Respiratory medicine – Introduction

The respiratory system is one of the most important topics for nursing competitive exams including AIIMS Nursing Officer, NORCET, RRB Staff Nurse, ESIC, DSSSB, and all State PSC nursing exams. Whether you are a GNM graduate or a BSc Nursing student, a solid understanding of respiratory system anatomy and physiology forms the foundation of your preparation in the Medical-Surgical Nursing and Anatomy & Physiology sections.This article covers everything you need to know — the organs of the respiratory system, the muscles and nerves that control breathing, the composition of inhaled and exhaled air, and the critical role of alveoli in gas exchange.

What Is the Respiratory System?
The respiratory system is the organ system responsible for bringing oxygen into the body and removing carbon dioxide from it. This process — called external respiration — is what keeps every cell in the human body alive. In nursing exams, questions from this topic appear regularly in the Anatomy & Physiology section as well as in Medical-Surgical Nursing.

Composition of Inhaled and Exhaled Air — Values You Must Know
One of the most commonly asked MCQ areas in nursing competitive exams is the percentage composition of the gases in inhaled versus exhaled air. These are the values to memorize:
Inhaled Air
Oxygen (O₂)- 21%
Carbon Dioxide (CO₂)- 0.04%
Nitrogen (N₂)- 78%
Exhaled Air
Oxygen (O₂)- 16%
Carbon Dioxide (CO₂)- 4%
Nitrogen (N₂)- 78%

Nitrogen remains unchanged because the body neither uses nor produces it during normal breathing.
This is a commonly tested fact in AIIMS and RRB nursing exams.

Organs of the Respiratory System — Nose to Alveoli
The respiratory tract is divided into the upper respiratory tract and the lower respiratory tract.

Upper Respiratory Tract includes the nose and nasal cavity, pharynx, and larynx. The nose filters, warms, and humidifies incoming air. The pharynx serves as a shared passage for both air and food, while the larynx, also known as the voice box, contains the vocal cords and the epiglottis which prevents food from entering the airway.

Lower Respiratory Tract begins at the trachea — a cartilaginous tube approximately 10–12 cm long — which divides at the carina into the right and left bronchi. The right bronchus is shorter, wider, and more vertical than the left, which is why aspirated objects more commonly lodge on the right side. This is a high-yield fact for nursing exams. The bronchi continue to divide into bronchioles, which end in the terminal bronchioles and finally the respiratory bronchioles.

Alveoli — Structure and the Process of Gas Exchange
The alveoli are tiny air sacs at the end of the respiratory bronchioles and are the functional units of the lungs. There are approximately 300 million alveoli in the adult human lungs, providing a massive surface area of around 70 square meters for gas exchange — roughly the size of a tennis court.

Gas exchange at the alveoli happens through the process of diffusion across the alveolocapillary membrane. Oxygen moves from the alveoli into the blood, and carbon dioxide moves from the blood back into the alveoli to be exhaled. This process is driven by the difference in partial pressures of the gases on either side of the membrane (Dalton's Law and Henry's Law — important for exam theory).

Muscles of Respiration
The primary muscle of respiration is the diaphragm — the dome-shaped muscle separating the thoracic and abdominal cavities. During normal quiet breathing (eupnea), the diaphragm does approximately 70–80% of the work of breathing.The external intercostal muscles assist during inspiration by elevating the ribs and expanding the thoracic cavity. During forced expiration or respiratory distress, accessory muscles like the sternocleidomastoid, scalenes, and abdominal muscles are recruited.

The Phrenic Nerve — Control of the Diaphragm
The phrenic nerve, arising from cervical roots C3, C4, and C5 (remembered by the mnemonic "C3, 4, 5 keeps the diaphragm alive"), is the primary nerve controlling the diaphragm. Injury at or above C3 results in loss of spontaneous breathing, requiring mechanical ventilation — a critical point in neurological nursing questions.

Watch the Full Video Lecture
Dr. Sameer Jain explains the complete anatomy and physiology of the respiratory system using whiteboard diagrams in this free video lecture on the Medusera YouTube channel. The lecture is specifically designed for nursing students preparing for AIIMS NORCET, RRB, ESIC, DSSSB, and State PSC nursing competitive exams.