MSc Physiology Lecture 12: Respiratory System

Respiratory System

• The primary function of the respiratory system is to supply oxygen to the blood, which delivers oxygen to all parts of the body.
• The respiratory system performs this function while breathing, exchanging oxygen and carbon dioxide through the alveoli.
• The average adult's lungs contain approximately 600 million alveoli, which are surrounded by capillaries.

Principle Functions of the Respiratory System

• Ventilate the lungs
• Extract oxygen from the air and transfer it to the bloodstream
• Excrete carbon dioxide and water vapour
• Maintain the acid-base balance of the blood

Composition of Inspired and Expired Air

• Inspired air: approximately 79% nitrogen, 20% O2, 0.04% CO2, and water vapour/trace gases
• Expired air: approximately 79% nitrogen, 16% O2, 4% CO2, and water vapour/trace gases

Structure of the Respiratory System

• Respiration takes place with the aid of the mouth, nose, trachea, lungs, diaphragm, and intercostal muscles.
• Oxygen enters the respiratory system through the mouth and nose, then passes through the larynx and trachea.
• The trachea splits into two bronchi, which divide into smaller bronchial tubes that lead to the alveoli.

Lungs

• The lungs are paired, cone-shaped organs that take up most of the space in the chest.
• They have two main functions: to take oxygen into the body and to remove carbon dioxide.
• The right lung has three lobes, while the left lung has two lobes due to the heart taking up space in the left side of the chest.
• The lungs can be divided into smaller portions called bronchopulmonary segments, each with its own blood supply and air supply.

Function of Alveoli

• Air enters the lungs through the bronchi and eventually forms little thin-walled air sacs or bubbles called alveoli.
• The alveoli are where gas exchange takes place between the air and the blood.
• Capillaries surround each alveolus, allowing oxygen to diffuse from the air into the blood and carbon dioxide to diffuse from the blood into the air.

Blood Supply

• The lungs receive a large blood supply from the pulmonary arteries, which carry blood low in oxygen and high in carbon dioxide.
• The newly oxygen-rich blood then travels back through the pulmonary veins into the left side of the heart.

Additional Functions of the Lungs

• Alter the pH of blood by facilitating alterations in the partial pressure of carbon dioxide
• Filter out small blood clots formed in veins
• Filter out gas micro-bubbles occurring in the venous blood stream
• Influence the concentration of some biologic substances and drugs used in medicine in the blood
• Convert angiotensin I to angiotensin II by the action of angiotensin-converting enzyme
• Serve as a layer of soft, shock-absorbent protection for the heart