Respiratory System – NDA Biology Notes

What is the Respiratory System?

The respiratory system takes oxygen from the air and delivers it to the blood. At the same time, it removes carbon dioxide from the blood and breathes it out.

Every cell in the body needs oxygen to produce energy. Carbon dioxide is the waste product of this energy production. The respiratory system manages both.

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Pathway of Air — From Nose to Alveoli

Air enters the body and travels through a series of structures before reaching the lungs. Each structure has a specific role.

Nose (Nasal Cavity)

Air enters through the nose. Fine hairs and mucus inside the nose filter dust and microbes. The nose also warms and moistens the air before it goes deeper.

Pharynx

After the nose, air passes into the pharynx. The pharynx is the common passage for both air and food. From here, air goes towards the larynx.

Larynx (Voice Box)

The larynx contains the vocal cords. It produces sound when we speak. A flap called the epiglottis closes during swallowing to prevent food from entering the airway.

Trachea (Windpipe)

Air passes from the larynx into the trachea. The trachea is reinforced with C-shaped rings of cartilage. These rings keep the trachea open so air can always flow through. [NDA 2024-II]

Bronchi

The trachea divides into two bronchi — one going into each lung. Bronchi also have cartilage rings. They contain smooth muscle. [NDA 2024-II]

Bronchioles

The bronchi further divide into smaller tubes called bronchioles. Bronchioles have no cartilage — only smooth muscle walls. [NDA 2024-II]

This is an important exam fact. Bronchi have cartilage. Bronchioles do not.

Alveoli

At the end of the bronchioles are tiny air sacs called alveoli. This is where gas exchange happens. Oxygen from the air passes into the blood. Carbon dioxide from the blood passes into the air. The alveolar walls are extremely thin. Blood supply is very rich. This makes the exchange fast and efficient.

Structure	Cartilage Present?	Key Role
Nose	Yes	Filter, warm, moisten air
Trachea	Yes (C-rings)	Air passage; kept open by cartilage
Bronchi	Yes	Air passage into each lung
Bronchioles	No	Small air passages to alveoli
Alveoli	No	Gas exchange — O₂ in, CO₂ out

Bowman’s capsule does NOT take part in breathing. Bowman’s capsule is a structure in the kidney. It has nothing to do with respiration. It is used as a distractor in respiratory system questions. [NDA 2018-I]

Breathing Mechanism — How the Diaphragm Works

Breathing is a mechanical process. The body changes the size of the chest cavity to move air in and out.

The main muscle of breathing is the diaphragm — a dome-shaped muscle at the bottom of the chest cavity.

Inhalation (breathing in):

• The diaphragm contracts and flattens downward.
• The rib cage expands upward and outward.
• The chest (thoracic) cavity becomes larger.
• The pressure inside the lungs drops below the atmospheric pressure outside.
• The pressure in the pleural space (surrounding the lungs) becomes more negative. [NDA tested via CDS]
• Air flows in from outside because pressure outside is higher.
• Inhalation is an active process — muscles work.

Exhalation (breathing out):

• The diaphragm relaxes and domes upward.
• The rib cage moves inward and downward.
• The chest cavity becomes smaller.
• The pressure inside rises.
• Air is pushed out.
• Normal exhalation is a passive process — no active muscle effort needed.

Key exam facts from CDS 2013:

• During inhalation, the diaphragm contracts — it does NOT relax. [NDA tested via CDS]
• During exhalation, the thorax cavity contracts — it does NOT expand.
• Normal exhalation is passive — not active.
• During inhalation, intra-pleural pressure becomes more negative. This was the only correct statement in the CDS question. [NDA tested via CDS]

Gas Exchange and Haemoglobin

Gas exchange happens at the alveoli in the lungs.

Oxygen diffuses from the air inside the alveoli into the blood in the surrounding capillaries. Carbon dioxide diffuses from the blood into the alveoli to be exhaled.

