Gas Exchange in Animals

Gas Exchange Overview

• Gas exchange in animals with lungs involves breathing, gas transport, and exchange with tissue cells.
• The three phases of gas exchange are: breathing, transport of oxygen and carbon dioxide in the blood, and body tissues taking up oxygen while releasing carbon dioxide.
• Cellular respiration requires a constant supply of oxygen and carbon dioxide disposal.

Exchange Surfaces

• Respiratory surfaces must be thin and moist for the diffusion of oxygen and carbon dioxide.
• Earthworms and other animals use their skin for gas exchange.
• Most animals use specialized body parts, for gas exchange.
• Examples are gills (in fish and amphibians), tracheal systems (in arthropods), and lungs (in tetrapods that live on land).

Gills

• Gills are extensions of the body, with a lot of surface area for gas exchange.
• Gas exchange in fish is enhanced by ventilating the gills (moving water) and countercurrent flow between water and blood.

Tracheal System

• Compared to water, using air to breathe has two big advantages.
• Air contains higher concentrations of O2 and air is lighter and easier to move.
• Air-breathing animals lose water through their respiratory surfaces.

Evolution of Lungs

• Tetrapods likely evolved in shallow water.
• The first tetrapods on land diverged into three major lineages.
• Amphibians use small lungs and their body surfaces.
• Nonbird reptiles have lower metabolic rates and simpler lungs.
• Birds and mammals have higher metabolic rates and more complex lungs.

Human Respiratory System

• In mammals, air is inhaled through the nostrils into the nasal cavity.
• Air is filtered by hairs and mucus, warmed and moisturized, and sampled for odors.
• Air passes from the nasal cavity, to the pharynx, then the larynx, past the vocal cords.
• Then it goes to the trachea that is held open by cartilage rings, into paired bronchi, into bronchioles, and finally to the alveoli.
• Alveoli are grapelike clusters of air sacs where gas exchange occurs.

Alveoli

• Alveoli are well adapted for gas exchange, they have a lot of surface areas in general and for the capillaries.
• O2 diffuses into the blood and CO2 diffuses out of the blood in alveoli.

Smoking

• Mucus and cilia in the respiratory passages can be damaged by smoking.
• Smokers must cough to clear dirty mucus from the trachea without healthy cilia.
• Smoking has many negative outcomes like it can cause lung cancer, heart disease, and emphysema.
• Smoking increases the risk of heart attacks and strokes, raises blood pressure, and increases harmful types of cholesterol.
• Annually, in the USA, smoking causes ≈440,000 deaths.

Breathing

• Breathing is the alternating inhalation and exhalation of air (ventilation).
• Inhalation occurs when the rib cage expands, the diaphragm moves downward, and the pressure around lungs decreases causing air to be drawn into the respiratory tract.
• Exhalation occurs when the rib cage contracts, the diaphragm moves upward, the pressure around the lungs increases and air is forced out of the respiratory tract.

Breathing Control

• Breathing is usually under automatic control.
• Breathing control centers in the brain sense and respond to CO2 levels in the blood.
• A drop in blood pH increases the rate and depth of breathing.

Transport of Gases

• The heart pumps blood to two regions.
• The right side of the heart pumps oxygen-poor blood to the lungs.
• The left side pumps oxygen-rich blood to the body.
• Blood picks up O2 and drops off CO2 in the lungs.
• Blood drops off O2 and picks up CO2 in the body tissues.
• Gases move from areas of higher concentration to areas of lower concentration.

Hemoglobin

• Most animals transport O2 (oxygen) bound to proteins called respiratory pigments.
• Some animals use copper-containing pigments.
• Almost all vertebrates use iron-containing hemoglobin.
• Hemoglobin helps transport CO2 and buffers the blood.
• Most CO2 in the blood is transported in the plasma as bicarbonate ions.