arch passes nearly laterally, but it soon arches strongly dorsally. As it passes dorsal to the lung it curves medially and posteriorly, giving off two branches, the occip- itovertebral artery and the subclavian artery, which continues onto the brachium as the brachial artery (Figure 6.16). After giving off these two branches, the systemic arch enters the pleuroperitoneal cavity.Within the cavity each systemic arch passes posterome- dially. Just anterior to the level of the kidneys, the right and left systemic arches unite to form the dorsal aorta, which continues posteriorly along the middorsal wall of the cavity (Figures 6.13 and 6.16). Immediately after its origin, the dorsal aorta sends off a large branch, the celi- acomesenteric artery, to the abdominal viscera. This vessel soon bifurcates into the celiac artery, which mainly supplies the liver, gall bladder, stomach, and pancreas, and the mesenteric artery, which supplies the intestines and spleen.
Between the kidneys, several (usually between four and six) smaller, paired vessels, the urogenital arteries, extend laterally from the dorsal aorta to supply the kidneys and gonads, as well as fat bodies and urogenital ducts (Figure 6.16). Near the posterior end of the kidneys the dorsal aorta bifurcates into right and left common iliac arteries (Figures 6.12 and 6.16). In this region the arteries lie dorsal to the veins, which are also more prominent. Follow one of the common iliac arteries posteriorly. It gives off two smaller branches in quick succession from its lateral surface. These are the hypogastric artery, which mainly supplies musculature in this region and the urinary bladder, and the femoral artery, which helps supply several muscles and the skin in this region.
The common iliac artery then continues into the hind limb as the sciatic artery (Figure 6.16). Its many branches supply the leg.Return to the heart, lift the ventricle, and examine the sinus venosus, which leads into the right atrium (Figure 6.15). Note the large vessels, the right anterior vena cava and the left anterior vena cava, extending along the lateral edge of the atria and passing into the sinus venosus. The venae cavae collect blood from the head and forelimbs, as well as the skin. Many of the vessels that enter the venae cavae lie dorsal to the arterial vessels and should be injected with blue latex. In a few specimens, however, these will have been infiltrated by the latex of the arteries and will be partly or completely red.
Trace an anterior vena cava. The pattern described here is the general pattern, but there is variation. Indeed, the branching patterns of the right and left venae cavae may vary. The anterior vena cava collects blood from several vessels and empties into the sinus venosus. The main vessels forming the anterior vena cava are the external jugular, innominate, and subclavian veins (Figures 6.14 and 6.17). These three may join together. The external jugular vein extends almost directly anteriorly and drains the tongue and lower jaw. For most of its length it passes nearly parallel and just medial to the external carotid artery. The innominate vein may be quite short or it may extend laterally for a longer distance before receiving its tributaries, the internal jugular and subscapular veins. The internal jugular, draining the eye, brain, and upper jaw, extends anterolaterally, whereas
FIGURE 6.16 Schematic illustration of the arterial system of the frog in ventral view superimposed on the body outline.
the subscapular vein, draining mainly the muscles associated with the pectoral girdle, passes nearly laterally. Finally, the subclavian vein passes posterolaterally, formed by the confluence of the brachial vein from the forelimb and the musculocutaneous vein, noted earlier, from the pectoral musculature and the deep surface of the skin.
The pulmonary veins, returning blood to the heart from the lungs, enters the left atrium (Figure 6.17). The pulmonary vein, one on each side, passes dorsal to the anterior vena cava. Right and left pulmonary veins then unite just anterior to the heart to form a short single vessel that extends posteriorly into the left atrium.
FIGURE 6.17 Schematic illustration of the venous system of the frog in ventral view superimposed on the body outline. Right atrium, ventricle, and bulbus cordis of heart removed.
Much of the blood posterior to the heart (that from the skin being the main exception) is returned via the large posterior vena cava, the large vessel extending between the kidneys (Figures 6.12-6.14). Here it receives several renal veins from the kidneys (the fat bodies generally drain into the more anterior renal veins) and spermatic veins from the testes or ovarian veins from the ovaries. The posterior vena cava extends anteriorly through the liver, receiving from it several hepatic veins (Figure 6.17), to reach the posterior end of the sinus venosus. The posterior vena cava also receives blood from much of the hind limbs and dorsal body musculature by way of the paired renal portal veins, which enter the kidneys (Figures 6.12, 6.13, and 6.17). This blood then makes its way through the kidneys to reach the posterior vena cava.
Examine a renal portal vein. It extends mainly along the dorsolateral surface of the kidney. It is formed posteriorly by the union of the iliac and sciatic veins (Figures 6.13 and 6.17), the latter from the medial side of the thigh. The iliac vein is formed by the femoral vein, the large vessel from the lateral side of the thigh, and the pelvic vein. A connection between the femoral and sciatic veins, the communicating iliac vein, extends from the femoral, curving dorsally and then medioventrally to meet the sciatic vein (Figure 6.17).
The pelvic vein extends ventromedially to join the pelvic vein from the other side of the body. Their union forms the ventral abdominal vein, already identified above but which will be described shortly. A small vesicular vein, draining the urinary bladder, enters the ventral abdominal vein just after its origin. The renal portal vein passes anteriorly, sending numerous branches into the kidney. The dor- solumbar vein, which drains the dorsal and lateral abdominal walls, consists of numerous branches that collect usually into a single vessel that enters the renal portal vein at about the midlength level of the kidney (Figures 6.12-6.14 and 6.17).As noted above, the ventral abdominal vein passes anteriorly along the midventral wall of the pleuroperitoneal cavity and extends between the lobes of the liver. It then arches dorsally and then posteriorly. It gives off three branches, two of which enter the right and left lateral lobes of the liver, and the third continuing to join the hepatic portal vein, which, as described below, enters the liver (Figures 6.14 and 6.17). Blood from the hind limb may thus return to the heart through the posterior vena cava by several routes. It may pass through the renal portal system or the hepatic veins. In the latter instance it may pass through the branches of the ventral abdominal vein that enter the liver directly or through the hepatic portal vein via that branch of the ventral abdominal vein joining the hepatic portal vein. The hepatic portal vein drains the abdominal viscera. It is formed mainly by the two following vessels: the gastric vein, which collects several vessels and drains the stomach and part of the esophagus, and the intestinal vein, formed by vessels that drain most of the small intestine and large intestine (Figures 6.14 and 6.17). The hepatic portal then receives a branch from the ventral abdominal vein before entering the liver.
Key Terms: Cardiovascular System
brachial artery brachial vein bulbus cordis carotid arch carotid body celiac artery celiacomesenteric artery common iliac arteries cutaneous artery dorsal aorta dorsolumbar vein external carotid artery external jugular vein femoral artery femoral vein gastric vein heart hepatic portal vein hepatic veins hypogastric artery iliac vein innominate vein internal carotid artery internal jugular vein intestinal vein kidneys left anterior vena cava left atrium
mesenteric artery musculocutaneous vein occipitovertebral artery ovarian veins pelvic vein pericardial cavity pericardium posterior vena cava pulmocutaneous arch pulmonary artery pulmonary veins renal portal veins renal veins right anterior vena cava right atrium sciatic artery sciatic vein sinus venosus spermatic veins subclavian artery subclavian vein subscapular vein systemic arch truncus arteriosus urogenital arteries ventral abdominal vein ventricle