The blood vascular system of frog is closed and consists of Heart, arterial system, venous system, blood and lymphatic system. Its main function is to transport all liquid and gaseous materials to the living tissues and also to bring away from them the liquid and gaseous wastes of metabolism to organs of elimination.
Heart : It is a muscular organ and acts as pumping station and pumps blood.
External Features : It lies mid-ventrally in the anterior region of body cavity. It is reddish in colour and is triangular in shape with broad anterior and narrow posterior end.
It lies inside thin, transparent, two layered sac called as pericardium. The outer layer is called as parietal layer and the inner layer is called as visceral layer. The cavity between these two pericardial layers is called as pericardial cavity which contains pericardial fluid which protects heart from friction and mechanical shocks and keeps it moist.
It consists of 3 chambers; two auricles and one ventricle. The auricles are two anterior dark coloured chambers and are right auricle and left auricle. The two auricles remain demarcated by a longitudinal inter-auricular groove. The ventricle is posterior, conical pink coloured chamber. The auricles and ventricle remain demarcated with each other by a transverse auriculo-ventricular groove or coronary sulcus.
The frog’s heart has two additional chambers; sinus venosus and truncus arterious. The sinus venosus is dark- coloured, thin walled and roughly triangular chamber present on dorsal surface of heart. It is formed by union of two anterior precavels and one posterior postcavel. The truncus arteriosus is a tubular chamber like structure on ventral surface and arises from anterior right ventral side of ventricle and is a tubular structure. It bifurcates anteriorly into two branches or trunks, each further divides into three arches; carotid, systemic and pulmocutaneous.
Internal Structure : Its internal structure is studied with the help of vertical section from ventral side. It is hollow, muscular with different chambers. The different chambers remain separated by valves to keep one way flow of blood.
The two auricles are thin walled and remain separated from each other by a thin vertical inter-auricular septum. Right auricle is larger than the left. The sinus venosus opens into dorsal wall of right auricle by a large oval aperture, sinu-auricular aperture. It lies at middle close to interauricular septum and remains guarded by a pair of flap like valves to prevent back flow of blood. The common pulmonary vein opens into left auricle, near interauricular septum above the level of sinu-auricular aperture, by a small opening without valves. As this opening is oblique, it automatically gets closed on contraction of left auricle.
Both auricles open into ventricle by a common large auriculo-ventricular aperture guarded by two pairs of flap like auriculo-ventricular valves. The flaps of these auriculo-ventricular valves remain attached with wall of ventricle by fine thread like structures called as chordae tendineae which keep auriculo-ventricular valves in their own position during ventricular systole.
The ventricle is thick walled, muscular and spongy in nature. Its inner surface has irregular ridges or folds called as the columnae carneae with depressions called as fissures. These folds reduce cavity of ventricle.
Working of heart
Heart constantly beats during life time to pump blood into blood vessels. The regular contraction and relaxation of heart is called as heart beat. Contraction of heart is called as systole, while relaxation is called as diastole.
On the basis of nature of origin of wave for the contraction of heart; two types of heart are found which are neurogenic heart and myogenic heart. The heart in which wave of contraction originates due to the stimulation of nerves is called as neurogenic heart. The heart in which wave of contraction originates from special muscle node present on the wall of heart itself is called as myogenic heart. In frog, myogenic heart is present. The wave of contraction originates from sino-auricular node (SA- node) which is also called as pacemaker.
When sinus venosus contracts, its non oxygenated blood passes into right auricle through sinu- auricular aperture. The blood from lungs comes into left auricle through pulmonary vein. The two auricles now contract almost simultaneously forcing their blood into ventricle through auriculo-ventricular aperture. For further circulation of blood, there are two views;
(a) Old view. According to the older view the ventricle contains only deoxygenated blood in its right side received from right auricle and only oxygenated blood in its left side received from left auricle, with some mixed blood in the middle region. The two kinds of blood could not mix to any great extent because of their viscous nature and also because of the network of columnae carneane. When ventricle starts contraction, at first the deoxygenated blood from the right side being nearer flows into the truncus arterious and the spiral valve directs this blood into the common opening of pulmocutaneous arches and carried to lungs and skin for oxygenation. Next follows the mixed blood from middle region of ventricle which is directed into cavum aorticum by Spiral valve and it also now closes the opening of pulmocutaneous arches and the blood passes into the systemic arches and sent to the body and limbs. Finally enters the oxygenated blood of left side and directed by spiral valve into carotid arches to the head. Thus, spiral valve in truncus plays an important role in directing blood into different arches.
