In this article we will discuss about the circulatory system of toad with the help of diagram.
Circulatory or vascular system is the transport system by which materials such as food, oxygen and wastes are carried from one part of the body to other parts. The conveying medium is a liquid called blood. This is forced by a central pumping organ, the heart, through channels called blood vessels.
The large blood vessels carrying blood away from the heart are the arteries. These break up into smaller branches as they pass farther away from the heart.
The smallest branches penetrate into the tissues of the body and break up into very fine, thin-walled channels called capillaries (capillus = hair). The capillaries unite with one another to form a close network surrounding the tissue cells.
The blood is collected from the other end of the network by another kind of vessels called veins. A vein receives other veins as tributaries, and thus increasing in size the veins bring the blood back to the heart.
Although the blood circulates through the tissues, yet it is not liberated into them because it is conveyed through a closed system of vessels. Only the liquid part of the blood carrying the material to be transported, passes out through the thin walls of the capillaries into the intercellular spaces.
This fluid is known as the lymph. It is collected by a special kind of vessels called lymphatics which ultimately return the lymph to the veins. The total volume of the blood is restored in this way (Fig. 25).
Composition of Circulatory System of Toad:
Thus the circulatory system is actually made up of two separate systems:
1. The blood vascular system and
2. The lymphatic system.
The arteries carrying blood away from the heart, and the veins carrying the same towards the heart make up the blood vascular system.
The lymphatics carrying the lymph away from the tissue spaces constitute the lymphatic system. The arteries and veins differ net only in function, but they also differ in structure.
The wall of an artery consists of three layers:
(i) The outermost layer is composed of fibrous tissue and is, therefore, known as the fibrous coat or tunica adventitia;
(ii) The middle or muscular coat consists of involuntary muscle fibres running round the vessel; this is the tunica media;
(iii) The innermost layer forms the tunica internal it is composed of elastic fibres and endothelium which make up the internal lining membrane of the artery.
A vein has all the three coats; its fibrous coat is thicker and the muscular coat is thinner. Moreover, a vein has no elastic fibre in the tunica interna. Thus a vein has a delicate, thin and non-elastic wall, whereas an artery has a tough, thick and elastic wall.
The lumen of a vein is larger in diameter than the lumen of the corresponding artery. There are valves inside larger veins for regulating and directing the blood streams towards the heart.
1. Blood-Vascular System:
The blood-vascular system has three main components:
(a) The blood,
(b) The heart, and
(c) The blood vessels, i.e. arteries and
The blood is a reddish opaque liquid flowing through the blood vessels. It consists of a straw-coloured ground substance called blood plasma and solid cells called blood corpuscles. The corpuscles are suspended in the plasma.
The blood plasma is mostly watery. Many substances are found in solution in the blood plasma. These include mineral substances, food, waste products, hormones, gases, etc. It is the chief transporting medium.
The corpuscles are of three kinds:
(a) Red blood cells or erythrocytes (erythros—red),
(b) White blood cells or leucocytes (leucos=white), and
(c) Blood platelets or thrombocytes.
The erythrocytes are oval, biconvex, nucleated cells about 15 to 20 micro in size (one micron = 1/1000 mm.). They carry the colouring matter of the blood, an iron-containing protein called haemoglobin.
The haemoglobin has a remarkable affinity for oxygen and is, therefore, useful in the transport of this gas during respiration. There are about 4 to 5 lakhs of erythrocytes in a cubic millimetre of blood which serve for transporting oxygen. The leucocytes are less numerous than the erythrocytes, numbering about 4 to 5 thousands in a cu. mm. of blood.
They are small, colour-less nucleated cells capable of changing their shape like an amoeba. They may creep out through the thin-walled capillaries to engulf and remove foreign bodies from the tissues. Thus the leucocytes are the scavengers of the body.
They destroy bacteria and protect the toad against invading microbes. Moreover, they can ingest fat globules from the small intestine and carry them into the circulating blood.
Leucocytes, therefore, are scavengers, soldiers and porters of the body. The thrombocytes are small spindle-shaped nucleated cells which are often designated as the blood platelets. They break down when the blood is shed, and release an enzyme which helps in the clotting or coagulation of blood.
