Blood Circulatory System

The blood circulatory system (BCS) is a complex cardiovascular system comprising the heart, blood and blood vessels. The BCS circulates blood throughout the vertebrates’ bodies, including humans. The BCS is divided into two major circuits: the pulmonary circular and the systemic circular. The blood vessels form a huge network of arteries, veins and capillaries. The BCS is said to be closed in the case of vertebrates. It means that the blood never comes out of its vessels while being circulated in the body. Invertebrates, on the contrary, are said to have open BCS.

What is blood?
It is a fluid comprising of 

• Red blood cells or RBC (erythrocytes) – to transport oxygen, minerals and other substances to various body parts,

•  White blood cells or WBC (leukocytes) – to provide a defence system by warding off pathogen and other foreign microbes,

•  Platelets (thrombocytes) – to help in clotting during an injury, and 

• Plasma which carries oxygen and other nutrients to various parts of the body. Due to BCS, essential nutrients are transported. Gases like oxygen, carbon dioxide and certain hormones are circulated throughout the body. Because of the presence of BCS, homeostasis is also maintained by temperature and pH stabilisation. 

Organs of the BCS

The BCS has four major organs performing specific functions. 

1. Heart: The heart is a muscular organ located between the lungs in the chest’s cavity. It is enclosed by the pericardium, which has the pericardial fluid that absorbs mechanical shocks. The heart in humans is four-chambered – two atria in the upper chambers and two ventricles in the lower chambers.
2. Blood: Blood is a fluid connective tissue whose primary role is circulating hormones, nutrients, gases, minerals to various parts of the body. It is composed of plasma, RBC, WBC, platelets.
3. Blood vessels: This is the network through which the blood is circulated. It is categorised into three main types – arteries, veins and capillaries. 

• Arteries transport oxygenated blood from the heart to various body parts except for the pulmonary artery, which carries deoxygenated blood. They have a narrow lumen and thick, strong walls because of the high pressure which flows to distant body parts against gravity. 
• Veins, on the contrary, transport deoxygenated blood from various body parts back to the heart except for pulmonary veins, which carry oxygenated blood. They are thin-walled and have wide lumen because of minimal or no such pressure.
• Capillaries connect very small arteries called arterioles and small veins called venules. They have very thin walls for gases, minerals and other wastes to pass through them.

1. Lymph: Lymph is known as tissue fluid. It is colourless, contains salt, water, proteins that are circulated in the intercellular spaces of the tissues. It flows passively.

Mechanism of BCS

The BCS constantly works to circulate blood to the rest of the body. It does so in the following way:

1. Lungs get deoxygenated blood from the right ventricle. The pulmonary artery does the work for it. In the lungs, it gets oxygenated and comes back to the heart via pulmonary veins.
2. Ventricles get blood from the atria. The left ventricle pumps the blood to the rest of the body via arteries.
3. As the blood moves, it provides the hormones, gases, nutrients, minerals, etc.
4. Veins carry deoxygenated blood and CO2 back to the heart.
5. The heart pumps it back to the lungs. 
6. This cycle is repeated in endless loops.

Circuits of BCS

The blood circulatory system has two major circuits, although a third circuit is also considered.

1. Pulmonary circuit: This circuit carries deoxygenated blood from heart to lungs via pulmonary artery and oxygenated blood from lungs to heart via the pulmonary vein.

2. Systemic circuit: This circuit carries oxygenated blood from the heart to the rest of the body and brings deoxygenated blood from body parts back to the heart.

3. Coronary circuit: These are the heart’s arteries. They provide oxygenated blood to the heart muscles and bring deoxygenated blood to the atria. 

Functions of BCS

• Sustenance of all organ systems.

• BCS protects cells from foreign molecules or pathogens, aiding in disease protection.

• It provides an interface for cell-to-cell communication.

• Substances in blood repair damaged tissues aiding in the healing process.

• Transport of gases like oxygen and carbon dioxide, nutrients, hormones, metabolic waste products via blood.

• BCS maintains a steady level of homeostasis in the body through temperature and pH stabilisation.

Conclusion

The blood circulatory system (BCS) is a system that includes the heart, blood and its vessels, which circulates blood throughout the vertebrates’ bodies. The human body is a complex system requiring many processes to perform simultaneously. All essential elements must be transferred throughout the body to run the processes non-stop. The blood circulatory system solely does this. The role of BCS is to transport various gases, nutrients, minerals, hormones throughout the body and retrieve waste products from metabolism and throw them out from the body. Multiple organs of the BCS do all these actions.