Respiration and Circulation – Basic Life Processes

Respiratory Organs

Human respiration includes a network of tissues and organs working together to introduce oxygen to our body and remove carbon dioxide. The respiratory organs consist of the airway, the respiratory muscles (intercostal muscle and diaphragm) and the lungs. The airway includes the following:

  • Nose
  • Mouth
  • Larynx
  • Pharynx
  • Bronchioles
  • Bronchi


Lungs are the chief respiratory organs seen in most vertebrates. Humans have two lungs, a bigger right lung and a comparatively smaller left lung. They are located in the thoracic cavity, where the trachea branches into the bronchial airways. The terminal bronchioles lead to tiny vascularised sacs called alveoli. These alveoli are the primary sites for gas exchange. The lungs rest on a sheet of muscle called the diaphragm. This diaphragm separates the thoracic cavity from the abdominal cavity.

Mechanism of Breathing

Breathing involves inspiration and expiration. There is a pressure gradient between the lungs and the atmosphere that facilitates the movement of air through the lungs. When the intra-pulmonary pressure (within lungs) is less than the atmospheric pressure, then the inspiration process occurs. Likewise, expiration happens when the intra-pulmonary pressure is high. The diaphragm and intercostal muscles aid in this inspiration and expiration process. The intercostal muscles are attached between the ribs. The external intercostal manipulates the size of the ribcage and assist in inhalation. Likewise, the internal intercostal helps in exhalation by slightly lowering the ribcage.

Steps involved in Respiration

  • Initially, the breathing mechanism by which the oxygen is drawn in and the carbon dioxide-rich alveolar air is released out.
  • Diffusion (across the alveolar membrane) and transportation (blood) of oxygen and carbon dioxide.
  • Diffusion of gases between the tissues and blood.
  • Oxygen is utilised by the cells, and carbon dioxide is released out (cellular respiration).

These sequential steps provide oxygen to our body. While the respiratory system helps in the exchange of gases, there is another system performing vital activities – the circulatory system. It brings about the flow of blood throughout the body. Both these have to interrelatedly function for the survival of humans. Let us learn more about the circulatory system.

Circulatory System

The human circulatory system includes a network of blood vessels, blood and most importantly, the heart. The heart is also situated in the thoracic cavity. It is slightly tilted to the left and is seen between the two lungs. The human heart is four-chambered:

  • Upper left Atrium
  • Upper right Atrium
  • Lower left Ventricle
  • Lower right Ventricle

The heart is protected by the pericardial sac, which also contains the pericardial fluid. The opening between the right atrium and right ventricle is guarded by the tricuspid valve. Likewise, the left atrium and left ventricle are guarded by a bicuspid valve. These valves prevent the backflow of the blood.

Function of Heart

The heart helps in the circulation of blood throughout the body. Maintaining blood pressure is another vital function of heart. It primarily transports O2 and returns CO2 and deoxygenated blood to the pulmonary circuit. During pulmonary circulation, they receive O2 from the lungs and deliver CO2 for expiration. Usually, the right heart (right atrium and right ventricle) collects deoxygenated blood. Likewise, the left heart pumps oxygenated blood through the aorta to the rest of the body.

Cardiac Cycle

The sequential contraction and relaxation of the heart (heartbeat) constitutes the cardiac cycle. The contraction of the heart muscles is called the systole, and the relaxation is called the diastole. The emptying of blood happens during systole, and the refilling is done during diastole. The two prominent sounds (lub-dub) are produced during each cardiac cycle.

The total volume of blood pumped by the heart in a unit of time is called the heart output or the cardiac output.

To explore human respiration visually, register at BYJU’S YouTube channel.

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