Introduction : This system is concerned with the circulation of body fluids to distribute various substances to various body parts.

  Functions of Circulatory System.                                                                                                                    

• Transport of various substances such as nutrients, waste products, respiratory gases, metabolic intermediates (Such as lactic acid from muscle to liver), vitamins hormones etc.
• Regulation of body pH by means of buffer, body temperature homeostasis, water balance etc.
• Prevention of disease by means of antibodies and antitoxins.
• Support or turgidity to certain organs like penis and nipples.

  Types of Circulation.                                                                                                                                           

Circulatory system in various groups of animals can be classified as follows :

1. Intracellular circulation : Occurs inside the individual cells where the distribution of substances is through cyclosis of cell cytoplasm. Example – Protozoans.
2. Extracellular circulation : When the distribution of the substances occurs inside the body through extracellular or intracellular fluids. This is of following types –

1. Extra organismic circulation : When the water of the external environment circulate through body. This is also called as water circulation system. Example – canal system in porifera, water vascular system in Echinoderms and gastrovascular system in coelenterates.
2. Intra-organismic circulation : It involves circulation of body fluids. It is of following types :
   1. Parenchymal circulation : In platyhelminthes, the fluid filled spaces present in the mesodermal parenchyma tissue between body wall and internal organs are used in the distribution of substances.
   2. Coelomic circulation : Coelomic fluid is concerned with the transport of substances. Example – pseudocoelomic fluid in the roundworms and haemolymph in Arthropods.
   3. Blood vascular system : It contains blood and a pumping structure (heart) for circulation of materials inside the body. It is of following types –

1. Open circulatory system
2. Closed circulatory system

Differences between open and closed circulatory system

  Circulatory system in multicellular animals                                                                                               

1. In protozoans : Distribution of nutrients takes place by cyclosis (streaming movement) of cytoplasm.
2. In poriferans : The vascular system of poriferans is the canal system. A simplest canal system involves ostia (mouth), spongocoel and on osculum (Anus).

Route followed by water current in sponges :

Outside     through

Dermal ostia

Incurrent canals

through prosopyles

Radial canals

through apopyles

through

Osculum

Spongocoel

through

gastric ostia

Excurrent canals

3. In coelenterates : Hydra has a single large internal cavity called coelenteron or gastrovascular cavity. It has single opening the mouth. It also extends into the hollow tentacles. It lacks a mesodermal epithelial covering (peritoneum) and a coelomic fluid. It is concerned with first extracellular and then intracellular digestion of food.
4. In platyhelminthes : Vascular system is absent but circulation occurs with the help of parenchyma hence called parenchymal circulation. Example – Fasciola hepatica.
5. In annelids : Vascular system in annelids is a closed circulatory or blood vascular system which comprises four parts : blood, blood glands, blood vessels and hearts.

1. Blood : Red, due to respiratory pigment haemoglobin dissolved in plasma. The blood cells are colourless and nucleated like the leucocytes of vertebrates.
2. Blood glands : Reddish bodies present on alimentary canal in segments 4, 5 and 6 and are thought to produce blood corpuscles and haemoglobin.
3. Blood vessels : Lack endothelium. The arrangement of blood vessels in first 13 segments is different from that is rest of the body. I^st 13 segments have five longitudinal vessels – dorsal, ventral, a pair of lateral oesophageal and a supraoesophageal vessel. Behind 13^th segment has 3 longitudinal vessels – dorsal, ventral and subneural.
4. Hearts : Four pairs, one pair in each of 7^th, 9^th, 12^th and 13^th segment. Two anterior pairs receiving blood from dorsal vessel only are called lateral hearts and two posterior pairs receiving blood from dorsal vessel as well as

supra oesophageal vessel are called latero-oesophageal hearts. All hearts possess muscular pulsafile walls to pump blood into ventral vessel. Valves present to prevent back flow of blood.

• Anterior loops : 2 pairs, one pair in each of the 10^th and 11^th segment, carry blood from lateral oesophageal vessel to supra oesophageal vessel.
• Lymph glands : Two, small, whitish, located on each side of the dorsal blood vessel in each of the segments 26 and those behind it. They produce phagocytic cells which are occasionally liberated into the coelomic fluid to phagocytise harmful bacteria and other invaders.

Circulation in earthworm :

Behind 13^th segment

Subneural vessel (from body wall)

I^st 13 segments Dorsal vessel

Lateral hearts

Lateral oesophageals

Anterior loops Supra oesophageal

Ventral vessel : Distributing vessel. Subneural vessel : Collecting vessel. Lateral oesophageal : Collecting vessel. Supra oesophageal : Collecting vessel.

