The circulatory system of vertebrates consists of blood, blood vessels, and a muscular, pumping heart. Blood vessels are hollow organs that serve to carry blood throughout the body. Their walls are composed of tissue layers called “tunics”. The innermost is the tunica intima/tunica interna which is in direct contact with the blood. The second, middle, tunic is the tunica media. The outermost tunic is the tunic externa/tunica adventitia. There are three classes of blood vessels: arteries, veins, and capillaries. Arteries serve to carry blood from the heart to the tissues of the body. Since they are carrying blood at its highest pressure, arteries have proportionately the thickest walls with the tunica media being very thick. Capillaries are
microscopic blood vessels where materials are exchanged between the tissues and the blood. To facilitate this, capillaries are very thin walled to maximize diffusion. They are microscopically small and will not be a feature of your dissection. Veins serve to carry blood back to the heart from the body’s tissues. Their walls are proportionately thinner than are those of arteries since they carry blood at a lower pressure. Also, due to the lower pressure, veins will have numerous valves projecting into the lumen to prevent the backflow of blood.
The heart is a muscular pump. It is located in the pericardial cavity and covered by a serous membrane called the pericardium. The heart evolved from blood vessels. As a result, the heart wall is composed of three tissue layers that are similar to the three tunics of large blood vessels. The inner lining is termed the endocardium. The middle and thickest layer is the myocardium. It is rich in cardiac muscle tissue to pump the blood. The epicardium is the outer layer of the heart. The heart wall will receive its own blood supply through the coronary blood vessels. Vertebrate hearts can be divided into single circuit and double circuit hearts. Single circuit hearts are found in fishes. Blood passes form the heart to the gills, from the gills to the rest of the body, and from the rest of the body back to the heart. Blood picks up oxygen from the gills and delivers it to the rest of the body where the cells will use the oxygen for respiration.
The deoxygenated blood will then be returned to the heart. Double circuit hearts are found in amniotes. There are two currents or circuits. One is the pulmonary circuit which carries
deoxygenated blood from the heart to the lungs to pick up oxygen. The oxygenated blood is then carried from the lungs back to the heart. The other is the systemic circuit which carries
oxygenated blood from the heart to all of the tissues of the body. There oxygen is used for respiration and carbon dioxide is dumped into the blood. The deoxygenated blood is returned to the heart.
Part E1: The Circulatory System in Squalus
The heart of Squalus is typical for that of most fishes. It has four portions (in sequence of receiving blood): sinus venosus, atrium, ventricle and conus arteriosus. Fish are said to have a two-chambered heart, one atrium and one ventricle. The sinus venosus receives deoxygenated blood from the body’s tissues. It is a thin walled vein having little muscle and being composed mostly of fibrous connective tissue. The sinus venosus primarily serves to collect deoxygenated
Arteries of the Squalus Oropharynx
1. Internal Carotid Artery 2. Hyoidean Epibranchial Artery 3. Radix Aorta 4. Vertebral Artery 5-8 Efferent Branchial Arteries (I-IV) 9. Esophageal Artery 10. Dorsal Aorta
Squalus Arteries
1. Dorsal Aorta 2. Celiac 3. Gastrohepatic 4. Gastric
5. Hepatic 6. Pancreaticomesenteric 7. Intraintestinal 8. Duodenal
9. Pyloric 10. Anterior Intestinal 11. Gastrosplenic 12. Posterior Intestinal
13. Posterior Mesenteric 14. Iliac 15. Femoral 16. Caudal
17. Annular
Squalus Veins
1. Lateral Abdominal 2. Renal Portal 3. Posterior Cardinal 4. Dorsal Aorta 5. Hepatic Portal 6. Gastric 7. Leinomesenteric 8.Posterior Intestinal 9. Posterior Splenic 10. Pancreaticomesenteric 11. Pyloric 12. Intraintestinal 13. Anterior Splenic 14. Anterior Intestinal 15. Annular
Note: Systemic veins are in blue and veins of the hepatic portal system are in yellow.
Squalus Deep Vessels & Heart
1. Afferent Branchial a. 2. Ventral Aorta 3. Conus Arteriosus 4. Ventricle
5. Atrium 6. Sinus Venosus 7. Internal Jugular v. 8. External Jugular v.
9. Subclavian v. 10. Brachial v. 11. Lateral Abdominal v. 12. Posterior Cardinal Sinus 13. Genital Sinus 14. Dorsal Aorta 15. Posterior Cardinal v. 16. Renal Portal v.
