Section V-Cardiovascular System

Heart and Arterial Circulation

Heart

The pericardial cavity contains the heart, the muscular pump that drives the blood around the cardiovascu­lar system. To expose the pericardial cavity, continue the incision into the pleuroperitoneal cavity forward through the coracoid bar and the hypobranchial muscu­lature.

The ventricle is the most conspicuous structure of the heart. It is a large, oval, muscular chamber; a coronary artery, supplying the heart, will be seen on its surface. Lift the ventricle’s posterior end to expose the sinus venosus and atrium. The sinus venosus is the thin, tri­angular, posterior, sac-like chamber attached to the transverse septum. It receives venous blood from the body and passes it anteriorly into the atrium. Anteriorly the ventricle narrows into the fourth chamber of the heart, the muscular, tube-like conus arteriosus, through which blood leaves the heart.

Arteries of the Branchial Region

Once blood leaves the heart, it enters the ventral aorta and passes through the afferent branchial arteries on its way to being aerated in the gills. To trace this arterial route, continue dissecting forward from the conus arte­riosus. The ventral aorta gives off five pairs of afferent branchial arteries. The most posterior are the 4th and 5th afferent branchial arteries, which may arise sepa­rately or by a short common trunk. The 3rd afferent branchials emerge further anteriorly. Trace the ventral aorta anteriorly to its bifurcation. Each of the divisions divides again into the 1st and 2nd afferent branchial arteries.

Blood passing through the afferent branchial arteries enters the gill lamellae, where gas exchange occurs. The aerated blood is then collected again so it may be sent to the rest of the body. To view the system that recol­lects the blood, delicately remove the membrane lining the roof of the oral cavity and pharynx and the area around the uncut internal branchial openings (Figures 3.28-3.30). Aerated blood is initially collected from the lamellae into the pretrematic and posttrematic arteries. These arteries join dorsally and ventrally to form a com­plete collector loop around a pharyngeal slit (Figure 3.23). Note that there are only four collector loops, as the last slit has only a pretrematic artery. From the dorsal end of each loop, an efferent branchial artery carries blood away from a gill (Figure 3.28). The four pairs of arteries pass posteromedially and empty into the dorsal aorta on the roof of the pharynx, which passes posteriorly to supply most of the body with aerated blood. Its branches are considered below.

Examine first the vessels that supply the head. A hyoidean artery arises from the anterodorsal part of the first collector loop, just anterior to the first efferent branchial artery, and extends anteriorly. Paired dorsal aortae arise from the first efferent branchial arteries. They pass anteriorly and then veer laterally and join a hyoidean artery. The union of the hyoidean and a paired dorsal aorta forms the internal carotid artery. The inter­nal carotid extends anteriorly a short distance and, near the level of the spiracle, gives rise to the stapedial artery, which extends anterolaterally a short distance before passing into the chondrocranium. Past the origin of the stapedial artery, the internal carotids from either side extend anteromedially and meet middorsally to form a single vessel that enters the chondrocranium.

The afferent spiracular artery arises from near the middle of the pretrematic artery of the first loop. Do not dissect for it here. It was noted earlier (page 41; Figure 3.17) as it passed deep to the hyomandibular nerve on the levator hyomandibulae muscle. Return to the dis­section of the muscles, and trace the afferent spiracular artery anteriorly into the pseudobranch of the spiracle and posteriorly into the collector loop. The efferent spiracular artery collects blood from the pseudobranch. Find it on the roof of the oral cavity. It extends antero- medially, passing ventral to the stapedial artery, and enters the chondrocranium. The lower jaw is supplied by the external carotid artery, which arises from the anteromedial corner of the first collector loop (Figure 3.30).

The hypobranchial artery (Figure 3.30) usually arises from the ventral end of the second collector loop, but

FIGURE 3.27 Head and branchial region of the shark in ventral view, with the hypobranchial musculature dis­sected and reflected to expose the heart and ventral aorta.

branches from the other loops may contribute to it. The hypobranchial passes posteriorly to the conus arterio­sus, where it divides into the coronary and the pericar­dial arteries. The former is clearly observed on the conus arteriosus and ventricle (Figure 3.27), the latter on the wall of the pericardial cavity. The narrow, sinuous pharyngoesophageal artery arises from the second effer­ent branchial artery and extends posteriorly to give off branches to the pharynx and esophagus (Figure 3.28). The rest of the blood flow passing back to the body goes through the dorsal aorta, which gives off various large branches.

