Gaseous exchange in mammals

Gaseous exchange in mammals

The components of the gaseous exchange system in mammals include the nostril, trachea, lungs, intercostals muscles, diaphragm and ribs.

Adaptations and functions of parts of the mammalian respiratory system

Part	Adaptive Features	Functions
Nose and Nasal Cavity	Mucus lining and hairs (cilia)	Trap dust and microorganisms
Glottis	Presence of epiglottis	Closes the trachea during swallowing to prevent food from entering the respiratory system
Trachea, Bronchus and Bronchioles	Blood vessels near the surface; Rings of cartilage; Mucus lining and cilia	Warm the air; Prevent collapse of the respiratory tract; Trap and filter dust and microorganisms
Lungs	Spongy texture with air spaces (alveoli)	Main organ for gaseous exchange; air spaces hold inhaled air
Alveoli	Numerous in number; Thin membranes; Moist surface; Dense network of capillaries; Constantly contain air	Provide large surface area for gaseous exchange; Reduce diffusion distance; Allow gases to dissolve before diffusing; Transport gases to and from tissues; Maintain shape and prevent collapse
Pleural Membrane	Contains pleural fluid	Lubricates the lungs, allowing smooth movement over the thoracic cavity during breathing
Ribs	Made of hard bone tissue	Protect the lungs from mechanical injury
Intercostal Muscles	Work antagonistically: when one contracts, the other relaxes	Enable expansion and contraction of the thoracic cavity during breathing
Diaphragm	Muscular sheet of tissue	Separates the thorax from the abdomen and enables breathing by flattening or doming

The mechanism of gaseous exchange in mammals

Inhalation (Inspiration)

1. Diaphragm Contraction: The diaphragm contracts and moves downward.
2. External Intercostal Muscles: These muscles contract, lifting the ribcage upward and outward.
3. Volume Increase: The combined action of the diaphragm and ribcage increases the volume of the thoracic cavity.
4. Pressure Decrease: The increase in volume results in a decrease in air pressure inside the thorax.
5. Air Inflow: This pressure difference causes air to rush into the lungs through the nostrils, trachea, and bronchioles to equalize the pressure.

Exhalation (Expiration)

1. Diaphragm Relaxation: The diaphragm relaxes and returns to its dome-shaped position.
2. External Intercostal Muscles Relax: The ribcage moves inward and downward.
3. Volume Decrease: The thoracic cavity's volume decreases due to the movements of the diaphragm and ribcage.
4. Pressure Increase: The decrease in volume increases the air pressure inside the thorax.
5. Air Expulsion: The higher pressure forces air out of the lungs, passing through the bronchioles, trachea, and nostrils, and out of the body.

Breathing in (inhalation)	Breathing out (exhalation)
External intercostal muscles contract	The external intercostal muscles relax
Internal intercostal muscles relax	The internal intercostal muscle contract
The ribcage is lifted outward and upward	The ribcage moves inward and downward
The diaphragm contracts and flattens	The diaphragm relaxes and becomes dome-shaped
The volume of thoracic cavity increases as pressure decreases. This allows air to enter the thoracic cavity	The volume of thoracic cavity decreases as pressure increases
Air enters the alveoli through the nostrils, pharynx, glottis, trachea, bronchioles and finally alveoli	Air leaves the alveoli through the bronchioles, trachea, glottis, pharynx and finally nostrils

Gaseous exchange across the alveolus

The actual exchange of oxygen and carbon dioxide takes place in the alveoli. One mammalian lung has millions of alveoli. The alveoli are surrounded by network of capillaries.

Diffusion of gases

Inhalation and Oxygen Diffusion

1. When we breathe in, air fills the alveoli. The concentration of oxygen in the alveoli is higher than in the blood, so oxygen diffuses from the alveoli into the blood in the surrounding capillaries.
2. The oxygen then binds to haemoglobin in red blood cells, forming oxyhaemoglobin, which is carried through the bloodstream to the tissues.

Oxygen Release in the Tissues

1. In the tissues, oxyhaemoglobin breaks down to release oxygen, which is used by the cells for cellular processes.
2. As the tissues use oxygen, they release carbon dioxide, increasing its concentration in the tissues.

Carbon Dioxide Diffusion into the Blood

1. With higher levels of carbon dioxide in the tissues than in the blood, carbon dioxide diffuses into the blood in the capillaries.
2. Carbon dioxide combines with haemoglobin to form carbaminohaemoglobin.

Transport of Carbon Dioxide to the Alveoli

1. The blood transports the carbaminohaemoglobin to the alveoli.
2. The concentration of carbon dioxide is now higher in the blood than in the air in the alveoli, causing carbon dioxide to diffuse from the blood into the alveoli.

Exhalation and Carbon Dioxide Removal: The carbon dioxide in the alveoli is then carried through the bronchioles, trachea, glottis, pharynx, and finally expelled from the body through the nostrils into the atmosphere

Composition of inspired and expired air

Gas	Inspired air	Expired air
Oxygen	20.95%	16.40%
Carbon dioxide	0.03%	4.00%

Factors affecting the rate of gaseous exchange

i. Concentration of Carbon Dioxide

A high concentration of carbon dioxide (CO₂) in the blood increases the rate of gaseous exchange. This helps the body quickly remove excess CO₂ while allowing more oxygen (O₂) to reach the tissues. The higher the carbon dioxide buildup, the faster the breathing rate.

ii. Concentration of Haemoglobin

Haemoglobin is a protein in red blood cells that carries oxygen from the lungs to body tissues and returns carbon dioxide to the lungs. Adequate haemoglobin levels allow efficient gas transport. In conditions like anaemia (low haemoglobin levels), the body increases the rate of gaseous exchange to compensate for the reduced oxygen-carrying capacity.

iii. Physical Activity

During exercise or physical exertion, muscles require more oxygen and produce more carbon dioxide. This increases the rate of breathing and gaseous exchange to meet the higher metabolic demands of active tissues.

iv. Health Status of the Body

Certain illnesses or infections may increase the body's metabolic rate and oxygen demand, leading to a higher rate of gaseous exchange. However, some respiratory diseases (e.g., asthma, bronchitis) may reduce the efficiency of the lungs and slow down breathing and gas exchange.

v. Altitude

At high altitudes, the air contains less oxygen and atmospheric pressure is lower. This reduces the partial pressure of oxygen, requiring faster and deeper breathing to take in enough oxygen for the body's needs. Over time, the body also increases red blood cell production as an adaptation.

vi. Age

Younger individuals tend to have higher metabolic rates and greater physical activity levels, which require more oxygen and thus a faster rate of gaseous exchange. In contrast, older people may have a slower metabolism and reduced respiratory efficiency.