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Circulatory Pathways

The circulatory patterns are of two types - open or closed.Open circulatory system is present in arthropods and molluscs in which blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. 

Annelids and chordates have a dosed circulatory system in which the blood pumped by the heart is always circulated through a closed network of blood vessels. This pattern is considered to be more advantageous as the flow of fluid can be more precisely regulated.

All vertebrates possess a: muscular chambered heart.

Fishes have a 2-chambered heart with an atrium and it ventricle.

Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle, whereas crocodiles, birds and mammals possess a 4-chambered heart with two atria and two ventricles.

In fishes the heart pumps out deoxygenated blood which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart (single circulation).

In amphibians and reptiles, the left atrium receives oxygenated blood from the gills/lungs/skin and the right atrium gets the deoxygenated blood from -other body parts. However, they get mixed up in the single ventricle which pumps out mixed blood (incomplete double circulation).

(A) Open Circylatory System and (b) Closed Circulatory System

In birds and mammals, oxygenated and deoxygenated blood received by the left and right atria respectively passes on to the ventricles of the same sides. The ventricles pump it out without any mixing up, i.e., two separate circulatory pathways are present in these organisms, hence, these animals 'have double circulation.

1. Heart of vertebrates:

Pisces (=Branchial heart),

Cyclostomata: ( Labeo,Scoliodon)

Thick, muscular, made of cardiac muscles, has two chambers (i) auricle and (ii) ventricle. The heart is called venous heart since it pumps deoxygenated blood to gills for oxygenation. This blood goes directly from gills to visceral organs (single circuit circulation). A sinus venosus and conus arteriosus is present. Lung fishes have only one auricles and one ventricle. 
Heart of vertebrates

2. Amphibians, (Lung fish, Frog, Toad, Neoceratodus or Dipnoi) Heart-consists of:

(i) Two auricles

(ii) Undivided ventricle

(iii) Sinus venosus

(iv) Truncus arteriosus

(conus + proximal part of aorta) Right auricle receives blood from all the ·visceral organs (deoxygenated) via precaval and post caval. Pulmonary artery carries deoxygenated blood to lungs for oxygenation. This blood returns to left auricle via pulmonary vein (Double circuit circulation)

(v) S.A. node insinus venosus

(vi) Trunchus arteriosus divided into synangium, pylangium.
Heart of Amphibians

3. Reptiles (Lizards, Snakes, Turtles):

Heart consists of:

(i) Left and right auricle

(ii) Incompletely divided ventricle (Ventricle in crocodiles, gavialis, and alligator is completely divided)

(iii) Sinus venosus

(iv) Conus arteriosus divided into right systemic, left systemic and pulmonary arch. (Double circulation)

(v) Foramen panizzae at crossing of right-left systemic arch.

(vi) Only SA node in right auricle 

Heart of Reptiles

4. Aves (Pigeon)

Exhibit double circulation:

Heart consists of

(i) Left and right auricle

(ii) Left and right ventricle

(iii) Complete separation of arterial and venous circulation

(iv) Only right systemic arch is present

(v) Sinus venosus and truncus, arteriosus absent

(vi) Two pace maker SA node and AV node

(vii) Mitral valve present.

Heart of Aves

5. Mammals (Rabbit, man): Same as bird except that mammals have left systemic arch.

Human Heart

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