17. Afferent renal v. 18. Efferent renal v. 19. Cloacal 20. Iliac
21. Femoral 22. Caudal a. 23. Caudal v. 24. Common Cardinal v.
blood from the tissues of the body and has little contractile action. (Blood is pulled into the sinus venosus due to pressure caused by ventricular contractions.) Blood travels from the sinus
venosus, through the sinoatrial aperture, into the atrium. A pair of unidirectional valves guards the sinoatrial aperture. These valves prevent the backflow of blood. Blood moves into the atrium when it relaxes after emptying (systole). The atrium receives blood from the sinus venosus and pushes it into the ventricle. This chamber is a thin walled muscular sack. Contractions of its muscular component will push the blood through the atrioventicular aperture into the ventricle.
The atrioventicular aperture is also guarded by a pair of unidirectional valves which also prevent the backflow of blood. The ventricle is a muscular, thick walled chamber that serves as the main pumping chamber of the heart. It pumps blood into the conus arteriosus. The ventricle generates the main force for fish circulation. The conus arteriosus receives blood from the ventricle and conducts it to the gills by way of the ventral aorta. The conus arteriosus extends to the cranial most extent of the pericardial chamber and joins with the ventral aorta. The wall of the conus arteriosus is primarily composed of cardiac muscle and elastic connective tissue. The cardiac muscle component of the conus arteriosus allows for steady blood pressure in the ventral aorta. It also has a series of semilunar valves to prevent the backflow of blood into the ventricle. The conus arteriosus is different between cartilaginous and bony fishes. Cartilaginous fishes, such as Squalus, have a relatively longer conus arteriosus. Bony fishes, particularly teleosts, have a shorter conus arteriosus. Since the conus arteriosus is shorter it has a muscular swelling at it s base to compensate called the bulbus arteriosus.
Squalus Heart
The arteries serve to carry blood from the heart to the tissues of the body. Typically this blood will be oxygenated but that is not always the case. In fishes, such as Squalus, arterial flow
begins with the ventral aorta receiving blood from the conus arteriosus and bringing it to the gills. This blood is deoxygenated. Capillary beds in the gills will serve as the site of gas exchange. The oxygenated blood will be recovered by the dorsal aorta and delivered to the throughout the body. During development the ventral aorta extends craniad, under the pharynx, and connects with the developing aortic arches. In the primitive gnathostome condition there were six pairs of aortic arched connecting the ventral aorta and dorsal aorta. The first aortic arches to develop are those of the mandibular arch. The other five arches develop shortly after.
However, before the sixth pair fully forms, the first pair will disappear leaving only branches called the spiracular arteries. The second pair will sprout buds that will develop into the first pretrematic arteries. Other buds will sprout off of the third through sixth pairs of aortic arches.
These buds will become the posttrematic arteries. The posttrematic arteries will give rise to branches that will become the other pretrematic arteries. Soon during development arches 2 through 6 will develop occlusions where portions of the vessel will be lost. Segments located ventral to the occlusions will give rise to the afferent branchial arteries while the segments located dorsal to the occlusions will give rise to the efferent branchial arteries. At the same time capillary beds will begin to form in the demibranchs. These capillary beds are special in that they are drained by arterioles instead of venules (which is the typical condition). They are an example of a rete mirabilia.
Esophageal A. Radix Aorta (poorly injected) Mock up of a Radix Aorta
Efferent Branchial Arteries Hyoidean Epibranchial A. Internal Carotid A.
Squalus Arteries of the Doral Oropharynx
In your dissection you should be able to observe the ventral aorta passing through the floor of the oropharynx and branching into four pairs of afferent arteries going to the gills. These structures tend to be poorly injected and will be hard to find. However, the roof of the
oropharynx will be fairly well injected and you should be able to observe its arteries. There will be four pairs of efferent branchial arteries receiving oxygenated blood from the gills. They will drain into the dorsal aorta that can be observed dorsal to the pleuroperitoneal cavity and at the caudalmost extent of the oropharynx. The anterior extensions of the dorsal aorta are the paired radix aortae (which are sometimes referred to as the “paired dorsal aortae”). They are small “L-shaped” vessels located anterior to the first pair of efferent branchial arteries and immediately lateral to the vertebrae. Immediately medial to these arteries are the difficult to find vertebral arteries. Extending anteriorly from each first efferent artery will be a small artery that receives blood directly from the first gill pair. This is the hyoidean epibranchial artery. The hyoidean epibranchial artery will extend anteriorly along the roof of the oropharynx. Part way along its length it will meet with the radix aorta. At its anterior extent it will branch into the stapedial artery (which services the extrinsic ocular muscles) and the internal carotid artery. Coming off of the second efferent branchial artery and extending caudally will be a wavy artery called the esophageal artery. The subclavian arteries will arise from the dorsal aorta between the third and fourth efferent branchials and will extend towards the pectoral fins. One of its branches will be the brachial artery of the pectoral fin. Another will be the lateral artery that runs along the lateral line and lateral abdominal vein. The vein is easy to find but the artery typically injects poorly.