Further bleeding is thus prevented when a blood vessels is injured. In the adult toad, the blood corpuscles are mainly manufactured by the bone-marrow.
The heart is the central pumping station for the circulation of blood. It is a hollow, pear-shaped, muscular organ which is situated in the anterior part of the body cavity, in front of the liver. It is completely enclosed in a transparent bag of membrane, the pericardium. The broad base of the heart is directed forwards, whereas the narrow apex points towards the posterior end and lies between the two main lobes of the liver.
The heart is mainly composed of three chambers: a thick-walled, conical ventricle and two thin-walled auricles, right and left. There are two other smaller chambers: a thin-walled triangular sinus venosus on the dorsal side, opening into the right auricle, and a thick-walled tubular conus arteriosus ventrally, connected to the base of the ventricle.
The sinus venosus is a thin-walled sac-like chamber which is situated on the dorsal side of the heart. It is more or less triangular in shape with the three caval veins opening into its three corners. These veins carry deoxygenated blood into the sinus.
It communicates with the right auricle through a slit-like sinuauricular aperture, the edges of which are guarded by the sinuauricular valves. The valves permit entry of blood from the sinus into the right auricle, but no backflow or regurgitation is allowed.
The two auricles, right and left, form the base of the heart. They are separated from the ventricle by a narrow groove, the coronary sulcus. The auricles are completely separated from one another by a strong vertical partition, the inter auricular septum. The right auricle is larger than the left; it receives deoxygenated blood from the sinus venosus through the sinuauricular aperture.
The left auricle receives oxygenated blood from the lungs through a small opening, the aperture of the common pulmonary vein. The two auricles communicate with the ventricle by a common opening, the auriculo-ventricular aperture, which is guarded by the auriculo- ventricular valves.
The membranous cusps of this valve hang down like curtains to allow the passage of blood from the auricles into the ventricle, but not in a reverse direction. The free edges of the cusps are attached to the ventricular wall by means of a number of fine threads called chordae tendineae.
The ventricle is the thick-walled conical chamber which is situated behind the auricles. Its posterior bluntly pointed portion forms the apex of the heart. The ventricular cavity is greatly reduced by a number of interlacing muscle fibres which arise from its own wall.
The ventricle, therefore, is spongy and when it is cut, its interior looks like a honeycomb. The sponginess of the ventricle prevents admixture of the two kinds of blood in the ventricular cavity, the oxygenated kind from the left auricle and the deoxygenated kind from the right auricle.
The conus arteriosus is the stout tube which arises ventrally from the base of the ventricle and passes obliquely towards the left. It is continued forwards as the truncus arteriosus which is the base of the main artery for carrying the blood away. The truncus however is not a part of the heart; it belongs to the arterial system. The conus is separated from the truncus by a set of pocket-like semilunar valves.
A similar set of three semilunar valves guards the opening between the ventricle and the conus. These valves prevent backflow into the ventricle. Inside the conus is a twisted, longitudinal flap, the spiral valve. This divides the cavity of the conus into a right channel, the cavum aorticum, and a left channel, the cavum pulmocutaneum.
The spiral valve, the semilunar valves and the spongy ventricular cavity co-operate with one another in guiding the two kinds of blood. The deoxygenated blood is pumped through the cavum pulmocutaneum and the oxygenated kind through the cavum aorticum. The two kinds of blood enter different arterial arches and are carried to different places.
1. Arterial System:
The arteries and their branches form the arterial system. The truncus arteriosus is the main artery originating from the conus. It at once divides into right and left branches.
Each of these trunks splits into three arterial arches:
(i) The carotid anteriorly,
(ii) The systemic in the middle, and
(iii) The pulmocutaneous posteriorly.
Thus, there are three pairs of arterial arches for supplying blood to different parts of the body. The carotids supply the head region, the systemics supply the trunk and limbs, and the two pulmocutaneous supply the lungs and skin.
