Chapter 22 Gas Exchange

I. Mechanisms of Gas Exchange

1. The three phases of gas exchange in humans involve:

(1)  Breathing (inhale oxygen (O₂) and exhale carbon dioxide (CO₂))

(2)  Transport of gases by the circulatory system

(3)  Exchange of gases with body cells (your cells take up O₂ from the blood and release CO₂ to the blood)

2. Gills are adapted for gas exchange in aquatic environments.

3. There are 2 big advantages to breathing air:

(1)  Air contains a much higher concentration of O₂ than water does.

(2)  Air is much lighter and easier to move than water.

4. The tracheal system of insects provides direct exchange between the air and body cells.

II. The Human Respiratory System

1. In mammals, branching tubes convey air to lungs in the chest cavity (= thoracic cavity).

2. In all mammals, lungs locate in thoracic (chest) cavity which is separated from the abdominal cavity by the diaphragm.

3. From the nasal cavity (or mouth), air passes to pharynx, larynx, trachea, two bronchi (singular form bronchus), bronchioles, and alveoli (singular form alveolus).

4. Each of your lungs contains millions of these tiny sac alveoli. Alveoli have a surface area of about 100 m², 50 times that of your skin.

5. Ventilation occurs by negative pressure breathing, a system in which air is pulled into the lungs.

(1)  During inhalation, the ribs move upward and muscles between the ribs contract, and the diaphragm contracts and moves downward.

(2)  During exhalation, the ribs move downward and muscles between the ribs relax, and the diaphragm relaxes and moves upward.

6. Breathing is automatically controlled by involuntary mechanisms. Sensors in the medulla (medulla oblongata) monitor the pH of the cerebrospinal fluid as an indicator of CO₂ level in the blood.

III. Transport of Gases in the Human Body

1. Blood of the circulatory system (a closed circulation) transports respiratory gases.

2. Hemoglobin in red blood cells (= erythrocytes) carries O₂, help transport CO₂, and buffers the blood. Almost all vertebrates and many invertebrates use hemoglobin, an iron-containing pigment that turns red when it binds O₂ (oxyhemoglobin, 97%). The rest disolves in plasma (3%).

3. Hemoglobin is a multipurpose molecule. It carries O₂, helps transport CO₂, and buffers the blood. A hemoglobin molecule contains 4 polypeptide chains of two (α and β) different types.

4. Most of the CO₂ that diffuses from tissue cells into a capillary enters red blood cells (70% in bicarbonate ion converted by carbonic anhydrase), where some of it combines with hemoglobin (23% in carbaminohemoglobin). The rest reacts with water, forming carbonic acid (H₂CO₃), which then breaks apart into a hydrogen (H⁺) and a bicarbonate ion (HCO₃^-) in the plasma (7%). Hemoglobin binds most of the H⁺ (deoxyhemoglobin) produced by the reaction between CO₂ and water, minimizing the change in blood pH (buffering the blood).