Branches of the Dorsal Aorta

The paired subclavian arteries are the first major branches of the dorsal aorta (Figures 3.28 and 3.29). These arise usually between the 3rd and 4th efferent brachial arteries.

FIGURE 3.28 Roof of the oral cavity and pharynx in the shark in ventral view, showing the pattern of the arte­rial circulation. The left side visceral arches have been cut. The floor of the oral cavity and of the pharynx have been swung open.

the posterior ventrolateral artery (a branch of the iliac arteries; see page 55).

After giving off the subclavian arteries, the dorsal aorta continues posteriorly into the pleuroperitoneal cavity. Return to this cavity to examine the following vessels (Figures 3.29 and 3.31). The first branch of the dorsal aorta in the cavity is the celiac artery, a large, unpaired vessel that continues posteriorly along the right side of the stomach.

FIGURE 3.29 Schematic illustration showing the pattern of the arterial system of the shark superimposed on ventral (top figure) and left lateral (bottom figure) views of the body outline.

FIGURE 3.30 Schematic illustration showing the pattern of the arterial system in the head and branchial region of the shark superimposed on ventral (top figure) and left lateral (bottom figure) views of the head.

where it branches into the dorsal and ventral gastric arteries to the dorsal and ventral parts of the stomach, respectively. Return to the pancreaticomesenteric artery and trace it posteriorly as it passes dorsal to the pylorus and onto the ventral side of the intestine as the anterior intestinal artery. Before doing so it gives off several smaller branches near the pylorus (which you do not need to name).

Trace the dorsal aorta further posteriorly. At about the level of the spleen, two arteries arise close together. The anterior one is the anterior mesenteric artery, which extends onto the intestine as the posterior intestinal artery. The posterior artery is the gastrosplenic artery, which mainly supplies the spleen and the posterior part of the stomach. Note that the anterior mesenteric and gastrosplenic arteries lie in the posterior edge of the greater omentum.

The final branches of the dorsal aorta are the paired iliac arteries and the caudal artery (Figure 3.29). At about the level of the cloaca, the iliac arteries arise and pass posterolaterally dorsal to the kidneys. They emerge from under the kidneys and extend toward the pelvic fins. Before entering the fin, each iliac branches into the femoral artery, which enters the fin, and the posterior ventrolateral artery, which turns anteriorly along the body wall and eventually unites with the anterior ven­trolateral artery, noted above. The caudal artery is the posterior continuation of the dorsal aorta into the tail. Finally, the dorsal aorta gives rise to numerous inter­segmental arteries (Figures 3.29, 3.34) to the axial mus­culature. Free the lateral margin of a kidney and lift it to observe these arteries.

Venous Circulation

Hepatic Portal System

The hepatic portal system is the venous system that returns blood from the alimentary tract to the liver (where raw nutrients in blood are processed before the blood returns to the heart). Essentially, it drains the structures supplied ultimately by the celiac (except for the gonads), anterior mesenteric, gastrosplenic, and posterior mesenteric arteries. Thus, the branches of the hepatic portal system closely follow many of the branches of these arteries, and it is convenient to study

FIGURE 3.31 Pleuroperitoneal cavity of the shark in ventral view, showing the pattern of the arteries and veins.

them following your identification of the arteries. In some specimens the hepatic portal system is injected with yellow latex, which greatly facilitates its study. You may follow its branches in an uninjected specimen, but also briefly examine a shark that has the system injected.

The main vessel of the hepatic portal system is the hepatic portal vein (Figure 3.32), a large vein that lies in the lesser omentum alongside the hepatic artery and anterior part of the bile duct. The hepatic portal vein is formed by the confluence of three main vessels, the gastric, pancreaticomesenteric, and Iienomesenteric veins. They unite to form the hepatic portal near the anterior tip of the dorsal lobe of the pancreas. Recall that the celiac artery splits into its branches very near this point as well. Occasionally, the gastric and lieno- mesenteric veins join to form a very short vessel that then unites with the pancreaticomesenteric to form the hepatic portal vein.

