Circulatory System

Introduction

In unicellular organisms simple diffusion is enough for the transportation of nutrients and gases because the entire cell surface is in contact with the outer environment but multicellular organisms rely on a more intricate method of transportation comprising the circulatory system. This is because in the case of multicellular organisms, cycling the nutrients, gases, and waste effectively and efficiently throughout the entire body cannot be achieved with simple diffusion. Circulation in animals enables the bulk flow of essential nutrients and gases to individual cells and thus promotes their growth. The bigger animals are, the higher is the number of cells in their body resulting in the need for a more complex circulatory system. 

Body  

The animal circulatory system is made up of an extensive network of cylindrical vessels commonly known as arteries, veins, and capillaries. All these vessels are connected to a pumping organ, known as the heart that pushes the circulatory fluid through it. The circulatory system varies according to the skeletal structure of the animals. So, while the system is quite simple in invertebrates, it becomes increasingly complex in the vertebrates as per the complexity of their body structure.

Animals can have either an open or a closed circulatory system, both of which are explained briefly as follows.

Open circulatory system

In animals with an open circulatory system, all the fluids are transported by the blood vessels into a cavity. The movement of animals causes the blood inside this cavity to move freely in every direction and across the entire body, This ensures that every organ in the body is soaked in the blood and thus able to receive oxygen besides getting any wastes removed. 

The speed of blood flow is quite slow as there are no smooth muscles present to cause the contraction of blood vessels.  For effective circulation in animals with an open system, an elongated beating heart is present for blood pulsing through the body. 

Invertebrates such as crabs, insects, snails, etc., are the most common animals with open circulatory systems. In the case of larger and more complex crustaceans, including lobsters, artery-like vessels help push blood through their bodies.   

Closed circulatory system

In a closed circulatory system, the blood flows only in the blood vessels and is transferred between them continuously without ever being released into the cavity. The blood flows in a single direction in specific blood vessels and delivers the nutrients and oxygen to the cells in the process while also removing any waste products.  

Closed circulatory systems are a common means of transportation in vertebrates, although major differences exist in the structure of this system amidst different groups of vertebrates. These differences are primarily in the structure of the heart and how blood flows during circulation in animals having this transportation system. These changes have mostly been caused due to the adaptation of these animals during the process of evolution as well as the resulting differences in their anatomy.

Closed circulatory systems can be further divided into two main types, as discussed below. 

Single Circulatory System 

In this system of circulation in animals, the blood passes through a single circuit and involves the completion of only a single cardiac cycle. Such animals have a double-chambered heart with a single atrium and a ventricle. 

This type of circulation is found in animals such as fish. The heart in the animal circulatory system in the fishes pumps deoxygenated blood to the gills where it gets oxygenated and is then supplied to the entire body. At the same time, the deoxygenated blood is returned to the heart.

This results in a significant disadvantage in the form of a low oxygen delivery rate, which, in turn, results in lowering the metabolism rate of these animals.

Double Circulatory System

This form of the animal circulatory system is found mostly in birds and mammals and is considered to be the most effective transportation method. The heart of animals with this circulatory system has four chambers: the left atria, left ventricle, right atria, and right ventricle. The heart is  connected to the lungs through the pulmonary artery and pulmonary vein. Heart is also connected to different body organs by arteries and veins. This creates a double pathway for the flow of blood, with one route for oxygenated blood and the other for deoxygenated blood. These two circuits are known as the pulmonic and systemic circulatory systems.  

The name double circulation is derived from the fact that the blood follows two routes. The blood vessels carry the deoxygenated blood from the body to the right atrium, and the right ventricle sends it to the lungs for getting oxygenated. The oxygenated blood from the lungs is sent to the left atrium by pulmonary veins and is then pumped out by the left ventricle to other parts of the body. 

Conclusion

Circulation in animals is one of the most important life processes as it is directly linked to the survival and growth of organisms.Through circulation nutrients reach to all the body cells and help in their well being. Being aware of the different types of circulatory systems present in various animals is a critical part of learning about their metabolism and other related body functions. It also helps to better understand the classification of animals into various categories and how they have evolved over the years. Studying the various organs that comprise the circulatory system can help further gain a better understanding of this vast topic.