CHAPTER 32PAGES

Respiration

Why Exchange Gases?

The act of breathing is called respiration Cellular respiration converts the energy in nutrients

into the ATP used by cells, requires oxygen and generates carbon dioxide as waste

The circulatory system works in with the respiratory system

The circulatory system extracts oxygen from the air in your lungs, carries it within diffusing distance of each cell, then picks up carbon dioxide for release from the lungs

Cellular respiration depletes O2 levels, creating a concentration gradient that favors the diffusion of CO2 out of cells and the diffusion of O2 into them

Requirements for Diffusion

Animal respiratory systems are diverse but all meet three requirements that facilitate diffusion

1. Respiratory surfaces remain moist so gases can diffuse across cell membranes

2. Cells lining respiratory surfaces are very thin, to facilitate diffusion of gases through them

3. Respiratory systems have a sufficiently large area in contact with the environment to allow adequate gas exchange

Evolutionary Adaptations for Gas Exchange

Some animals in moist environments lack specialized respiratory structures

The outside of the body is covered by a thin, gas-permeable skin, which provides an adequate surface area for the diffusion of gases

If the body is small and elongated (microscopic roundworms) gases need to diffuse only a short distance to reach all cells

An animal’s body may be thin and flattened, (flatworms) most cells are close to the moist skin

Invertebrate Gas Exchange

The slow rate of gas exchange by diffusion may suffice for a larger, thicker bodied organism if energy demands are low, as sea jellies, which can be large but require little O22

Another adaptation for gas exchange involves bringing the watery environment close to each cell - Sponges circulate seawater through channels within their bodies

Earthworms

For O2 delivery to cells, some animals combine a large skin surface area with well-developed circulation

In earthworms, gases diffuse through moist skin and are distributed throughout the body by a circulatory system

Blood in the skin capillaries rapidly carries off O2 that has diffused through the skin, maintaining a concentration gradient that favors the inward diffusion of oxygen

The worm’s elongated shape ensures a surface area relative to its internal volume

Respiratory Systems

Facilitate gas exchange by diffusion

Most animals have evolved specialized respiratory systems that interface with circulatory systems to exchange gases between their cells and the environment

Transfer of gases between environment and body cells usually occurs in stages that alternate between bulk flow and diffusion

During bulk flow, liquids or gases move through large spaces, from areas of higher to lower pressure

This contrasts with diffusion, where molecules move individually from higher to lower concentrations

Gas Exchange Occurs in Stages

For animals with well-developed respiratory systems Air or water moves past a respiratory surface by bulk flow

(down a pressure gradient), this is usually facilitated by muscular movements such as breathing

O2 and CO2 are exchanged through the respiratory surface by diffusion O2 diffuse into the capillaries of the circulatory system CO2 diffuses out

Gases are transported between the respiratory system and tissues by the bulk flow of blood as it is pumped throughout the body

Gases are exchanged between tissues and the circulatory system by diffusion; at the tissue level, O2 moves out of the capillaries into tissues, and CO2 moves from the tissues to the capillaries

Gas Exchange in Mammals

O2

O2

CO2

alveoli(air sacs)

Oxygenated bloodDeoxygenated blood

Gases move in and out of the lungs by breathing

O2 and CO2 areexchanged in thelungs by diffusion

Gases dissolvedin the blood are transported by the circulatory system

2

3

O2

O2

1

CO2

left ventricle

leftatrium

O2 and CO2

are exchangedin the tissuesby diffusion

4

O2

rightatrium

rightventricle

CO2

CO2

CO2

CO2 CO2

O2

Gas Exchange in Aquatic Environments

Gills are the respiratory structures of some aquatic animals

The simplest gills (amphibian) are thin projections of the body surface that protrude into the surrounding water

Elaborately branched or folded to increase surface area, have dense profusion of capillaries

Fish Gills

Protected by a bony flap or operculum

Fish create a continuous current over their gills by pumping water into their mouths and ejecting it through the operculum

Countercurrent exchange - water and blood flow in opposite directions within the gill, maintaining a concentration gradient

Terrestrial Animals & Internal Structures

Internal respiratory structures are used by terrestrial animals to help keep the respiratory surfaces moist

Two examples are the tracheae in insects and lungs in vertebrates

Insects Respire Using Tracheae

Tracheae are elaborately branched internal tubes that deliver air to the body cells

Air enters tracheae though abdominal openings or spiracles

The spiracles open into tracheae that branch into smaller tubes (tracheoles), which deliver air close to each body cell for O2 and CO2 exchange

Some insects use abdominal contractions to enhance air movements into and out of spiracles