After picking up oxygen in the alveoli, the blood travels through the pulmonary vein to the left atrium — the first heart chamber to receive oxygenated blood from the lungs. [NDA tested via CDS]

Haemoglobin carries both O₂ and CO₂:

Haemoglobin in red blood cells carries both oxygen and carbon dioxide — not just oxygen. [NDA tested via CDS]

• In the lungs: haemoglobin loads oxygen.
• In the body tissues: haemoglobin releases oxygen and picks up some carbon dioxide.
• Back in the lungs: haemoglobin releases carbon dioxide and loads oxygen again.

Effect of acidic blood on haemoglobin:

If blood becomes acidic (lower pH), the oxygen-carrying capacity of haemoglobin decreases. [NDA tested via CDS]

This happens during intense exercise — lactic acid is produced, which makes the blood more acidic.

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Altitude Sickness

At high altitudes, the air is thinner. The partial pressure of oxygen is low.

This means each breath contains less oxygen. Haemoglobin cannot pick up as much oxygen per breath. The body becomes oxygen-starved. This causes dizziness, breathlessness, and fatigue — called altitude sickness.

The cause is low partial pressure of oxygen — not low haemoglobin levels, not high CO₂. [NDA tested via CDS]

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Lung Volumes

The lungs can hold different amounts of air.

Tidal Volume is the amount of air breathed in and out during normal resting breathing. This is approximately 500 mL. [NDA 2025-I]

Tidal volume is not the maximum — it is just normal, relaxed breathing.

Term	Meaning
Tidal Volume	Air in and out during normal (resting) breathing — exam answer
Vital Capacity	Maximum air exhaled after maximum inhalation
Inspiratory Volume	Extra air that can be inhaled beyond tidal volume
Total Lung Capacity	Absolute maximum air the lungs can hold

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Anaerobic Respiration — Lactic Acid and Muscle Cramps

This is the most tested sub-topic in this chapter — 5 questions across NDA and CDS all point to the same answer.

What happens during intense exercise:

During vigorous exercise, the muscles need a lot of energy very quickly. The heart and lungs cannot supply oxygen fast enough.

When oxygen supply is not enough, muscle cells switch to anaerobic respiration — energy production without oxygen.

In anaerobic respiration:

• Glucose is broken down to pyruvate (a 3-carbon molecule).
• Pyruvate is then converted to lactic acid. [NDA 2025-I]
• Lactic acid builds up in the muscles.
• This causes the burning sensation and muscle cramps. [NDA 2006-I] [NDA 2009-II] [NDA tested via CDS]

End product of anaerobic respiration in human muscles = Lactic acid. Not ethanol, not CO₂, not pyruvic acid.

Ethanol is the end product of anaerobic respiration in yeast (fermentation) — not in human muscles. Students must not confuse these two.

Lactic acid is produced from pyruvate, not directly from ATP or glucose. [NDA 2025-I]

Pyruvate has 3 carbon atoms. [NDA tested via CDS]

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ATP — Energy Currency of the Cell

ATP (Adenosine Triphosphate) is the molecule that stores and delivers energy for all cellular processes. It is called the energy currency of the cell. [NDA tested via CDS]

When a cell needs energy, ATP releases it and becomes ADP (Adenosine Diphosphate).

ATP is not the same as glucose. Glucose is the fuel. ATP is the currency — the actual form in which energy is used.

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Respiration in Other Organisms

Frog breathes through skin:

The frog uses its skin as a respiratory organ. Frogs breathe through both their lungs and their moist skin. Skin respiration supplements lung breathing. [NDA 2008-I] [NDA tested via CDS]

Other animals:

• Fish — gills
• Cockroach — spiracles (tiny openings in the body wall)
• Whale — lungs
• Shark — gills

Frog is the correct answer to “which animal breathes through the skin.”

Smoke Inhalation

A person trapped in a burning, smoke-filled building should be checked for respiratory burn once they are out. [NDA 2011-II]

Smoke damages the lining of the airways directly. Carbon monoxide in smoke binds to haemoglobin and reduces oxygen delivery. The injury is respiratory in nature — not a surface burn, not an internal organ injury.

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Aircraft and Lung Disease

Crew and passengers of aircraft suffer from chronic obstructive pulmonary disease due to particulate pollutants in the aircraft cabin environment. [NDA tested via CDS]

Not solar radiation, not ozone concentration, not nitrogen oxide. Particulate matter in the cabin air is the cause.

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