Blood vascular system of Unio.

Blood vascular system of Unio.

The blood vascular system is well developed and is an open type. It consists of

(i) the heart and pericardium,

(ii) arteries

, (iii) sinuses,

(iv) veins.

(i) Heart and Pericardium.

It is situated in front of the posterior adductor muscle and placed the dorsal mid-line. It is found enclosed in a thin-walled triangular chamber called pericardium, which encloses the heart.The pericardium is lined by an epithelium and filled with the pericardial or coelomic fluid, represents a portion of coelom and communicates with the suprabranchial chambers through the kidney.The heart is highly contractile and consists of three chamber heaving one one ventricle and two auricles.The two auricle, lying one on either side of the ventricle , are transparent, thin-walled, highly distensible, triangular chambers one on either side of the ventricle. Each auricle is attached to the pericardial wall by the broad base and opens into the ventricle by an auriculo-ventricular aperture having valve opening towards the ventricle.The auricle receives blood from the gills, ctenidia, kidney, and mantle, and pour it into the ventricle. The single ventricle is a large, thick-walled, muscular and horizontal chamber, wrapped around the rectum.In most lamellibranch’s the ventricle has become folded around the rectum so that the pericardium encloses not only the heart but also a part of the alimentary canal.It pumps blood to the body. The muscular heart beats about 20-100 times per minutes. The circulation is rather slow but seems to be adequate for such a sedentary animal.

(ii) Arteries.

Each end of the ventricle pump blood, both forward and backwards, through two main arteries, known as anterior and posterior aortae, respectively. The anterior aorta run anteriorly dorsal to the intestine passes over the rectum and divides into three main branches.

• An anterior pallial artery which supplies branches to the mantle lobes,
• A pedal artery which supplies the foot and
• A visceral artery which supplies the various visceral organs through numerous branches, of which the main are a gastric branch to the stomach, a hepatic branch to the digestive gland, an intestinal branch to the intestine, and a gonadal or genital branch to the gonad.

The posterior aorta runs posteriorly ventral to the rectum.It sends out fine branches to the pericardium and kidney, a prominent rectal artery to the rectum and a posterior pallial artery to the mantle.

(iii) Sinuses.

The arteries break up into a network of the vessel called sinuses all over the body. The vessel instead of forming small capillaries, lead into irregular cavities, the blood sinuses and lacunae which lack the epithelium lining of a true blood vessel.There are no capillaries in molluscs, except in cephalopods, the blood from the arteries seeps into lacunar spaces in the connective tissue from where blood is taken up by the veins. Therefore, the Unio’s circulatory system is said to be an open type.

(iv) Veins.

The venous blood from the visceral organs is collected by smaller veins; gonadal vein from gonad, an intestinal vein from the intestine, hepatic vein from liver and gastric vein from the stomach. All these veins together form the visceral vein. The blood from the pedal sinus in the foot is collected by the pedal vein to form a long vein called vena cava.The vena cava lie longitudinally beneath the pericardium between the kidney. Blood from the vena cava goes to the kidney through afferent renal veins where nitrogenous waste is removed from the blood.From the kidneys, blood is collected by efferent renal veins which finally form a pair of afferent branchial or ctenidial veins which sends branches into the filaments of the gill. Blood is oxygenated in gill and goes into longitudinal efferent branchial or ctenidial vein which returns the blood to one of the auricles of the heart.But some blood from the vena cava and kidneys goes directly to the heart without going to the gills, hence, the heart also receives some deoxygenated blood. The blood which had gone to the mantle is purified,i.e., oxygenated and then returned to the other auricle of the heart by pallial veins.

The mantle also serves as an important respiratory organ and it sends aerated blood directly to the auricle through posterior pallial veins.

[II] Blood.

The blood consists of colourless plasma containing a respiratory pigment, called haemocyanin. A large number of colourless stellate amoebocytes or corpuscles, also referred to as leukocytes are found in plasma.It is the circulatory medium and consists of plasma and corpuscles. The plasma is colourless probably in all lamellibranch’s but in some species, it is slightly bluish in colour due to the presence of a respiratory pigment, the haemocyanin. Haemocyanin is a copper-containing respiratory pigment hence, imparts bluish in colour to the plasma in contrast to haemoglobin which is iron-containing respiratory pigments. However, some bivalves like Solen possess haemoglobin as respiratory pigments. The leucocytes are of granular as well as non-granular in nature.The blood distributes oxygen and nutrients to all parts of the body and transfers co2 and other waste products of metabolism to the gills, mantle and kidney.

A course of circulation of blood.

Blood, from the heart, goes to the anterior and posterior parts of the body through anterior and posterior aortae.Through these aortae, a part of blood goes to the mantle where oxygenation occurs and finally oxygenated blood is conveyed to the auricle through a pallial vein. The other parts of the body are supplied by different branches of these aortae here it becomes deoxygenated.The deoxygenated blood is finally collected into vena cava through several veins.The vena cava gives blood to the kidney where nitrogenous wastes have removed the blood then goes to the gills for oxygenation. The oxygenated blood from the gills is conveyed to the heart by efferent branchial or ctenidia veins. Thus, circulation is completed and the heart again starts distributing the blood to the various organs. In this way, the cycle goes on.The blood is kept by travelling in the proper direction by means of valves which are present in the heart and the veins.

Reference.

Aggarwal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L..,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

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