(b) Modern view. Recent experimental studies conducted mostly by Vandervael and Foxon show that there is completely mixed blood in ventricle and from truncus blood flows simultaneously through the three pairs of arches to all parts of the body. This view is based on the fact that the blood received from skin and buccal cavity into sinus venosus and right auricle is in fact more oxygenated than that received from lungs into left auricle and the spiral valve in truncus is functionless in frogs. Studies by Delong indicate that the carotid arches receive highly oxygenated blood, the pulmocuataneous arches with least oxygen, and the systemic arches receive mixed blood. The precise mechanism is not understood.
1. Common carotid arch: It is a short vessel running forward and outward, but soon divides into 2 branches: external and internal carotids.
a. External carotid: It is the smaller inner branch carrying blood to the tongue and adjacent parts. It is also called lingual.
b. Internal carotid: It is the larger outer branch. At its base it forms a little swelling, the carotid labyrinth. Its lumen is converted into a labyrinth by folding of the walls. It is probably a sense organ and controls blood pressure in the internal carotid artery. The internal carotid divides into 3 branches – a palatine to the roof of buccal cavity, a cerebral to the brain, and an ophthalmic to the eye.
2. Systemic arch: It is the longest of the three arches and with greatest distribution of blood. The two systemic arches curve dorsally around the oesophagus and join with each other behind the heart to form the dorsal aorta. In its course each systemic arch gives off 3 arteries:
a. Oesophageal: A small inner artery to oesophagus.
b. Occipito-vertebral: A larger outer branch which immediately divides into an occipital branch to occiput or posterior part of head, and a vertebral branch to vertebral column and spinal cord.
c. Subclavain: It is a large outer artery supplying the shoulder region and extending into the forelimb as brachial artery.
Dorsal aorta: As already said, it is formed by the union of both the systemic arches. It runs posteriorly lying mid-dorsally just beneath the vertebral column. It gives off the following arteries:
a. Coeliaco-mesenteric: It is a single large artery arising from the left side of the junction of the two systemic arches. It has two main branches: the coeliac to stomach, pancrease and liver; and the anterior mesenteric to spleen and small intestine.
b. Gonadial: A pair of short arteries to gonads, called spermatic in male frog and ovarian in female frog.
c. Renal: While passing between the two kidneys, dorsal aorta gives 5-6 pairs of small renal arteries in a series into both the kidneys.
d. Posterior mesenteric: It arises from the posterior end of dorsal aorta, or sometimes from anterior mesenteric. It goes to large intestine or rectum.
e. Common iliacs: The dorsal aorta finally bifurcates posteriorly into two common liliacs, each supplying an epigastric to ventral body wall, rectovesicular to rectum and urinary bladder and enters inside the hind limbs of own side and divides into femoral which supplies to hip and upper thigh, and sciatic which supplies to lower leg.
3. Pulmocutaneous arch: It divides into two main arteries, pulmonary to the lung and cutaneous to skin of dorsal and lateral sides.
(1) Pulmonary veins, (2) Caval veins and (3) portal veins.
1. Pulmonary veins: Oxygenated blood from two lungs is collected by right and left pulmonary veins which unite to form a common pulmonary vein opening directly into the left auricle on the dorsal side.
2. Caval veins: Deoxygenated blood from rest of the body travels towards heart in three large vessels, two anterior precavals and single posterior postcaval, all the three opening into sinus venosus.
a. Anterior venacava or precavals: The right and left precavls or anterior venacava collect venous blood from the anterior part of body. Each precaval is formed by the union of 3 major veins which are:
i. External jugular: It is formed by the union of lingual form tongue and mandibular from outer margin of lower jaw.
ii. Innominate: It is formed by the union of internal jugular from cranial cavity and orbit and subscapular from shoulder and back of arm.
iii. Subclavian: It is formed by the union of brachial from forelimb and the musculo-cutaneous from muscles & skin of side of body and head.
b. Posterior venacava or postcaval: The single postcaval is a large, dark-coloured vein lying ventral to dorsal aorta. Its posterior end is formed between the two kidneys by the union of 5-6 pairs of renal veins collecting blood from kidneys. It also receives a pair of genital veins (spermatic in male and ovarian in female) from gonads. The postcaval then runs forwards, dorsally to the liver and receiving from it a pair of short hepatic veins, before opening into the posterior part of sinus venosus.
