It pumps the blood at one end and receives it at the other end by the returning vessels or the veins. To keep the blood circulating in a uniform direction it is provided with valves which prevent the backward flow of the blood.
The rhythmic contraction of heart may be triggered entirely through nervous impulses (neurogenically), or independently of the nervous system through modified muscles (myogenically) or through a myogenic mechanism which subject to regulation by nerves.
Morphologically the following four distinct types of heart have been recognized in the animals:
1. Pulsating hearts:
In annelids and Amphioxus many blood vessels are contractile in nature and show rhythmic peristaltic waves, such vessels referred to as pulsating hearts.
The peristaltic activity of these vessels actually imparts motion to the blood. This pushes the blood forward which is circulated throughout the body.
In general the pulsations are not as regular as the beats of the chambered heart.
2. Tubular hearts:
Such types of hearts are found in arthropods. They are in the form of contractile muscular tubes.
In some cases there is an specialized area in the dorsal blood vessel which functions as heart.
These are usually anchored at several corners and receive blood through paired ostia which are provided with valves.
In Branchipus, Artemia and insects the heart is tubular which extends for a considerable length of the body. In crustaceans the heart is pulsating muscular sac (maynard, 1960).
Tubular hearts are invariably surrounded by pericardium. In many insects the heart is suspended by variously arranged alary muscles which maintain tension on the heart and by their contraction lateral openings (ostia) on the heart are closed.
Contraction of the alary muscles usually coincides with contraction of the heart. These contractions together with the closing of ostia result in the expulsion of blood to tissues in the anterior region of the animal.
Another result of the contractile actions is the creation of negative pressure within the pericardial chamber. This vaccum, along with muscular movements throughout the body provides the means by which a new supply of blood may enter the heart from the haemocoel.
From the heart one or two arteries generally take their origin towards the anterior end but sometimes posterior and lateral arteries also take their origin from the heart.
3. Ampullar or booster hearts:
Such types of hearts are found in some crustaceans, insects, and other animals like cephalopods.
These are in the form of dilated portions of the blood vessels and are commonly known as “blood pumps” or “booster hearts”.
In crustacea these are compressed by contractions of somatic muscles which have their origin and insertions outside the heart and run through the heart or its wall or lie in close proximity to it.
These somatic muscles are secondarily adapted to the problems of circulation ; in some cases they contract rhythmically (Maynard, 1960). In insects they are connected with the circulation of wings (Diptera and Odonata), the antennae (Orthoptera) and the legs (Hemiptera).
Lymph hearts of fishes, amphibians and reptiles are other examples of such types of hearts.
These hearts help in collecting the tissue and discharging the same into veins at many points. These are composed of striated anastomosing fibres and may have valves which prevent the back flow of lymph.
In cephalopods branchial hearts is found in the form of circular structure beneath the gills which simply pumps the blood into the gills.
4. Chambered hearts:
Such type of hearts is characteristic feature of most vertebrates and molluscs. In vertebrates these hearts are made up of either two chambers (as in fishes) or three chambers (as in amphibians) or four chambers (as in birds and mammals).
The heart in each case is provided with valves which regulate the direction of the flow of blood and also prevent backward flow of blood.
In fishes the heart is an ‘S’ shaped structure having only two chambers; one auricle and one ventricle.
These two chambers lie in the same plane. In amphibians the heart is three chambered consisting two auricles and one ventricle.
The two auricles are separated into left and right with the help of interauricular septum, while the ventricle is an undivided single chamber.
The oxygenated blood from the lungs returns to the left auricle while the deoxygenated blood coming from different parts of the body enters the right auricle.
As there is only one ventricle the oxygenated and deoxygenated blood get mixed in the ventricle.
In most reptiles the heart is also three chambered having two completely divided auricles and an incompletely divided ventricle.
Higher reptiles such as crocodiles show four chambered heart consisting of two auricles and two ventricles.
In birds and mammals the heart is distinctly four chambered consisting of two auricles and two ventricles.
Each ventricle has a valve at its inlet, and a valve at its outlet. The inlet valves are termed the atrioventricular valves.
On the left side it is also known as the mitral valve while on the right side it is known as the tricuspid valve. The mitral valve has two cusps while tricuspid three cusps.
The outlet valves have three cusps and are known as the semilunar valves. The valve on the left side of the heart is also known as the aortic valve while that of the right side pulmonary valve. These are structures which allow the blood to flow in one direction only.
Blood returning from the body tissues enters the right auricle through two large veins, the superior and inferior venae cavae.
Blood returning from the lungs enters the left auricle through the pulmonary veins. The auricles expand as they receive the blood.
Both auricles then contract side by side, pushing the blood through the open valves (atrioventricular valves) into the ventricles.
Then ventricles contract simultaneously; the atrioventricular valves become closed by pressure of the blood in the ventricles.
The right ventricle propels the blood into the lungs through the pulmonary arteries and the left ventricle propels it into the aorta, from which it travels to the other body tissues.
The valves in the openings of the ventricles into the pulmonary artery (pulmonary valve) and the aorta (aortic valve) close after the ventricles contract, thus preventing backflow of blood.