THE RESPIRATORY SYSTEM Human Biology & Physiology — Unit 5
DEPARTMENT OF BIOLOGICAL SCIENCES
FIRST YEAR COURSE: HUMAN BIOLOGY & PHYSIOLOGY
Course Notes | Unit 5
THE RESPIRATORY SYSTEM
Gas Exchange, Breathing Mechanics, and Gas Transport
Figure 1. Overview of the Human Respiratory System showing major anatomical structures.
LEARNING OBJECTIVES
By the end of this chapter, students should be able to:
1. Describe the structural components of the respiratory system.
2. Explain how gas exchange occurs at respiratory surfaces.
3. Outline the mechanics of inhalation and exhalation.
4. Describe how oxygen and carbon dioxide are transported in the blood.
5. Apply knowledge of respiratory physiology to clinical contexts.
First Year Course Notes — For Academic Use Only Page
,THE RESPIRATORY SYSTEM Human Biology & Physiology — Unit 5
1. Introduction
Every living cell requires a continuous supply of oxygen (O ₂) to sustain metabolic processes and must
simultaneously eliminate carbon dioxide (CO₂), a primary waste product of cellular respiration. In
unicellular organisms and very small multicellular organisms, gas exchange can occur by simple diffusion
across the cell membrane. However, in large, complex organisms such as humans, a dedicated organ system
— the respiratory system — has evolved to meet these demands efficiently.
The human respiratory system is a highly organised network of organs and structures that facilitates the
movement of air into and out of the body, brings oxygen into intimate contact with the bloodstream, and
removes carbon dioxide from the body. The system works in close coordination with the cardiovascular
system to ensure that gases are delivered to, and collected from, every tissue in the body.
This chapter examines the structural components of the respiratory system, the principles governing gas
exchange, the mechanics of breathing, and the mechanisms by which gases are transported in the blood and
body fluids.
1.1 Anatomical Overview of the Respiratory Tract
The respiratory tract is conventionally divided into two regions:
• Upper Respiratory Tract: Consists of the nasal cavity, pharynx (throat), and larynx (voice box).
This region filters, warms, and humidifies incoming air before it passes deeper into the tract.
• Lower Respiratory Tract: Comprises the trachea, bronchi, bronchioles, and the lungs — including
the alveoli, which are the primary sites of gas exchange.
1.2 Structural Features of the Lungs
Each lung is enclosed within a double-layered membrane called the pleura. The space between these two
layers, the pleural cavity, contains a thin film of fluid that reduces friction during breathing and maintains
negative pressure essential for lung expansion.
The right lung is divided into three lobes (superior, middle, and inferior), while the left lung, which is
slightly smaller to accommodate the heart, has two lobes (superior and inferior). Air reaches the lungs
First Year Course Notes — For Academic Use Only Page
, THE RESPIRATORY SYSTEM Human Biology & Physiology — Unit 5
through a branching network of airways — the bronchial tree — that progressively narrows from the main
bronchi, to secondary bronchi, tertiary bronchi, bronchioles, and ultimately to the terminal bronchioles
leading to the alveoli.
First Year Course Notes — For Academic Use Only Page
DEPARTMENT OF BIOLOGICAL SCIENCES
FIRST YEAR COURSE: HUMAN BIOLOGY & PHYSIOLOGY
Course Notes | Unit 5
THE RESPIRATORY SYSTEM
Gas Exchange, Breathing Mechanics, and Gas Transport
Figure 1. Overview of the Human Respiratory System showing major anatomical structures.
LEARNING OBJECTIVES
By the end of this chapter, students should be able to:
1. Describe the structural components of the respiratory system.
2. Explain how gas exchange occurs at respiratory surfaces.
3. Outline the mechanics of inhalation and exhalation.
4. Describe how oxygen and carbon dioxide are transported in the blood.
5. Apply knowledge of respiratory physiology to clinical contexts.
First Year Course Notes — For Academic Use Only Page
,THE RESPIRATORY SYSTEM Human Biology & Physiology — Unit 5
1. Introduction
Every living cell requires a continuous supply of oxygen (O ₂) to sustain metabolic processes and must
simultaneously eliminate carbon dioxide (CO₂), a primary waste product of cellular respiration. In
unicellular organisms and very small multicellular organisms, gas exchange can occur by simple diffusion
across the cell membrane. However, in large, complex organisms such as humans, a dedicated organ system
— the respiratory system — has evolved to meet these demands efficiently.
The human respiratory system is a highly organised network of organs and structures that facilitates the
movement of air into and out of the body, brings oxygen into intimate contact with the bloodstream, and
removes carbon dioxide from the body. The system works in close coordination with the cardiovascular
system to ensure that gases are delivered to, and collected from, every tissue in the body.
This chapter examines the structural components of the respiratory system, the principles governing gas
exchange, the mechanics of breathing, and the mechanisms by which gases are transported in the blood and
body fluids.
1.1 Anatomical Overview of the Respiratory Tract
The respiratory tract is conventionally divided into two regions:
• Upper Respiratory Tract: Consists of the nasal cavity, pharynx (throat), and larynx (voice box).
This region filters, warms, and humidifies incoming air before it passes deeper into the tract.
• Lower Respiratory Tract: Comprises the trachea, bronchi, bronchioles, and the lungs — including
the alveoli, which are the primary sites of gas exchange.
1.2 Structural Features of the Lungs
Each lung is enclosed within a double-layered membrane called the pleura. The space between these two
layers, the pleural cavity, contains a thin film of fluid that reduces friction during breathing and maintains
negative pressure essential for lung expansion.
The right lung is divided into three lobes (superior, middle, and inferior), while the left lung, which is
slightly smaller to accommodate the heart, has two lobes (superior and inferior). Air reaches the lungs
First Year Course Notes — For Academic Use Only Page
, THE RESPIRATORY SYSTEM Human Biology & Physiology — Unit 5
through a branching network of airways — the bronchial tree — that progressively narrows from the main
bronchi, to secondary bronchi, tertiary bronchi, bronchioles, and ultimately to the terminal bronchioles
leading to the alveoli.
First Year Course Notes — For Academic Use Only Page