3. Portal Veins: A vein which collects blood from one organ of body and supplies blood to another organ instead of going to heart is called as portal vein. Thus, a portal vein is bicapillary vein. All veins associated with a portal vein forms portal system. Frog consists of two portal systems.
a. Renal Portal system: The veins which carry blood to a capillary system in kidneys constitute the renal portal system. Blood of each hind leg is collected by two veins, an outer fermoral and an inner sciatic. On entering the abdominal cavity the femoral divides into a dorsal renal portal and a ventral pelvic vein. The sciatic unites with the renal portal of its own side and while running along there outer border of kidney of its side it receives blood from lumbar region by a dorso-lumbar vein. Renal portal vein enters the kidney by several branches which break up into capillaries.
The importance of renal portal system is that kidney filters the blood and removes nitrogenous metabolic wastages from blood.
b. Hepatic portal system: It consists of hepatic portal vein and anterior abdominal vein.
A large hepatic portal vein is formed by the union of several branches from stomach, intestine, spleen, and pancreas. The veins which unite to form hepatic portal vein are:
• Gastric from stomach.
• Pancreatic from pancreas.
• Duodenal from duodenum.
• Anterior mesenteric from small intestine.
• Spleenic from spleen.
It carries blood of alimentary canal, laden with digested foodstuffs, to the liver into which it breaks up into capillaries. The liver converts excess glucose into glycogen and stores it as reserve food.
Anterior abdominal vein : The pelvic veins of both sides unite to form a median ventral or anterior abdominal vein. It receives blood from urinary bladder and ventral abdominal wall and runs forwards to enter liver into which it breaks up into capillaries. Before entering liver the anterior abdominal and hepatic portal veins are connected by a small loop.
Heart : It is a muscular organ and acts as pumping station and pumps blood.
External Features : It lies mid-ventrally in the anterior region of body cavity. It is reddish in colour and is triangular in shape with broad anterior and narrow posterior end.
It lies inside thin, transparent, two layered sac called as pericardium. The outer layer is called as parietal layer and the inner layer is called as visceral layer. The cavity between these two pericardial layers is called as pericardial cavity which contains pericardial fluid which protects heart from friction and mechanical shocks and keeps it moist.
It consists of 3 chambers; two auricles and one ventricle. The auricles are two anterior dark coloured chambers and are right auricle and left auricle. The two auricles remain demarcated by a longitudinal inter-auricular groove. The ventricle is posterior, conical pink coloured chamber. The auricles and ventricle remain demarcated with each other by a transverse auriculo-ventricular groove or coronary sulcus.
The frog’s heart has two additional chambers; sinus venosus and truncus arterious. The sinus venosus is dark- coloured, thin walled and roughly triangular chamber present on dorsal surface of heart. It is formed by union of two anterior precavels and one posterior postcavel. The truncus arteriosus is a tubular chamber like structure on ventral surface and arises from anterior right ventral side of ventricle and is a tubular structure. It bifurcates anteriorly into two branches or trunks, each further divides into three arches; carotid, systemic and pulmocutaneous.
Internal Structure : Its internal structure is studied with the help of vertical section from ventral side. It is hollow, muscular with different chambers. The different chambers remain separated by valves to keep one way flow of blood.
The two auricles are thin walled and remain separated from each other by a thin vertical inter-auricular septum. Right auricle is larger than the left. The sinus venosus opens into dorsal wall of right auricle by a large oval aperture, sinu-auricular aperture. It lies at middle close to interauricular septum and remains guarded by a pair of flap like valves to prevent back flow of blood. The common pulmonary vein opens into left auricle, near interauricular septum above the level of sinu-auricular aperture, by a small opening without valves. As this opening is oblique, it automatically gets closed on contraction of left auricle.
Both auricles open into ventricle by a common large auriculo-ventricular aperture guarded by two pairs of flap like auriculo-ventricular valves. The flaps of these auriculo-ventricular valves remain attached with wall of ventricle by fine thread like structures called as chordae tendineae which keep auriculo-ventricular valves in their own position during ventricular systole.
The ventricle is thick walled, muscular and spongy in nature. Its inner surface has irregular ridges or folds called as the columnae carneae with depressions called as fissures. These folds reduce cavity of ventricle.
