15. Circulatory system

The circulatory system aids in the circulation of various substances like oxygen, hormones, nutrients and cellular wastes.  

  

There are two types of the circulatory system present in animals:

 

1. Open circulatory system: The blood pumped by the heart passes through vessels into open spaces, or body cavities called the sinuses. This is called the open circulatory system. Invertebrates like arthropods and molluscs have an open circulatory system.

 

2. Closed circulatory system: The blood pumped by the heart circulated through a closed network of blood vessels. This is called the closed circulatory system. Annelids and all the vertebrates have a closed circulatory system.

The circulatory system of humans constitutes the closed type. It is composed of the blood vascular system and the lymphatic system.

 

The blood vascular system comprises the heart, blood and its vessels. The lymphatic system comprises the lymph, lymphatic vessels, lymphatic capillaries, lymphatic nodes and ducts.

The human heart is present between the lungs in the thoracic cavity. It is a hollow muscular organ of cardiac muscles that is a size of our fist.

 

  

The heart consists of three layers:

 

1. Pericardium - External layer

 

2. Myocardium - Middle layer

 

3. Endothelium - Internal layer

 

The blood transports both oxygen and carbon dioxide, and thus the heart has \(four\) different chambers that prevent the \(O_2\)-rich blood from mixing with the blood containing \(CO_2\). The four chambers include the:

The left and the right atria are separated by an interatrial septum, while an interventricular septum separates the left and right ventricles.

  

 

There is the presence of \(four\) valves in the heart. They include:

Contraction of the heart is called systole, while the relaxation of the heart is called diastole. The \(CO_2\)-rich blood must reach the lungs to eliminate the \(CO_2\), while the oxygenated blood is returned to the heart. The \(O_2\)-rich blood is subsequently circulated throughout the body. 

 

 

1. The right atrium receives deoxygenated blood from the different parts of the body through the superior and inferior vena cava. The right atrium relaxes during the process to collect the deoxygenated blood.

 

2. The right atrium contracts, and the right ventricle relaxes. Thus, the deoxygenated blood from the right atrium is sent to the right ventricle.

 

3. Next, the right ventricle contracts, and blood enters the pulmonary artery. Through the pulmonary artery, blood is pumped to the lungs for oxygenation. At the lungs, the exchange of gases takes place. The oxygen-rich blood is returned to the heart through the pulmonary veins.

 

4. The pulmonary veins carry the deoxygenated blood to the left atrium from the lungs. The left atrium relaxes during the process to collect the oxygenated blood.

 

5. From the left atrium, the blood reaches the left ventricle due to the contraction of the left atrium. The left ventricle relaxes, and blood from the left atrium reaches the left ventricle. The oxygenated blood is forced into the ventricle through the atrioventricular openings by the atrial contraction.

 

6. Next, the left ventricle contracts and blood is pumped out to the various parts of the body through the aorta.

 

As you can see, the ventricles need to pump blood to the various organs, and thus they have thicker muscular walls than the atria.