(i) Carotid Arches and their Branches:
The carotid arch, on each side, proceeds outward and forward. It soon bifurcates into an inner branch called external carotid artery and an outer branch, the internal carotid artery. Immediately near the point of bifurcation is a small swelling called carotid labyrinth. Inside the labyrinth, the carotid arch breaks up into numerous minute vessels which reunite at the other end.
The carotid labyrinth offers an extra resistance to the passage of blood through the carotid arch. The external carotid artery supplies blood to the floor of the buccal cavity, tongue and outer side of the head.
The internal carotid artery turns backward and comes very close to the systemic arch of the same side, to which it is tied by a small amount of fibrous tissue, the carotid ligament. Finally, it turns forward to enter the skull through a foramen and is distributed to the brain and its coverings.
(ii) Systemic Arches and their Branches:
The systemic arch sweeps outward. It then curves round the oesophagus to reach the dorsal side, where it joins with its fellow of the opposite side to form a median artery called dorsal aorta. Thus, the two systemics form an arterial ring round the oesophagus.
Each systemic gives out the following branches:
(i) A short laryngeal artery to supply the voice-box;
(ii) An occipito-vertebral artery which breaks up into oraches for supplying the pharynx, the back of the head, the vertebral column and the spinal cord;
(iii) A stout subclavian artery which proceeds outward to supply the shoulder and forelimb of the same side;
(iv) the left systemic gives off an additional twig, the oesophageal artery, to the oesophagus. The right systemic has no oesophageal branch.
The dorsal aorta is formed in the mid-dorsal line by the union of the right and left systemic arches. It extends backward, lying in front of the vertebral column, and terminates at the posterior end of the body cavity by dividing into two iliac arteries. A stout coeliacomesenteric artery is given out from the commencement of the dorsal aorta.
This breaks up into a coeliac branch for supplying the stomach, pancreas, liver and gall-bladder, and a mesenteric branch for supplying the intestines, cloaca, spleen and mesentery. In passing through the space between the kidneys, the dorsal aorta gives off four to five pairs of renal arteries for supplying the urinogenital organs.
The iliac arteries are the terminal branches of the dorsal aorta. Each iliac gives off art epigastricovesical branch to supply the urinary bladder and the ventral body-wall. After this the iliac artery enters the hind limb of the same side where it divides into femoral and sciatic branches.
(iii) Pulmocutaneous Arches and their Branches:
Two pulmocutaneous arches carry deoxygenated blood to the lungs and skin for aeration. They are the hindermost and the shortest of the arterial arches. Each arch passes outward and then backward. A very slender branch is given out to supply the skin. This is the cutaneous artery. The main trunk then runs into and supplies the lung of the same side as the pulmonary artery.
2. Venous System:
The veins and their tributaries constitute the venous system. The arteries break up into smaller and smaller branches, and these branches merge into a network of thin-walled, hair-like vessels, called capillaries. The capillaries merge into small veins, which join to form larger veins.
The venous system of toad may be subdivided into three separate groups:
(i) The pulmonary,
(ii) The systemic, and
(iii) The portal.
(i) Pulmonary Veins:
Two pulmonary veins carry oxygenated blood from right and left lungs. The right and left pulmonary veins unite to form a common pulmonary vein, which opens into the left auricle on the dorsal side.
(ii) Systemic Veins:
These are represented by the three large veins or venae cavae draining into the corners of the triangular sinus venosus. They carry deoxygenated blood from all parts of the body except lungs. Two of the venae cavae are situated anteriorly; these are the right and left precavals. A single postcaval is found posteriorly, opening into the apex of the sinus venosus.
Each precaval vein is formed by the union of three tributaries:
(a) External jugular,
(b) Innominate, and
(a) The external jugular vein is formed by the union of two tributaries—a lingual vein bringing blood from the tongue, and a faciomandibular vein from the jaws and snout.
(b) The innominate vein is also formed by the union of two tributaries—an internal jugular from the interior of the skull, and a subscapular from the back of the shoulder.
