NOTES FOR BIOLOGY 1002

SECTIONS 004, 005, 006

Spring 2006

DR. STEVEN POMARICO

CHAPTER 40

RESPIRATION

The process cellular respiration in organisms (what the mitochondria do) requires O₂ and generates CO₂ as a waste product

Because this cellular respiration is where animals generate most of their ATP (energy) it’s important to keep this process going.

In order to keep the supply of O₂ flowing to the cells and remove the CO₂ waste respiration is required.

In most animals, the O₂ and CO₂ are transported in the blood by bulk flow and exchange into and out of the cells by diffusion.

Gas exchange between blood and external environment (air or water) also occurs by diffusion.

The respiratory system provides a large, moist surface area so that blood and the external environment may be exchange O₂ and CO₂ by diffusion.

The Evolution of Respiratory Systems

1. Respiration without a respiratory or circulatory system (See fig 40.4).

Respiration without specialized respiratory structures occurs in small, thin animals in moist environments. (Example: nematodes or flatworms, each have a relatively large, moist external surface area for gas exchange)

2. Respiration without a respiratory system but with a circulatory system

Earthworms have well-developed closed circulatory systems to carry gases throughout their bodies but still rely on gas exchange by diffusion in and out of the circulatory system through the moist skin surface.

3. Respiration with a respiratory and a circulatory system

Respiratory systems and gas exchange use two processes:

         Bulk flow

         Diffusion

Respiration using gills - aquatic animals.

The gills contain a dense profusion of capillaries inside the filaments (See fig 40.6)

In fish, a flap called the operculum covers the gills.

Fish create a continuous current past the gills by gulping water into the mouth and sending it out the opercular openings.

Gills are too delicate for land animals. They would dry out and collapse.

Land animals have evolved 3 solutions:

         -book lungs

         -tracheae

         -lungs

We’ve seen two of these before in arthropods. Book lungs in arachnids and tracheae in insects.

Lungs are delicate, moist, respiratory chambers deep within the body in land snails and vertebrates.

In the vertebrates lungs first evolved in freshwater fish as outpockets of the digestive tract.

Some amphibians use gills and lungs at different stages of their life cycles (frog tadpoles vs. terrestrial adults).

However reptiles have less of a connection with water and also have better developed lungs.

Birds and mammalian respiratory system are even more developed but remain simple in design.

The mammalian respiratory systems has only 2 parts

         -Conducting portion: passageways that carry air to and from the lungs

         -Gas exchange portion: sacs called alveoli.

The conducting portion carries air to the lungs (and CO₂ from the lungs).

Air flow (See fig 40.10):

Through the mouth and nose > pharynx > larynx with vocal cords > trachea > bronchi > Lungs with repeated smaller branches called bronchioles > Microscopic alveoli (tiny air pockets where gas exchange occurs)

The pharynx is the chamber where the paths from the mouth and nose meet.

The larynx is the first part of the pathway that is exclusively for gas flow (no food).

The larynx also houses the vocal cords that create sound when exhaled air causes them to vibrate.

The trachea is a rigid tube leading to the lungs; this tube is designed with cartilage rings for support and flexibility.

The trachea splits into 2 large branches called bronchi (singular is bronchus) which in turn each split into many smaller bronchioles.

As air moves towards the lungs it is warmed and moistened. Most dust and microbes are trapped in mucous that is secreted from cells lining the airway.

The mucus is swept towards the pharynx by ciliated cells of the bronchioles, bronchi, and trachea (smoking interferes with this process).

In the pharynx, mucus is coughed up or swallowed into the digestive tract.

The gas exchange portion occurs in the alveoli. (See fig 40.10)

The alveoli cluster about the end of each bronchiole like a bunch of grapes. The clusters of alveoli are surrounded by capillary beds.

Because we’ve gotten back to small distances, diffusion is the process of gas exchange.

O₂ diffuses from the moist air through the one-cell thick alveoli and capillary walls into the bloodstream (which is low in O₂) where it is picked up by red blood cells.

CO₂ does the reverse.

Each lung has 2.5 million microscopic (0.2 mm) alveolar chambers, providing about 75 m² (800 ft²) of surface area for diffusion.

Emphysema is a condition in which the alveoli become brittle and rupture, causing decreased area for gas exchange

Carbon monoxide (CO) can fool hemoglobin and bind in place of O₂ and about 200 times as tighter than O_2.

The resulting hemoglobin is incapable of transporting oxygen.

The mechanics of breathing (See fig 40.12).

Inhalation: air is actively inhaled.

Exhalation: air is passively exhaled.

Inhalation is accomplished by making the chest cavity larger.

The diaphragm muscle is a dome-shaped muscle forming the floor of the chest cavity. The lungs are surrounded on the sides and top by the pleural membrane.

During inhalation, the diaphragm contracts, drawing downward. At the same time the intercostal muscles between the ribs also contract, lifting the ribs upward and outward.

The lungs expand within the chest cavity due to a vacuum created between lungs and chest cavity walls.

Exhalation occurs when diaphragm and intercostal muscles relax.

Control of breathing

The nerve impulses to move the diaphragm and the intercostal muscles originate in the respiratory center of the brainstem.

Receptor neurons in the brainstem monitor CO₂ concentration in the blood. If CO₂ levels rise, breathing rate and depth increase.

The respiratory center is regulated to maintain a constant level of CO₂ in the blood.

The respiratory center is much less sensitive to changes in the O₂ level in the blood stream. When blood O₂ levels fall, receptors in the aorta and carotid arteries stimulate the breathing center to increase breathing rate.

When the brain activates muscles during heavy exercise, it also stimulates the breathing center to increase breathing rate.