Gas Exchange in Humans

11.2 Gas Exchange in Humans

Gas Exchange Surfaces

Aerobic respiration requires glucose and oxygen. Cells obtain glucose as described in Chapters 6 and 7. Animals acquire sugars like glucose from carbohydrates in their diet, while plants produce it through photosynthesis.

Oxygen is sourced directly from the surroundings, whether from the air for terrestrial organisms or dissolved in water for aquatic organisms.

Carbon dioxide, a waste product of aerobic respiration, must be removed from the organism.

Gas exchange occurs in specialized areas where oxygen enters and carbon dioxide exits. These areas are called gas exchange surfaces.

Characteristics of Gas Exchange Surfaces

Gas exchange surfaces must be permeable for oxygen and carbon dioxide to pass through easily. They also have other characteristics:

• They are thin to allow gases to diffuse across them quickly.
• They are close to an efficient transport system to take gases to and from the exchange surface.
• They have a large surface area so that a lot of gas can diffuse across at the same time.
• They have a good supply of oxygen.

Key Words:

• Gas exchange: the diffusion of oxygen and carbon dioxide into and out of an organism's body.
• Gas exchange surface: a part of the body where gas exchange between the body and the environment takes place.

The Human Breathing System

The human gas exchange system includes the lungs, which contain numerous tiny air spaces called air sacs or alveoli. Oxygen diffuses into the blood at the surface of the alveoli, which is the gas exchange surface. Lungs are light and spongy because of the air spaces.

Air enters the lungs through the windpipe or trachea.

Key Words:

• Alveoli: tiny air-filled sacs in the lungs where gas exchange takes place.
• Trachea: the tube through which air travels to the lungs; it has rings of cartilage in its walls to support it.

Nose and Mouth

Air can enter the body via the nose or mouth.

• Hairs in the nose trap dust particles.
• Thin bones inside the nose are covered with cells, including goblet cells that produce a liquid containing water and mucus.
• Water in the liquid evaporates, moistening the air.
• Cilia, tiny hair-like projections, trap bacteria and dust particles in the mucus.
• Cilia are found along the trachea and bronchi.
• They sweep mucus containing bacteria and dust particles up to the back of the throat to prevent airway blockage.

Figure 11.4: Part of the lining of the respiratory passages.

The figure depicts ciliated and goblet cells in the respiratory passages. The goblet cells release mucus, which is then moved by the beating cilia.

Key Words:

• Goblet cells: cells found in the lining (epithelium) of the respiratory passages and digestive system, which secrete mucus.
• Cilia: tiny projections from some of the cells in the lining of the respiratory passages; the cilia of many adjacent cells beat rhythmically in unison.

Figure 11.3: The human gas exchange system.

The diagram illustrates the components of the human gas exchange system:

• Larynx
• Trachea (supported by cartilage)
• Bronchi
• Bronchioles
• Alveoli (air sacs)
• Pleural membranes
• Pleural fluid
• Diaphragm
• Left lung
• Rib cross-section
• Heart
• External and internal intercostal muscles

The Trachea

Air passes from the nose or mouth into the trachea. The larynx, or voice box, is located below the epiglottis and contains the vocal cords. Muscles tighten the vocal cords to produce sounds when air passes over them.

The trachea is reinforced with rings of cartilage, which prevent it from collapsing when air pressure inside is lower than outside.

The Bronchi

The trachea extends through the neck into the thorax, which is the upper part of the body from the neck to the bottom of the ribs and diaphragm. In the thorax, the trachea divides into the right and left bronchi. Each bronchus leads to a lung and branches into smaller tubes called bronchioles.

Key Words:

• Thorax: the chest; the part of the body from the neck down to the diaphragm.
• Bronchus: one of the two tubes that takes air from the trachea into the lungs.
• Bronchiole: a small tube that takes air from a bronchus to every part of the lungs.

Alveoli

Alveoli, tiny air sacs, are located at the end of each bronchiole. Gas exchange occurs here.

The walls of the alveoli serve as the gas exchange surface. Tiny capillaries surround the alveoli. Oxygen diffuses from the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli.

Figure 11.5: Alveoli in the lungs, and the blood vessels that are associated with them.

Blood vessels transport blood with low oxygen from the pulmonary arteries to the alveoli. Oxygenated blood returns to the pulmonary veins.

Figure 11.6: Gas exchange across the wall of an alveolus.

The diagram shows oxygen diffusing into the blood and carbon dioxide diffusing out.

The walls of the alveoli are adapted for efficient gas exchange:

• They are very thin, only one cell thick, as are the capillary walls. The small thickness allows oxygen to diffuse into the blood quickly.
• They have an effective transport system. Blood is constantly pumped to the lungs via the pulmonary artery, which branches into capillaries. Carbon dioxide diffuses from the blood into the alveoli, and oxygen diffuses into the blood. Blood returns to the heart via the pulmonary vein.
• They have a large surface area. The total surface area of the alveoli in the lungs is over 100 m^2.
• The inner surface is moist so that oxygen dissolves.
• They have a good supply of oxygen. Breathing movements (ventilation) keep the lungs well-supplied with oxygen.

Key Word:

• Ventilation: the movement of air into and out of the lungs, by breathing movements.