(c) The subclavian vein is similarly formed by the union of two veins—a brachial from the forelimb and a musculocutaneous from the skin and muscles. In toad, the skin is an accessory respiratory organ. Hence the blood carried by the musculocutaneous vein is comparatively rich in oxygen.
The postcaval vein has its roots in the kidneys. It is formed by the union of four to five pairs of renal veins which collect blood from the kidneys. The genital veins from the reproductive organs drain into some of the renal veins. The postcaval runs forward to enter the liver substance, and receives a pair of hepatic veins one from each lobe of the liver. It finally terminates by joining the posterior end of the sinus venosus.
(iii) Portal Veins:
A portal vein begins in capillaries and ends in capillaries before the blood which travels through it is returned to the heart.
There are two portal systems in the toad:
(A) Hepatic portal system, and
(B) Renal portal system.
In the hepatic portal system, venous blood collected from capillaries at the posterior part and from the gut filters through capillaries in the liver on its way back to the heart. In the renal portal system, venous blood collected from capillaries in the posterior part of the body passes through capillaries in the kidneys on its way back to the heart.
(A) Hepatic Portal System.
In this, the two principal veins are the hepatic portal vein and the anterior abdominal vein. The hepatic portal vein is formed by the union of several small veins returning blood from the stomach, intestines, pancreas, spleen, etc.
The main trunk of the hepatic portal comes to the under surface of the liver, where it receives the anterior abdominal vein. Thus enlarged, it enters the liver substance and breaks up into hair-like sinusoids which finally drain into the hepatic veins.
The anterior abdominal Vein is formed in the following manner. Blood from the hind limb is returned by two large veins, the femoral and the sciatic. On entering the body cavity, the femoral vein gives off a median branch called pelvic vein.
The right and left pelvic veins unite in the mid-ventral line to form the anterior abdominal vein, which runs forward along the middle line under cover of the ventral body-wall. It receives several small tributaries from the ventral body-wall and from the urinary bladder.
At the level of the liver, it turns inward to join the hepatic portal vein. The conjoined trunk then divides into vessels which enter the lobes of the liver and break up into sinusoids. The hepatic veins originate at the other end of these networks and thus the blood is finally drained into the postcaval.
(B) Renal Portal System:
Blood is returned from the hind limbs by femoral and sciatic veins. The femoral vein comes from the front of the thigh. On entering the body cavity, it gives off the median pelvic branch which joins with its fellow of the opposite side to form the anterior abdominal vein. The main trunk of the femoral now receives the sciatic vein from the back of the thigh and proceeds towards the kidney as the renal portal vein.
At the outer margin of the kidney, the renal portal is joined by two or three dorsolumbar veins bringing blood from the body-wall. It now enters the kidney and breaks up into sinusoids which ultimately drain into the renal veins, thus returning the blood into the postcaval.
It is to be noted that the postcaval vein of the toad is the product of the renal portal and hepatic portal system, and the anterior abdominal vein is joining the two portal systems.
Circulation of Blood:
The heart continues to beat, taking short rest between successive contractions, and drives the blood into the arteries. The heart muscles have an innate tendency to contract and relax with a definite rhythm. Each period of contraction or systole is followed spontaneously by a shorter period of relaxation or diastole.
The contraction starts at the sinus venosus, spreads to the auricles, then to the ventricle, and finally to the conus arteriosus. The heart of a freshly killed toad beats rhythmically for a long time even when it is taken out.
During diastolic period, the sinus venosus receives deoxygenated blood from two precavals and the single postcaval vein. The left auricle, at the same time, is filled with oxygenated blood through the common pulmonary vein.
With the commencement of the systole, the sinus contracts and the deoxygenated blood is pumped into the right auricle, through the sinuauricular aperture. This is followed by the auricular systole. The two auricles contract simultaneously driving their contents into the single ventricle, through the auriculo-ventricular aperture.
Regurgitation, of deoxygenated blood into the sinus venosus is prevented by the closure of the sinuauricular valves.
Thus both kinds of blood enter the ventricle at the same time. As the ventricular cavity is spongy, there is not much mixing. During the very short stay in the ventricle the deoxygenated blood is stored on the right side, the oxygenated blood on the left side, and there is a mixed column of blood in the middle of the ventricular cavity.