Insects Breathe Using Tracheae tracheae spiracles

air

air

spiracle

tracheae

tracheoles

(a) Insect respiratory system

(c) Gas exchange pathway(b) Spiracle and tracheae

bodycells

CO2O2O2

tracheae

spiracle

externalskeleton ofthe insect

Terrestrial Vertebrates Use Lungs

Lungs are chambers containing moist respiratory surfaces that are protected within the body, where water loss is minimized and the body wall provides support

The first lung probably developed to allow ancestral fish to survive in stagnant, oxygen-poor water

Amphibians use gills for respiration as aquatic larvae, and a simple, sac-like lung when they metamorphose into adult form

Reptiles and Mammals

Reptiles and mammals have relatively waterproof skin covered with scales, feathers, or fur – reducing water loss

This helps them survive in dry environments, but eliminates the skin as a respiratory organ

To compensate, the lungs of reptiles and mammals have a far larger surface area for gas exchange than do amphibians

Bird Lungs

Adaptations that allow exceptionally efficient gas exchange, providing O2 to support the demands of flight

Birds have 7-9 inflatable air sacs, which do not exchange gases but act as reservoirs

Bird lungs are rigid and filled with thin-walled tubes (parabronchi), that are open at both ends, allowing air to flow completely through the lungs

The parabronchi are surrounded by tissue riddled with microscopic spaces and a dense capillary network that allows gas exchange

Bird Lung Structure

The organization of the bird air sacs and lungs allows one-way flow of fresh, oxygenated air through the lungs, from posterior to anterior, both as the bird inhales and exhales

Inhalation inflates the air sacs, drawing fresh air to the posterior sacs via a route that bypasses the lungs

Air from the posterior air sac is pushed into the lungs, where O2 is extracted

Used air is pulled out of the lung, and as the bird exhales, the air sacs deflate, forcing the used air through the bird’s nostrils

Fresh air for the posterior sac then enters the lungs

Bird receives fresh air both when inhaling and when exhaling

The Bird Respiratory System

Human Respiratory System

Divided into two parts

Conducting portion, a series of passageways that carry air into and out of the gas-exchange portion of the respiratory system

Gas-exchange portion, where gases are exchanged with the blood in tiny sacs within the lungs

Conducting Portion

Carries air to the lungs and contains the apparatus that makes speaking possible

Air enters through the nose or mouth and passes through the nasal or oral cavity into the pharynx , travels to the larynx, or “voice box,”

The opening to the larynx is guarded by the epiglottis, a flap of tissue supported by cartilage which prevents food from entering the larynx when swallowing During normal breathing, the epiglottis is tilted upward,

allowing air to flow into the larynx During swallowing, the epiglottis folds downward and covers

the larynx, directing substances into the esophagus

(a) Human respiratory system (b) Alveoli with capillaries

bronchiole

pulmonary venule

alveoli

capillarynetwork

pulmonary veins

pulmonary arterydiaphragm

nasal cavity

pharynx

oral cavityepiglottis

larynx

esophagus

trachea

bronchi

bronchiolesrings ofcartilage

pulmonary arteriole

The Human Respiratory System

Don’t inhale and swallow at the same time.

The Heimlich maneuver

If an individual inhales and swallows at the same time, food can become lodged in the larynx, blocking air from entering the lungs

The use of the Heimlich maneuver clears the obstruction

Making Sound

Within the larynx are the vocal cords, bands of elastic tissue controlled by muscles

Muscular contractions cause the vocal cords to partially obstruct air passage through the larynx

Exhaled air causes the vocal cords to vibrate, producing the tones of speech or song

Stretching the cords changes the pitch of the tones,

which can be articulated into words by movements of the tongue and lips

Human Respiration Structure

Inhaled air travels past the larynx into the trachea, a flexible tube with walls are reinforced with semicircular bands of stiff cartilage

Trachea splits into 2 bronchi, one leading to each lung

Inside the lung, each bronchus branches repeatedly into smaller tubes called bronchioles

Bronchioles lead to microscopic alveoli, tiny air sacs where gas exchange occurs

Gas Exchange

Occurs in the alveoli Alveoli cluster at the end of each bronchioles

(think:grapes) , providing 1,500 square feet of surface area for diffusion A network of capillaries covers the alveolar surface The walls of the alveoli consist of a single thin layer

of epithelial cells The respiratory membrane, through which gases

diffuse, consists of epithelial cells of the alveoli and the endothelial cells that form the wall of the capillary, across which gas exchange occurs

Alveoli

Well adapted for gas exchange Alveolar walls and capillary walls are only one cell thick,

gases diffuse a short distance to move between the environment and blood

Alveoli are coated with a thin layer of watery fluid containing surfactant, which prevents the alveolar surfaces from sticking together and collapsing when air is exhaled

Gases dissolve in this fluid as they pass in and out of the alveolar air

Surfactant - compounds that lower the surface tension

from thepulmonaryartery

alveolarmembrane

respiratorymembrane

surfactantfluid

to the pulmonary vein

(air) CO2

O2

capillary

Oxygen diffuses intothe red blood cells

Carbon dioxide diffusesinto the alveolus

Gas Exchange Between Alveoli and Capillaries

Animation: Gas Exchange in the Lungs

How are O2 and CO2 Transported?

