The blood is the circulating fluid connective tissue. It is the system of blood, heart and blood vessels. The main function of this system is to distribute the digested food and oxygen to the different parts of the body, in order to release energy to carry out life activities. The circulatory media in frog is the blood and lymph which is divided into two parts
It consists of blood, heart and blood vessels. Blood is a liquid connective tissue that is opaque, sticky and red in colour due to the presence of haemoglobin in RBC. It consists of plasma, which is non- living and constitute 60% of the total blood. Mostly it contains water and few plasma proteins, organic and inorganic substances.
They are living cells as they are capable of undergoing metabolic changes. These are three types
RBCs or erythrocytes: They are also called red blood cells, which are oval shaped, nucleated and biconvex in a frog. They contain haemoglobin that is made up of iron and proteins. It combines with oxygen forming oxyhaemoglobin that breaks up into oxygen and haemoglobin in the tissue. It is a respiratory pigment. RBC are formed in liver, spleen, kidney, and in bone marrow when a frog is very active during rainy seasons. The lifespan of RBCs is 100to 120 days and its numbers are nearly 4-6 millions per cubic mm of blood.
WBCs or leukocytes: These are white blood corpuscles without haemoglobin and are small, nucleated and irregular shaped. They have short life span and are less in number then RBCs. They can change their shape and move independently by amoeboid movement through intracellular spaces among the tissues. They act as a scavenger, soldier and are phagocytes. They are formed in the liver, spleen, and yellow bone marrow in the frog. Depending upon the presence and absence of granules, they are divided into granulocytes, which forms 70% of the total leucocytes, and agranulocytes that form 30% of total leucocytes.
Heart of frog is dark red coloured, conical muscular organ situated ventrally to the liver in the pericardial cavity, which is enclosed in two membranes, inner epicardium and outer pericardium. Pericardial fluid is found between this membrane. It protects the heart from external injury, keeps heart moist and provides the frictionless motion of the heart.
External structure of heart
The heart is a triangular structure with the broad anterior part called auricles or atrium and narrow posterior part called ventricle which is separated by the coronary sulcus. Auricle is divided into left and right auricle by inter- auricular septum. Auricles are thick walled. The ventricle is the most conspicuous and prominent part of the heart. The heart is three chambered with two auricles and one ventricle.
On the dorsal surface, there is a thin walled dark triangular structure called sinus venous. It opens into right auricles. The right precaval left precaval and post caval veins open into sinus venous. From the right side of the ventral surface of the ventricle, a tubular structure arised called truncus arteries which divide into two parts called aortic trunks.
Internal structure of heart
Internally heart consist of three layers, which are outer epicardium, middle myocardium, and inner endocardium. The heart is three chambered with two auricles and one ventricle. The two auricles are thin walled and remain separated from each other by a thin vertical inter-auricular septum. Right auricle is larger than left one as it receives blood from different parts of the body.
The sinus venous opens into the dorsal wall of right auricle by a large oval aperture, sinu-auricular aperture. It lies at middle close to the inter-auricular septum and remains guarded by a pair of flap like values. The common pulmonary veins open into left auricle, near inter-auricular septum, by a small opening without valves. Both auricles open into ventricle by a common large auriculo-ventricular aperture guarded by two pairs of flap like auriculo-ventricular values. The ventricle is thick-walled, muscular and spongy in nature. Its inner surface has irregular ridges or folds called the columnae carneae with depressions, which is also known as fissures. These folds reduce cavity of the ventricle. The flaps of auriculo-ventricular values remain connected with the wall of the ventricle by a thread like chordae tending.
The opening of ventricle into truncus anterior is guarded by three semilunar values, which prevent the backflow of blood from truncus arteries into the ventricle. The spirally twisted cavity of truncus anterior remains divided by three semilunar valves into a long thick walled proximal conus arteriosus or pylangium and a short distal, thin walled bulbous arteries or synangium. A longitudinal spiral valve is present on pylangium which remains attached dorsally while free ventrally. It divides cavity of pylangium into left dorsal cavum pulmocutaneous and right ventral cavum aorticum. The common opening of two pulmocutaneous arches lies in cavum pulmocutaneous while separate openings of carotid and systemic arches lie in synangium.
Heart constantly beats during lifetime under nervous control to pump blood into blood vessels. Contraction of heart is called systole while relaxation is called as diastole. When sinus venous contracts, it's non-oxygenated blood passes into right auricle through the sinu-auricular aperture the blood from lungs come into left auricle through pulmonary vein. The two auricles now contract almost simultaneously forcing their blood into the ventricle through the auriculo-ventricular aperture.
According to the old view, the ventricle contains only deoxygenated blood on 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 lines of blood could not mix to any great extent because of their viscous nature and also because of the network of columnae carneae.
When ventricle starts contraction, first the deoxygenated blood from the right side being nearer flows into the truncus arteries and directed by the spiral valve into the common opening of pulmocutaneous arches and carried to lungs and skin for oxygenation. Spiral valve now closes the opening of pulmocutaneous arches. Next follows the mixed blood which is pushed through cavum aortic into the systemic arches and is sent to the body and limbs. Finally, enters the oxygenated blood of left side and directed through carotid arches to the head. Thus, according to the old view, the spiral value in truncus arteries plays an important role in directing blood into different arches.
Recent experimental studies conducted by Vandervael and Foxon show that it is actually completely mixed blood in ventricle and truncus which flows simultaneously through the three pairs of arches to all parts of the body. The blood received from the skin and the buccal cavity into sinus venous and right auricle is equally oxygenated to the blood received from lungs to left auricle.
According to this view, the inter-auricular septum and spiral value in truncus arteries have become functionless in frogs.
Blood vessels are the system of pipelines through which blood flows. Its wall is made up of three layers, outermost layer tunica externa, middle layer tunica media, and innermost layer tunica interna. Blood vessels are arteries, capillaries, and veins.
Arteries: Strong and thick walled vessels as they carry blood away from the heart to the different parts of the body under high pressure with great speed. They do not have valves.They are situated deeply. They appear pinkish in colour.
Capillaries: The structure of capillaries depends on upon its function and are found abundantly in those tissues and organs where excessive metabolism occurs, but absent or fewer in number where there is no metabolism. They are thin walled with only tunic intima but without tunica media and tunica external as they bring blood into the closed possible relation with the surrounding tissues and are so thin that an exchange of food materials, gases and waste substances take place between blood and the tissue under pressure.
Veins: They are thin walled and collapsible which are provided with semilunar valves that prevent the backflow of the blood. Blood flow through the veins under very low pressure. Onward flow in veins is maintained by semilunar valves, contraction of muscles of the body and y the sucking activity of the heart. Structurally, veins are like artery but with the thin and more elastic wall than the artery. Their lumen is large. They carry impure or deoxygenated blood except h those bringing blood from the respiratory organs i.e. lungs.
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