Ventral vessel

Latero oesophageal hearts

Dorsal vessel : Distributing in 1^st 13 segments and collecting in the region behind 13^th segment.

6. In arthropoda : Blood vascular system in arthropods is ‘Open type’ or “Lacunar type”.

1. Prawn : Blood vascular system of prawn includes pericardium, heart, arteries, blood lacunar/sinuses, blood channels and blood. Blood is colourless with phagocytic leucocytes and respiratory pigment haemocyanin dissolved in plasma. The blood has remarkable clotting properties. Heart is a muscular, triangular organ without auricle and ventricle but with ostia for inflow of blood into heart and arteries. Five of the arteries arise from the anterior end and one from the posterior end of heart. The prawn’s heart always contains oxygenated blood only and hence, also called as Arterial heart.

Circulation of blood in prawn :

Pumping of oxgenated blood by heart into arteries

Arteries open into blood sinuses and lacunae of haemocoel

Heart collects the oxygenated blood through a slit-like opening in its cardiac wall

Exchange of nutrients, gases and excretory wastes between blood in lacunae and sinuses and surrounding tissues

Oxygenated blood from gills returns to pericardial sinus

2. Cockroach : Cockroach has an open circulatory system. The body cavity is called

Oxygenation of blood in gills

CHAMBERS OF HEART

VALVES

DORSAL

DIAPHRAGM

Deoxygenated blood seeps out into ventral sinuses to be carried to the gills

OSTIA

ALARAY MUSCLES

ANTENNA

haemocoel filled with a fluid haemolymph. The heart lies in the pericardial sinus of haemocoel. The heart is 13 chambered, tubular dorsal vessel, pulsatile with an anterior aorta.

PERICARDIAL SINUS

VENTRAL DIAPHRAGM

PERIVISCERAL SINUS(HAEMOCOEL) PERINEURAL

SINUS

NERVE CORD

HEAD

NECK

PULSATORY AMPULLA

Each chamber is inverted funnel, shaped   provided        with valved

Fig. – Blood vascular system of cockroach (Diagrammatic)

lateral apertures called ostia. The heart is supported by 13 pairs of triangular fan like alary muscles. The blood sinuses are dorsal pericardial sinus, middle perivisceral sinus and the ventral perinural sinus or sternal sinus. These sinuses are separated from each other by dorsal diaphragm and ventral diaphragm. Blood or haemolymph is colourless contains haemcytes but is without respiratory pigment as it plays no role in respiration.

Circulation of blood in cockroach :

Heart         Aorta          Head sinuses

Pericardial sinus

Perivisceral sinus

Perineural sinus

7. In echinoderms : A true blood vascular system is absent. It is represented by a haemal system which is enclosed within a perihaemal system. haemal system includes oral haemal ring (a sinus), radical haemal sinuses or strands (present in the arms), axial glands, heart, brown glands (present within axial sinus of perihaemal system) and aboral haemal ring, from which arise, five pairs of genital haemal strands.

8. Blood circulation in vertebrates : Blood circulation was discovered by William harvey. In case of vertebrates, blood circulation is of closed type, which can be grouped into two categories :

(a) Single circulation                            (b) Double circulation

Differences between single and double circulation

Double circulation in mammals can be divided into three parts :

1. Cardiac circulation : The amount of blood present in the heart. Its value is 7%.
2. Pulmonary or lesser circulation : The amount of blood present in the surrounding of lungs and pulmonary blood vessels. Its value is 9%.
3. Systemic or greater circulation : The amount of blood which circulates in the rest part of the body. Its value is 84%. It can be divided into three parts –

Arterial circulation – 15%

Capillary circulation – 5% Venous circulation – 64%

SUPERIOR VENA CAVA

SYSTEMIC CIRCULATIO

PULMONARY CIRCULATION –

AORTA

HEART –

7%

ARTERIES –13%

INFERIOR VENA CAVA

ARTTERIOLES AND

CAPILLARIES –7%

Heart : The form, structure and function of heart exhibits much variation. The characteristics of heart of fishes, amphibians, reptiles,

VEINS, VENULES AND VENOUS SINUSOIDES –64%

Fig. – Distribution of blood volume in different parts of circulatory system

birds and mammals is presented in the following table.

Heart of vertebrates

   Heart.                                                                                                                                                                    

1. Shape and position : Reddish, roughly conical, highly muscular, mesodermal hollow organ of the size of one’s first. Its average weight in males is about 300 gm. and in females about 250 gm. It lies behind the sternum in the mediastinum space of thoracic cavity in between the two lungs. The broader base faces upward and backward. The narrower apex is directed downward, forward and slightly towards left, lying between 5^th and 6^th ribs and rests on the diaphragm.
2. Protective covering : Heart is enclosed in a tough, 2 layered fibroserous sac, the pericardium. The outer layer is non-distensible fibrous pericardium and inner layer is thin serous pericardium which further consists of outer parietal layer (attached to fibrous pericardium) and inner visceral layer (adhered to the heart).