The ventricle opens into truncus arterious from top right region. The opening of ventricle into truncus arteriosus is guarded by 3 semilunar valves (4-according to Sharma, an Indian biologist) which prevent back flow of blood from truncus into ventricle. The spirally twisted cavity of truncus arteriosus remains divided by 3 semilunar valves into a long thick walled proximal conus arteriosus or pylangium and a short distal, thin walled bulbous aorta or synangium. A longitudinal spiral valve is present on pylangium which remain attached dorsally while free ventrally. It divides cavity of pylangium into left dorsal cavum pulmoutaneum and right ventral cavum aorticum. The common opening of two pulmocutaneous arches lies in cavum pulmocutaneum while separate openings of carotid and systemic arches lie in synangium. Sharma describes a joint opening of carotid and systemic. All these openings are guarded by a pair of valves to prevent back flow of blood.
Working of heart
Heart constantly beats during life time to pump blood into blood vessels. The regular contraction and relaxation of heart is called as heart beat. Contraction of heart is called as systole, while relaxation is called as diastole.
On the basis of nature of origin of wave for the contraction of heart; two types of heart are found which are neurogenic heart and myogenic heart. The heart in which wave of contraction originates due to the stimulation of nerves is called as neurogenic heart. The heart in which wave of contraction originates from special muscle node present on the wall of heart itself is called as myogenic heart. In frog, myogenic heart is present. The wave of contraction originates from sino-auricular node (SA- node) which is also called as pacemaker.
When sinus venosus contracts, its non oxygenated blood passes into right auricle through sinu- auricular aperture. The blood from lungs comes into left auricle through pulmonary vein. The two auricles now contract almost simultaneously forcing their blood into ventricle through auriculo-ventricular aperture. For further circulation of blood, there are two views;
(a) Old view. According to the older view the ventricle contains only deoxygenated blood in its right side received from right auricle and only oxygenated blood in its left side received from left auricle, with some mixed blood in the middle region. The two kinds of blood could not mix to any great extent because of their viscous nature and also because of the network of columnae carneane. When ventricle starts contraction, at first the deoxygenated blood from the right side being nearer flows into the truncus arterious and the spiral valve directs this blood into the common opening of pulmocutaneous arches and carried to lungs and skin for oxygenation. Next follows the mixed blood from middle region of ventricle which is directed into cavum aorticum by Spiral valve and it also now closes the opening of pulmocutaneous arches and the blood passes into the systemic arches and sent to the body and limbs. Finally enters the oxygenated blood of left side and directed by spiral valve into carotid arches to the head. Thus, spiral valve in truncus plays an important role in directing blood into different arches.
(b) Modern view. Recent experimental studies conducted mostly by Vandervael and Foxon show that there is completely mixed blood in ventricle and from truncus blood flows simultaneously through the three pairs of arches to all parts of the body. This view is based on the fact that the blood received from skin and buccal cavity into sinus venosus and right auricle is in fact more oxygenated than that received from lungs into left auricle and the spiral valve in truncus is functionless in frogs. Studies by Delong indicate that the carotid arches receive highly oxygenated blood, the pulmocuataneous arches with least oxygen, and the systemic arches receive mixed blood. The precise mechanism is not understood.
Arterial system
It includes all the arteries in body. The arteries carry blood away from the heart. The arterial system in frog beings with the truncus arteriosus which arises from top right region of ventricle.The truncusascends, get curved towards left and divides into left and right branches or trunks, each of which subdivides into three major vessels or aortic arches: (1) Common carotid, (2) systemic and (3) pulmocutaneous. 
1. Common carotid arch: It is a short vessel running forward and outward, but soon divides into 2 branches: external and internal carotids.
a. External carotid: It is the smaller inner branch carrying blood to the tongue and adjacent parts. It is also called lingual.
b. Internal carotid: It is the larger outer branch. At its base it forms a little swelling, the carotid labyrinth. Its lumen is converted into a labyrinth by folding of the walls. It is probably a sense organ and controls blood pressure in the internal carotid artery. The internal carotid divides into 3 branches – a palatine to the roof of buccal cavity, a cerebral to the brain, and an ophthalmic to the eye.
2. Systemic arch: It is the longest of the three arches and with greatest distribution of blood. The two systemic arches curve dorsally around the oesophagus and join with each other behind the heart to form the dorsal aorta. In its course each systemic arch gives off 3 arteries:
a. Oesophageal: A small inner artery to oesophagus.
b. Occipito-vertebral: A larger outer branch which immediately divides into an occipital branch to occiput or posterior part of head, and a vertebral branch to vertebral column and spinal cord.
c. Subclavain: It is a large outer artery supplying the shoulder region and extending into the forelimb as brachial artery.