Oxygen and carbon dioxide are transported in blood using different mechanisms

Blood picks up oxygen from the air in the lungs and supplies it to the body tissues, simultaneously absorbing CO2 from the tissues and releasing it into the lungs

These exchanges occur because diffusion gradients favor them In the lungs, O2 is high and CO2 is low, whereas in body

cells, CO2 is high and O2 is low

Oxygen Transport

90% of O2 carried by the blood, bound to hemoglobin

Each hemoglobin molecule can carry up to four

O2 molecules, each bound to one of four iron-containing heme groups

As oxygen binds hemoglobin, the protein changes its shape, which alters its color Oxygenated blood is bright cherry-red Deoxygenated blood is maroon-red

(air inalveolus)

(extracellularfluid)

alveolarwall

surfactantfluid

redbloodcells

hemoglobin

(a) O2 transport from the lungs to the tissues

O2

O2

O2capillarywalls

(plasma)cells ofbody tissues

respiratorymembrane

Oxygen Transport

Animation: Oxygen Transport

Carbon Dioxide Transport

CO2 from cellular respiration in the body cells diffuses into nearby capillaries, then is carried in the bloodstream to the respiratory membranes of the alveoli

Alveolar capillaries have a higher CO2 concentration than that of the alveolar air

Thus, CO2 diffuses down a concentration gradient into the alveolar air, which is exhaled

Carbon Dioxide is Transported 3 Ways

As bicarbonate ions (70%) Bound to hemoglobin (20%) Dissolved in plasma as CO2 (10%)

Bicarbonate ions (HCO3–) are formed in red blood cells

when CO2 combines with water, using the enzyme carbonic anhydrase

CO2 + H2O CO2 + HCO3–

The reaction producing bicarbonate ions is reversed as the blood flows through capillaries surrounding the alveoli, where CO2 is low: H+ + HCO3

– CO2 + H2O

As CO2 leaves the blood and diffuses into the alveoli, it diffuses back into red blood cells, where it recombines with H+, regenerating CO2 and H2O

The CO2 then diffuses into the air in the alveoli, which is exhaled from the lungs while the H2O remains in the blood

(b) CO2 transport from the tissues to the lungs

CO2

CO2

CO2

CO2

CO2

CO2CO2

CO2

CO2

CO2

+H2O

H2O

+H+

H+ HCO3–

1

2

3

4

5 HCO3–

HCO3–

Carbon Dioxide Transport

Animation: Carbon Dioxide Transport

Inhalation, Exhalation

Air is inhaled actively and exhaled passively Breathing occurs in two stages

Inhalation, when air is drawn into the lungs Exhalation, when air is expelled from the lungs

Inhalation occurs when the chest cavity is enlarged The lower boundary of the chest cavity is formed by the

diaphragm, which domes upward when relaxed During inhalation, the diaphragm is contracted, which

pulls it downward, and the rib muscles contract, lifting the ribs up and outward

Exhalation

Exhalation occurs spontaneously, when the muscles that cause inhalation are relaxed

As the diaphragm relaxes, it domes upward; at the same time, the ribs fall down and inward

These movements decrease the size of the chest cavity and force air out of the lungs

Air moves in Air moves out

Rib cagecontracts Lungs

compress

Diaphragmrelaxes upward

Diaphragmcontracts downward

Rib cageexpands Lungs

expand

(a) Inhalation (b) Exhalation

The Mechanics of Breathing

Animation: Breathing Mechanism

Breathing Rate

Controlled by the respiratory center of the brain Located in the medulla portion of the brain, just above the

spinal cord Nerve cells in the respiratory center generate cyclic action

potentials that cause contractions (followed by passive relaxation) of respiratory muscles

The respiratory center receives input from sources and adjusts the breathing rate and volume to meet the body’s needs

Primarily modified by CO2 receptors located in the medulla that adjust the breathing rate to maintain a constant low level of CO2 in the blood, while also ensuring that O2 levels remain adequate

As a backup system, there are also O2 receptors in the aorta and carotid arteries that stimulate the respiratory center to increase the rate and depth of breathing if O2 levels in the blood drop