FIGURE 3.32 Schematic illustration showing the pattern of the venous system in the shark superimposed on ventral (top figure) and left lateral (bottom figure) views of the body outline. In the top figure, the hepatic portal system has been moved outside the body outline for clarity. The stars indicate the position of the hepatic portal system.

The gastric vein accompanies the gastric artery onto the dorsal and ventral surfaces of the stomach. Follow the Iienomesenteric vein as it runs along the dorsal lobe of the pancreas. Rotate the spleen toward the left to observe that the vein is formed by the confluence, near the posterior end of the dorsal lobe, of the posterior Iienogastric vein and the posterior intestinal vein (Figure

3.32). The former comes from the spleen and posterior part of the stomach (i.e., in parallel with the lienogastric artery), the latter from the posterior part of the intes­tine (recall that this region is supplied by the anterior mesenteric artery, which becomes the posterior intesti­nal artery). The pancreaticomesenteric vein accompa­nies the pancreaticomesentric artery to the beginning of the intestine. Here it is formed by various tributaries. Among these are the anterior intestinal and the anterior Iienogastric veins (Figure 3.32). As you might expect, the former vein extends parallel to the anterior intesti­nal artery. It is larger than the anterior lienogastric vein, which comes from the spleen and adjacent regions of the pyloric region of the stomach.

Renal Portal System

Blood from the tail passes through the kidneys through the renal portal system before returning, through the posterior cardinal veins and sinus, to the heart. The main vessels of the renal portal system are the caudal vein and the renal portal veins. The latter arise through bifurcation of the caudal vein (Figure 3.32). The caudal vein passes anteriorly through the hemal arches of the vertebrae. Make a partial transverse section—deep enough to cut through the vertebrae—of the tail just posterior to the level of the cloaca. Note the caudal artery, which will be injected, lying in the dorsal part of the hemal arch. The caudal vein, which should not be injected, lies ventral to the artery (see Figure 3.6). Make additional partial sections, spaced about 1 cm apart, anterior to the first. As these sections are anterior to the cloaca, they will be at the level of the posterior end of the body cavity. Thus, cut deeply enough to go through the caudal artery and vein, but avoid cutting the kidneys, which lie ventral to and on either side of the vertebrae. You will thus be able to observe, near the pos­terior end of the kidneys, the bifurcation of the caudal vein into left and right renal portal veins. The latter con­tinue anteriorly along the dorsolateral margins of the kidneys. Afferent renal veins, which you will not be able to observe (but see Figure 3.32), branch from the renal portal veins and carry blood to the sinuses of the kidneys.

Systemic Veins

The systemic veins (Figure 3.32) are those that drain most of the body other than the viscera. Each side of the head and branchial region are drained mainly by the anterior cardinal sinus and inferior jugular vein. The anterior cardinal sinus is a relatively large space (com­pared with the diameter of most veins) lying dorsal to the pharyngobranchial cartilages. It was exposed during the dissection of the musculature to find the dorsal and lateral interarcual muscles (page 43). The sinus receives vessels that drain the eye, brain, and head, and leads blood posteriorly toward the heart. The orbital sinus, for example, surrounds the eye, but it is impractical to attempt to find it. The inferior jugular vein is a thin vessel draining the floor of the branchial region and you will not see it. The anterior cardinal sinus and inferior jugular vein on each side of the head are connected via the hyoidean sinus. Pass a probe into the hyoidean sinus to determine its course. It lies along the posterior surface of the hyoid arch, and so it was sectioned during the cut made to open the oral cavity and pharynx.