The ventricle now contracts. Blood cannot flow back into the auricles because the auriculo-ventricular aperture is tightly closed by the auriculo-ventricular valves. Since the conus arises from the right side, the first blood to enter the conus will be the deoxygenated kind in the right side of the ventricle.
This is now guided by the spiral valve into the cavum pulmocutaneum and from there, through the pulmocutaneous arches, to the lungs and skin. Moreover, the pulmocutaneous arches are the shortest of the arterial arches.
They, therefore, offer least resistance to the entry of deoxygenated blood into them at the very beginning of the ventricular systole. The deoxygenated blood is aerated in the lungs and brought back to the left auricle through the pulmonary veins—thus completing the ‘pulmonary circuit’.
The portion of blood, which becomes oxygenated in the skin, is returned to the sinus venosus through the musculocutaneous vein. So this portion of oxygenated blood is mixed up with the deoxygenated blood stream before it is returned back to the heart.
As the force of the ventricular contraction is increased, the mixed blood in the middle of the ventricular cavity is pumped through the cavum aorticum into the systemic arches. This mixed blood cannot enter the carotid arches because of the high resistance offered by the carotid labyrinths. Thus the mixed blood is distributed to the trunk and limbs.
Lastly, the pressure exerted by the carotid labyrinths is overcome and the oxygenated blood in the left side of the ventricle is now forced into the carotid arches, through the cavum aorticum.
It is distributed to the head region of the animal. It is to be clearly understood that the spiral valve directs the various courses of the three kinds of blood streams. During ventricular diastole, the blood cannot flow back from the arterial arches because of the pocket-like semilunar valves at the root of the conus.
The systemic and carotid arches take the blood away to every part of the body, except lungs and skin. Food and oxygen are carried to the tissues in this way. Exchange of materials occurs by osmosis through the thin wall of the capillary networks.
The blood is finally brought back to the heart by the three caval veins—thus completing the ‘systemic circuit’. While returning, the blood collects carbon dioxide and nitrogenous waste products from the tissues.
Further, the part of the blood stream which is returning from the alimentary canal becomes loaded with absorbed food. The carbon dioxide is mixed up with the general stream of systemic circuit and eliminated by the lungs and skin.
Blood rich in nitrogenous waste products is directed through the renal portal system into the kidneys for exertion. Lastly, the blood rich in food materials is diverted to the liver via the hepatic portal system. The liver cells are thus provided with a better chance of extracting food and storing the same for future use.
Vandervael and Foxon have experimentally demonstrated that there ii no separation of blood streams in the frog’s heart and the blood flowing out through the main arterial arches is nearly of the same composition. They injected a radio-opaque substance called thorotrast into the sinus venosus of a frog.
Subsequently a series of X-ray photographs were taken, which revealed that the opaque substance was uniformly distributed in all the three arterial arches. Similar results were obtained by injecting thorotrast through the common pulmonary vein.
It appears that there is no separation of the oxygenated and deoxygenated blood streams in the heart of frogs and toads. As aeration of blood occurs through the skin and mouth, the blood in the sinus venosus cannot be completely deoxygenated, because the cutaneous veins open into it.
Although a separation of the oxygenated and deoxygenated streams would be far better and more efficient, toads and frogs can somehow manage without it.
2. Lymphatic System:
The blood is not directly poured into the tissues. While circulating through capillary networks, the blood plasma exudes into the intercellular spaces, coming into direct contact with the cells. This fluid is known as the lymph. It is a colourless liquid containing a few leucocytes, but no erythrocyte. The lymph is collected from the intercellular spaces by small vessels called lymphatics.
These drain into lymph sacs. Some of the lymph vessels are contractile and are known as lymph-hearts. The lymph-hearts pump the lymph back into the veins. There are a pair of lymph-hearts near the urostyle, opening into the femoral veins; another pair, below the scapulae, drain into the subscapular veins.
The lymph transports food and oxygen from the blood to the cells of the body. Waste products are carried by the lymph into the blood stream.