Dorsal aorta: As already said, it is formed by the union of both the systemic arches. It runs posteriorly lying mid-dorsally just beneath the vertebral column. It gives off the following arteries:
a. Coeliaco-mesenteric: It is a single large artery arising from the left side of the junction of the two systemic arches. It has two main branches: the coeliac to stomach, pancrease and liver; and the anterior mesenteric to spleen and small intestine.
b. Gonadial: A pair of short arteries to gonads, called spermatic in male frog and ovarian in female frog.
c. Renal: While passing between the two kidneys, dorsal aorta gives 5-6 pairs of small renal arteries in a series into both the kidneys.
d. Posterior mesenteric: It arises from the posterior end of dorsal aorta, or sometimes from anterior mesenteric. It goes to large intestine or rectum.
e. Common iliacs: The dorsal aorta finally bifurcates posteriorly into two common liliacs, each supplying an epigastric to ventral body wall, rectovesicular to rectum and urinary bladder and enters inside the hind limbs of own side and divides into femoral which supplies to hip and upper thigh, and sciatic which supplies to lower leg.
3. Pulmocutaneous arch: It divides into two main arteries, pulmonary to the lung and cutaneous to skin of dorsal and lateral sides.
Venous system
The venous system includes veins or those blood vessels which carry blood of body to the heart. In frog it consists of three types of veins: (1) Pulmonary veins, (2) Caval veins and (3) portal veins.
1. Pulmonary veins: Oxygenated blood from two lungs is collected by right and left pulmonary veins which unite to form a common pulmonary vein opening directly into the left auricle on the dorsal side.
2. Caval veins: Deoxygenated blood from rest of the body travels towards heart in three large vessels, two anterior precavals and single posterior postcaval, all the three opening into sinus venosus.
a. Anterior venacava or precavals: The right and left precavls or anterior venacava collect venous blood from the anterior part of body. Each precaval is formed by the union of 3 major veins which are:
i. External jugular: It is formed by the union of lingual form tongue and mandibular from outer margin of lower jaw.
ii. Innominate: It is formed by the union of internal jugular from cranial cavity and orbit and subscapular from shoulder and back of arm.
iii. Subclavian: It is formed by the union of brachial from forelimb and the musculo-cutaneous from muscles & skin of side of body and head.
b. Posterior venacava or postcaval: The single postcaval is a large, dark-coloured vein lying ventral to dorsal aorta. Its posterior end is formed between the two kidneys by the union of 5-6 pairs of renal veins collecting blood from kidneys. It also receives a pair of genital veins (spermatic in male and ovarian in female) from gonads. The postcaval then runs forwards, dorsally to the liver and receiving from it a pair of short hepatic veins, before opening into the posterior part of sinus venosus.
3. Portal Veins: A vein which collects blood from one organ of body and supplies blood to another organ instead of going to heart is called as portal vein. Thus, a portal vein is bicapillary vein. All veins associated with a portal vein forms portal system. Frog consists of two portal systems.
a. Renal Portal system: The veins which carry blood to a capillary system in kidneys constitute the renal portal system. Blood of each hind leg is collected by two veins, an outer fermoral and an inner sciatic. On entering the abdominal cavity the femoral divides into a dorsal renal portal and a ventral pelvic vein. The sciatic unites with the renal portal of its own side and while running along there outer border of kidney of its side it receives blood from lumbar region by a dorso-lumbar vein. Renal portal vein enters the kidney by several branches which break up into capillaries.
The importance of renal portal system is that kidney filters the blood and removes nitrogenous metabolic wastages from blood.
b. Hepatic portal system: It consists of hepatic portal vein and anterior abdominal vein.
A large hepatic portal vein is formed by the union of several branches from stomach, intestine, spleen, and pancreas. The veins which unite to form hepatic portal vein are:
• Gastric from stomach.
• Pancreatic from pancreas.
• Duodenal from duodenum.
• Anterior mesenteric from small intestine.
• Spleenic from spleen.
It carries blood of alimentary canal, laden with digested foodstuffs, to the liver into which it breaks up into capillaries. The liver converts excess glucose into glycogen and stores it as reserve food.
Anterior abdominal vein : The pelvic veins of both sides unite to form a median ventral or anterior abdominal vein. It receives blood from urinary bladder and ventral abdominal wall and runs forwards to enter liver into which it breaks up into capillaries. Before entering liver the anterior abdominal and hepatic portal veins are connected by a small loop.