Return to the heart, and slit the ventral wall of the sinus venosus from side to side. Gently probe its walls and note that there are a number of openings that lead into it. Probing its posterior wall just to either side of the sagittal plane will lead into the opening of a hepatic vein and then into a hepatic sinus, which collects blood from the liver (Figures 3.31 and 3.32). Each posterolat­eral corner of the sinus venosus leads to a short though large vessel, the common cardinal sinus, which receives the rest of the main vessels leading blood back to the heart (Figure 3.32). Pass a blunt, curved probe along the posterior wall of the common cardinal sinus. It should lead, without much effort, into the posterior cardinal sinus (described below). The anterior cardinal sinus enters the common cardinal at about the same level, but it is much more difficult to probe for by this method. Here, merely note that the tip of the probe veers ante­riorly and dorsally.

The other vessels that enter the common cardinal are the inferior jugular vein and the subclavian vein (Figure

3.32). Gently probing the anterior wall of the common cardinal will lead into the inferior jugular vein. You may follow its course by palpating for the probe in the hypobranchial region. The subclavian vein enters the common cardinal just lateral to the entrance of the infe­rior jugular.

Return to the pleuroperitoneal cavity. Lift the gonad and anterior part of the liver so that the roof of the cavity may be observed. Lying dorsolateral to the esoph­agus is the posterior cardinal sinus (noted above), a large space bounded by a thin-walled membrane. It is filled with blue latex and therefore appears bluish. Note that the sinus curves toward the dorsal midsagittal line. Slit open the sinus and carefully remove the latex. Probe gently to verify that this sinus is continuous with the sinus from the other side of the body. Veins from the gonads and esophagus empty into the sinus, but it is impractical to trace them.

Follow the sinus posteriorly, moving the viscera to one side as you do so (Figure 3.32). The sinus narrows into a posterior cardinal vein that lies laterally to the dorsal aorta. The posterior cardinal veins appear as thin, translucent vessels because they are probably not injected, and so may be difficult to discern. Do not confuse them with the oviducts of a female or the archinephric duct of immature males. These thin, flat­tened, and straight structures are easily identified because they continue anteriorly to lie on the posterior cardinal sinus. Note that the veins lie along the medial margins of the kidneys. They receive efferent renal veins that collect blood from the kidneys and the segmentally arranged intersegmental veins that drain blood from the body wall. These veins, however, may not be easily observable.

A lateral abdominal vein extends along the inside of the ventrolateral body wall on each side (Figures 3.31 and

3.32). Trace one of these veins posteriorly. It is formed, at about the level of the cloaca, by the confluence of the cloacal vein (from the cloaca) and the femoral vein (from the pelvic fin). Trace the lateral abdominal forward to the pectoral girdle. Here, you will observe a number of veins coming together. You will probably see a conspicuous subscapular vein (Figure 3.31), which essentially runs parallel to the subclavian artery (but does not share its name). Find the brachial vein on the medial surface of the fin, in company with the brachial artery. It will probably be uninjected. Trace the brachial vein toward the body. It unites with the lateral abdom­inal to form the subclavian vein, already noted as entering the common cardinal sinus. The subscapular vein actually joins the brachial vein, but the brachial, subscapular, and lateral abdominal veins all come together very near each other in forming the subclavian vein, the vessel that continues the blood’s journey toward the heart. Follow the subclavian vein as it arches dorsomedially.

Key Terms: Cardiovascular System

common cardinal sinus conus arteriosus coronary artery dorsal aorta dorsal gastric artery efferent branchial artery efferent renal veins efferent spiracular artery external carotid artery femoral artery femoral vein gastric artery gastric vein gastrohepatic artery gastrosplenic artery hepatic artery hepatic sinus hepatic portal vein hepatic vein hyoidean artery hyoidean sinus hypobranchial artery

(commissural artery) iliac arteries inferior jugular vein internal carotid artery intersegmental arteries intersegmental veins lateral abdominal vein lienomesenteric vein orbital sinus ovarian artery

paired dorsal aortae pancreaticomesenteric artery pancreaticomesenteric vein

parietal pericardium pericardial artery pericardioperitoneal canal pharyngoesophageal artery

posterior cardinal vein posterior intestinal artery

posterior intestinal vein posterior lienogastric vein

posterior mesenteric artery

posterior ventrolateral artery (posterior epigastric artery)

renal portal vein sinus venosus stapedial artery subclavian arteries subclavian vein subscapular vein testicular artery transverse septum ventral aorta ventral gastric artery ventricle visceral pericardium