Circulatory system is the transport system of body that carries oxygen, carbon dioxide, nutrients, hormones and other materials from the place where they are produced or absorbed to the place where they are needed by the tissues. Chemical energy must be constantly produced in the body to carry out all biological processes and to defeat entropy.
Energy is produced by oxidation of glucose and other nutrients in mitochondria. Nutrients are absorbed in intestine after digestion of food material but must be transported to liver for assimilation before getting distributed throughout body via circulatory system. Similarly oxygen is absorbed in lungs or gills, from where it is taken up by the blood vascular system and carried to the tissues for oxidative processes.
Waste products generated by the chemical processes and break down of tissues, being toxic, should be quickly excreted out of the body. Circulatory system removes these noxious substances to the ports of exit such as lungs, kidneys and intestine. Moreover, hormones produces by glands must use circulatory highways to reach their destinations. Hence, circulatory system is necessary to maintain connectivity among different parts of body. Following are the chief functions of the circulatory system:
1. Maintenance of osmotic balance.Water being a major component of body mass, great solvent, tissues must maintain optimum level of water to carry out metabolic processes. Blood absorbs water from the gut and distributes it to all organs. Loss of water from blood as happens in the case of vomiting and diarrhoea, results in serious consequences, sometimes leading to death.
2. Transport of respiratory gases.Micro-organisms have more surface area as compared to the bulk and hence gases simply diffuse in and out of the body, requiring no specific respiratory organs. However, in larger animals oxygen must be absorbed in specialized respiratory organs and then transported via an oxygen-carrying pigment in blood, such as haemoglobin or hemocyanin and delivered to the tissues. Carbon dioxide takes a reverse route to the outside.
3. Distribution of nutrients.Food is digested and absorbed in the intestine and then transported via the blood and lymphatic circulation to the liver, where it is assimilated and then supplied to all parts of body.
4. Temperature regulation.If excess metabolic heat is generated in the body, blood circulation carries it to the body surface where it is dissipated via the skin. In endotherms metabolic heat is evenly distributed throughout the body by blood, while in ectothermic animals heat is absorbed from atmosphere by skin and then transported to all parts of body evenly so that metabolic activities can take place.
5. Transport of hormones and other chemicals.Hormones are secreted in endocrine glands but the target organs of these hormones are located in different parts of body. Blood carries these hormones from endocrine glands to target organs. Pituitary located on hypothalamus of brain secretes hormones such as TSH, GH, Gonadotropin, prolactin, ADH etc. which are transported by blood to other endocrine glands, which are stimulated to produce their own hormones.
6. Defence against invaders.Blood contains granular and agranular leucocytes which attack and kill pathogens that manage to enter the body. T-cells kill the invaders chemically while B-lymphocytes produce antibodies to inactivate pathogens. Monocytes or macrophages ingest foreign materials and tissue debris by endophagy and digest it.
7. Disposal of wastes.Carbon dioxide and nitrogenous wastes are constantly produces all over the body, and being toxic must be removed from body quickly. Blood carries them to the excretory organs such as kidneys, nephridia, malpighian tubules, lungs etc. from where they are released outside into the atmosphere. Toxins and excess salts are also carried via blood to the skin, where they are excreted via the sweat glands.
8. Diagnostic material. Many kinds of chemical transformations and production of antibodies take place in blood, which is also a carrier of a variety of chemicals and hence can be uses as an ideal material to analyse and know the physiological condition of the body. Therefore, blood test for leucocyte count, haemoglobin, liver, kidney and thyroid function tests and a variety of other tests can be carried out with ease using patient’s blood.
Circulatory system in vertebrates has two parts, the blood vascular system and lymphatic system, the former includes heart, arteries, veins and blood inside and the latter includes lymph channels and spaces, lymph hearts, lymph nodes and lymph flowing in them from the tissues towards heart. Lymph channels eventually open into larger veins.
Heart in simple terms is a highly muscular pumping organ that pumps blood into arteries and sucks it back through the veins. In vertebrates it has undergone transformation by twisting from a straight tube to a complex multichambered organ, enclosed in membranous pericardium.
In protochordates, a definite heart does not exist and major blood vessels contract rhythmically to maintain circulation. Amphioxus also has small bulbuli to pump blood into gills.
In fishes heart has three chambers, namely, sinus venosus, atrium and ventricle. Ventricle is connected with conusarteriosus that has a series of semilunar valves to prevent backflow of blood. In cyclostomes there is a single set of semilunar valves in conus arteriosus. Because of twisting of fish heart atrium is placed dorsally to the ventricle. Sinus venosus receives deoxygenated blood from all parts of body and ventricle pumps it into ventral aorta to be taken to gills for absorption of oxygen. In hagfishes there are no nerve fibres supplying heart muscles but in all other fishes there is a node of neural tissue in sinus venosus and another AV node at the junction of atrium and ventricle.
Outer layer of heart wall, called epicardium, is made of connective tissue and elastic fibres. Endocardium consists of flattened epithelial cells. Myocardium is a muscular layer. Between this layer and endocardium occur a layer of purkinje fibres which are made of nerve cells whose peripheral parts transform into striated muscle fibres.
In fishes atrium and ventricle are single chambered but in lungfishes both atrium and ventricle possess an incomplete septum dividing the lumen into two.
In bony fishes conus arteriosus is reduced with one or two sets of valves only and instead a thin walled bulbusarteriosus occurs at the base of ventral aorta which inflated when blood from ventricle is forced into it.
In lung fishes owing to the appearance of lungs a pulmonary artery is formed from the 6th aortic arch and a pulmonary vein from lungs opens into the right atrium. Therefore, a septum, albeit incomplete, is formed in atrium and also in ventricle to keep oxygen rich and oxygen deficient blood from mixing. Lungfishes also possess a spiral valve in the bulbus cordis.
Urodeles possess an incomplete septum in atrium and no septum in ventricle. Also there is no spiral valve in the truncus. But urodeles have a pulmonary artery and vein and a single set of semilunar valves in the truncus.
Anurans have a complete septum in atrium but no septum in ventricle. The inner lining of ventricle possesses spongy trabeculae. Truncus arteriosus displays a spiralvalve or septumbulbi, which divides its lumen into a dorsal and a ventral channel and also serves to shunt the oxygenated blood into systemic arches and deoxygenated blood to pulmonary arteries for carrying it to lungs.
There is a complete septum in atrium and an incomplete one in ventricle, with a foramenofPanizza, through which two kinds of blood mixes before it is pumped to the body organs. Also, ventral aorta in reptiles divides into two so that the right systemic arch emerges from the left ventricle and the left one emerges from the right ventricle. At the point of their crossing a foramen of Panizza allows the blood to flow from one arch to another. SA node in reptiles migrates near the junction of sinus venosus with atrium and the former is reduced in size.
In crocodiles, both atrium and ventricle are completely partitioned so that there is no mixing of blood in heart but the two systemic arched still have a foramen of Panizza connecting them and hence blood mixes, rendering crocodiles the status of cold blooded animals.
Birds are endothermic animals for which a complete separation of oxygenated and deoxygenated blood is necessary. So, there is a complete partition in atrium as well as in ventricle and left systemic arch disappears, allowing only oxygenated blood to be pumped to head region and to the rest of body organs. In birds there is single valve between right atrium and ventricle. Sinus venosus has disappeared in birds.