Squalus Early Branches of the Dorsal Aorta
Celiac Trunk Gastrohepatic A. Gastric A. Posterior Intestinal A. Gastrosplenic A.
Hepatic A. Pancreaticomesenteric A. Anterior Intestinal A.
Squalus Visceral Arteries 1
The dorsal aorta will extend through the pleuroperitoneal cavity immediately ventral to the vertebral column. It will give rise to the visceral arteries. The first branch will be the celiac artery. This is a prominent artery that will divide into two branches: the gastrohepatic artery and the pancreaticomesenteric artery. The gastrohepatic artery is a short vessel that will divide into the gastric artery, to service the stomach, and the hepatic artery, to service the liver. The pancreaticomesenteric artery gives rise to numerous conduits. One easily identified artery is the anterior intestinal artery. The anterior intestinal will travel along the ventral wall of the intestine and give rise to numerous annular arteries which run near the folds of the spiral valve. At the point where the anterior intestinal arises three other vessels will also arise. One will enter into the pyloric region of the stomach and is the pyloric artery. The second will be the duodenal artery supplying a small portion of the intestine prior to the spiral valves. The third is the intraintestinal artery which extends through the spiral valve. The second branch coming off of the dorsal aorta is the posterior intestinal artery. It will enter the dorsal wall of the intestine. It will also give rise to numerous annular arteries which will fuse to those that arose from the anterior intestinal artery. Branching close to the posterior intestinal artery will be the gastrosplenic artery. The gastrosplenic artery will enter into the spleen and traveling through the spleen will also send branches into the stomach. In some specimens the posterior intestinal and gastrosplenic arteries
Pyloric A. Anterior Intestinal A. Annular Arteries Posterior Mesenteric A.
Gastrosplenic A. Posterior Intestinal A. Dorsal Aorta (poorly injected) Squalus Visceral Arteries 2
Squalus Left Iliac Artery
arise from a common vessel that branches off of the dorsal aorta called the anterior mesenteric artery. Typically the fourth major artery arising from the dorsal aorta will be the posterior mesenteric which will service the rectal gland. Near the cloaca the dorsal aorta will branch into the paired iliac arteries and caudal artery. The iliac arteries run into the pelvic fins and give rise to the femoral arteries. The caudal artery runs through the tail and caudal fin.
Veins will drain blood from capillary beds. In a triple-injected specimen the veins will have two colors. Yellow vessels designate veins of the hepatic portal system. The hepatic portal system is a series of veins draining nutrient-rich blood from the digestive tract and carrying it to the liver for processing. Blue vessels designate systemic veins. Systemic veins usually convey blood that is high in carbon dioxide and other metabolic wastes to the heart and, subsequently, the gills. The systemic vessels will also contain the renal portal system. This venous pathway carries blood high in muscle waste products from the tail to the kidneys for cleansing. On occasion an error with specimen processing may cause the colors to be reversed.
Hepatic Portal Vein Gastric V. Anterior Splenic V. Posterior Splenic V.
Pancreaticomesenteric V. Annular V. Anterior Intestinal V.