Outer fibrous pericardium

Pericardium

Inner serous pericardium

Outer parietal layer Inner visceral layer

Between the parietal and visceral layers, occurs a

narrow potential space, the pericardial cavity which is derived from coelom and is filled with serous pericardial fluid for frictionless movement and protection from shock and mechanical injury.

3. Histology : The heart wall consists of connective tissue, blood vessels and cardiac muscle fibres in 3 different layers – Epicardium, Myocardium and Endocardium.

1. Endocardium : Innermost layer lining the cavity of heart and consisting of endothelium of squamous cells resting on thin basement membrane of loose connective tissue.

DIAPHRAG

Fig. – Diagram to show the layers of the pericardium

2. Myocardium : Middle, highly vascular layer, composed of cardiac muscle fibres joined together by intercalated disc. The connective tissue in myocardium acts as cardiac skeleton. Endocardium is thickest where the myocarduim is thinnest and vice-versa.
3. Epicardium : Visceral pericardium, joined to myocardium by connective tissue.

BRACHIOCEPHALIC TRUNK

RIGHT BRACHIOCEPHALIC VEIN

RIGHT SUBCLAVIAN ARTERY AND VEIN

SUPERIOR VENA CAVA

RIGHT LUNG

RIGHT ATRIUM

TRACHEA

LEFT COMMON CAROTID ARTERY

LEFT INTERNAL JUGLAR VEIN

LEFT BRACHIOCEPHALIC VEIN

ARCH OF AORTA

PULMONARY TRUNK LEFT PULMONARY

ARTERY

LEFT ATRIUM LEFT LUNG

LEFT VENTRICLE

DIAPHRAGM

PERICARDIUM

CORONARY SULCUS

RIGHT

APEX

INTERVENTRICLAR SULCUS

VENTRICLE

Fig. – Position of heart in our chest cavity

4. External structure : Human heart is 4- chambered and is divided by septa into two halves

• right and left. Each half has one darker, thin walled auricle in the broader upper region and one lighter, thick-walled ventricle in the narrower lower region.

1. Auricles (Atria) : Two in no., demarcated externally from ventricles by irregular groove called coronary sulcus and from each other by interatrial sulcus. When atria contract, small curtain like flaps called auricular appendages or appendices project from sides of auricles and overhang the corresponding ventricles.
2. Ventricles : Two in no. demarcated externally from each other by an oblique groove called

BRACHIOCEPHALIC TRUNK

SUPERIOR VENA CAVA

ASCENDING AORTA

RIGHT PULMONARY VEINS

RIGHT ATRIUM

CORONARY SULCUS

RIGHT VENTRICLE

INTERVENTRICULAR SULCUS

LEFT COMMON CAROTID ARTERY

LEFT SUBCLAVIAN ARTERY

ARCH OF AORTA PULMONARY

TRUNK

LEFT PULMONARY VEINS

LEFT ATRIUM GREAT

CARDIAC VEIN

LEFT VENTRICLE

APEX

interventricular sulcus which contains coronary blood vessels. The right ventricle does not reach apex.

Fig. – External features of human heart

3. Sinus venosus and conus arteriosus : Sinus venosus and conus/truncus/bulbus arteriosus are accessory chambers in the heart of lower vertebrates (fishes and amphibians). In rabbit, sinus venosus is formed in the embryo but later it becomes a part of wall of right auricle.

In frog, sinus venosus spreads upon most of the dorsal side of heart and conus arteriosus lies obliquely upon the ventral surface of right atrium.

LEFT ANTERIOR VENA CAVA

AOROTIC TRUNKS

RIGHT ANTERIOR VENA CAVA

RIGHT ATRIUM

SINUS VENOSUS PULMONARY VEINS

VENTRICLE

AORTIC TRUNKS

LEFT ANTERIOR VENA CAVA

LEFT ATRIUM

LEFT ATRIUM

CORONARY SULCUS

POSTERIOR

CONUS ARTERIOSUS

CORONARY SULCUS

DORSAL VIEW                                                                   VENTRAL VIEW

Fig. – Dorsal and ventral of frog's heart

(v)Internal structure

(a) Auricles : Atria are thin walled. They act as

SYNANGIUM

TO CAROTID

reserviors for blood entering the heart. Right auricle is bigger than left auricle and both are separated by a myomembranous partition called Interatrial or interauricular