Squalus Hepatic Portal System 1
The largest vein of the hepatic portal system is the hepatic portal vein. It will run along the common bile duct and the hepatic artery. It drains three veins: the gastric vein, the
pancreaticomesenteric vein, and the lienomesenteric vein. The gastric vein drains the capillary beds of the stomach and will run along the gastric artery. The pancreaticomesenteric and
lienomesenteric veins are easy to confuse at first since they both run near each other and towards the intestines. However, the pancreaticomesenteric vein runs with the pancreaticomesenteric artery while the lienomesenteric vein runs through the dorsal lobe of the pancreas. The pancreaticomesenteric vein has many of the same branches that you saw with the
pancreaticomesenteric artery. There is an anterior intestinal vein which is on the ventral aspect of the intestine (parallel to the anterior intestinal artery) and will drain the ventral portions o the annular veins. There is also a pyloric vein, draining the pyloric stomach, and an intraintestinal vein, draining the spiral valves. The pancreaticomesenteric vein will also drain the anterior splenic vein. The anterior splenic vein is a smaller vein running through the peritoneum along the pyloric portion of the greater gastric curvature and draining the spleen. It is easy to lose when removing the peritoneum. The lienomesenteric vein drains two fairly easily found veins: the posterior intestinal vein and the posterior splenic vein. The posterior intestinal vein runs along the dorsal aspect of the intestines (parallel to the posterior intestinal artery) and drains the posterior portions of the annular veins. The posterior splenic vein will drain the caudal portion of the spleen and typically runs parallel to the gastrosplenic artery.
Squalus Hepatic Portal System 2
The systematic veins anterior to the pleuroperitoneal cavity will not be injected and hard to find at best. However, many systematic veins in the pleuroperitoneal cavity should be fairly well injected and easier to work with. The renal portal system of the systemic vessels begins with the caudal vein carrying blood from the tail into the pleuroperitoneal cavity. The caudal vein
feeds into the right and left renal portal veins. The renal portal veins run along, and immediately lateral to, the mesenephric kidneys. Numerous small veins will branch off of the renal portal vein and enter into the kidney. These are the afferent renal veins. The afferent renal veins deliver the blood to the kidneys for the removal of metabolic wastes, excepting carbon dioxide.
(The tail is the main locomotory structure for swimming in sharks and the muscles will produce a good deal of waste.) The blood will be processed by the kidneys and will then drain into a series of small vessels called efferent renal veins. The afferent renal veins are on the lateral half of the kidney and the efferent renal veins are on the medial half of the kidney. These vessels will drain into the paired posterior cardinal veins. The posterior cardinal veins are located medial to the mesenephros and immediately lateral to the dorsal aorta. These paired vessels will carry the blood cranially to large venous sinuses. These are the right and left posterior cardinal sinuses. A sinus is a thin walled but large vein and so the posterior cardinal sinuses are fragile. They will be located behind the liver, near the gonads, in the cranial one fifth of the pleuroperitoneal cavity.
Immediately caudal to the posterior cardinal sinus, and draining into it, is the genital sinus. In most specimens it will be difficult to differentiate the genital sinus from the posterior cardinal sinus. The right and left posterior cardinal sinus will drain into the right and left common cardinal veins respectively. The common cardinal veins will return blood to the heart by feeding blood into the sinus venosus.
Afferent Renal V.
Squalus Renal Portal System
Squalus Pleuroperitoneal Vessels
The femoral vein can be observed by separating the pelvic depressor muscles. It will drain into the iliac vein which will run parallel to the iliac artery and drains into the lateral abdominal vein. The lateral abdominal veins will receive blood from veins of the lateral wall myomeres. In addition, the right and left lateral abdominal veins will both drain the cloacal veins. The cloacal veins enter into the lateral abdominal vein at the same point as the iliac veins.
The lateral abdominal vein will run through the lateral body wall near to the lateral line and lateral abdominal artery and will carry blood cranially. At the pectoral girdle it will drain the subclavian vein. The subclavian vein carries blood from the brachial vein of the pectoral fin. At its most cranial extent the lateral abdominal vein will drain into the common cardinal vein. The common cardinal veins will return blood to the heart from all of the systematic vessels. It drains the lateral abdominal, anterior cardinal, internal jugular veins and the posterior cardinal sinus.
Unfortunately it is usually difficult to observe this important vessel due to its location deep to the coracoid bar and its fragility.
Squalus Showing the Right Posterior Cardinal Sinus
Part E2: The Circulatory System in Necturus
The heart of Necturus, like most amphibians, will display modifications due to aerial respiration. These modifications allow oxygenated blood returning from the lungs or swim bladder to be separated in the heart from deoxygenated blood returning from the other organs.
There are typically four modifications in the amphibian heart over that of gill breathing fishes.
One modification was the establishment of a complete or partial interatrial septum. This establishes either the complete or partial separation of a right and left atrium. The interatrial septum is complete in anurans and some urodeles. The septum is lacking in lungless urodeles. It
One modification was the establishment of a complete or partial interatrial septum. This establishes either the complete or partial separation of a right and left atrium. The interatrial septum is complete in anurans and some urodeles. The septum is lacking in lungless urodeles. It