Essential idea: The lungs are actively

ventilated to ensure that gas exchange

can occur passively.

Understandings:

• Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in

alveoli and blood flowing in adjacent capillaries.

• Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas

exchange.

• Type II pneumocytes secrete a solution containing surfactant that creates a moist surface

inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing

surface tension.

• Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.

• Muscle contractions cause the pressure changes inside the thorax that force air in and out of

the lungs to ventilate them.

• Different muscles are required for inspiration and expiration because muscles only do work

when they contract.

Applications and skills:

• Application: Causes and consequences of lung cancer.

• Application: Causes and consequences of emphysema.

• Application: External and internal intercostal muscles, and diaphragm and

abdominal muscles as examples of antagonistic muscle action.

• Skill: Monitoring of ventilation in humans at rest and after mild and vigorous

exercise. (Practical 6)

The entire process of gas exchange

between the atmosphere and body

cells is called respiration.

Events of respiration include:

Movement of air into and out of the lungs-commonly

called breathing or ventilation

Gas exchange between air and blood in the lungs

Gas transport in blood between the lungs and body

cells

Gas exchange between blood and body cells.

Oxygen utilization and carbon dioxide production at

the cellular level is called cellular respiration.

The flow of air in and out of the alveoli is called

ventilation and has two stages:

Inspiration (or inhalation) and Expiration (or

exhalation).

Lungs are not muscular and cannot ventilate

themselves, but instead the whole thorax moves and

changes size, due to the action of two sets of

muscles

• the intercostal muscles

• the diaphragm.

Ventilation

This is the diffusion of gases (oxygen and carbon

dioxide)

There are two sites for gas exchange

(a)Alveoli: Oxygen diffuses into the blood from the

alveoli and carbon dioxide diffuses from the blood

into the alveoli

(b)Tissues: Oxygen diffuses from blood into the cells

and carbon dioxide diffuses from cells to the blood

Gas Exchange

Aerobic respiration uses oxygen in the

mitochondria and produces carbon dioxide

Anaerobic respiration does not use oxygen

but still produces carbon dioxide

Cell Respiration

Functions of the

Respiratory System

1. Provides extensive gas exchange surface area between air and circulating blood

2. Moves air to and from exchange surfaces of lungs

3. Protects respiratory surfaces from outside environment

4. Produces sounds-speaking, singing, olfactory sense

A ventilation system is needed to maintain

concentration gradients in the alveoli

Oxygen can always diffuse down its

concentration gradient from the air to the blood

Carbon dioxide can diffuse down its

concentration gradient from the blood to the

air.

The need for a ventilation

system

Ventilation

• Ventilation – process of bringing in fresh air and removing stale air

• Intercostal muscles in the rib cage and diaphragm increase and decrease the size of the lung cavity, causing you to inhale and exhale

• Remember gas laws

– Increased volume = decreased pressure

– Decreased volume = increased pressure

Inhalation (Inspiration)

• External intercostal muscles between ribs

contract, pushing ribcage out and up.

• Diaphragm contracts, pushes downward, and

enlarges the thoracic cavity.

• Pressure inside thoracic cavity drops below

atmospheric pressure

• Air flows into lungs until lung pressure rises

above atmospheric pressure.

Exhalation (Expiration)

• Internal intercostal muscles contract, pulling

ribcage down and in.

• Abdominal muscles contract

• Diaphragm relaxes, pushes up and shrinks

thoracic cavity

• Pressure in thoracic cavity rises above

atmospheric pressure.

• Air flows out of lungs until lung pressure

falls below atmospheric pressure.

Quick Review – Inhalation

and Exhalation

• Air moves from HIGH to LOW pressure

Inhalation:

• Increasing lung volume = drop in pressure

– therefore air rushes into the lungs

Exhalation:

• Decreasing lung volume = increase in pressure

– Therefore air is pushed out of the lungs

Organs of the respiratory

system

• Nose / mouth

• Nasal cavity

• Pharynx

• Epiglottis

• Larynx

• Trachea

• Bronchus (bronchi)

• Bronchioles

• Alveoli

• Pleura

• Diaphragm

Lungs

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

How Does the Human

Respiratory System Work?

The human respiratory system can

be divided into two parts

– The conducting portion, a series of

passageways that carry air into and

out of the gas-exchange portion of the

respiratory system

– The gas-exchange portion, where

gases are exchanged with the blood in

tiny sacs within the lungs

The conducting portion of the respiratory system

carries air to the lungs

– The 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 a chamber

called the pharynx

• It then travels to the larynx, or “voice box,” where

sounds are produced (vocal cords)

• The opening to the larynx is guarded by the

epiglottis, a flap of tissue supported by cartilage

How Does the Human

Respiratory System Work?

– Inhaled air travels past the larynx into the

trachea, a flexible tube whose walls are

reinforced with semicircular bands of stiff

cartilage

– The trachea splits into two bronchi, one leading

to each lung

– Inside the lung, each bronchus branches

repeatedly into ever small tubes called

bronchioles

– Bronchioles lead to microscopic alveoli, the tiny

air sacs where gas exchange occurs

How Does the Human

Respiratory System Work?

Alveoli – cup shaped structures at the end of the

bronchioles that resemble bunches of grapes; are in

direct contact with capillaries (gas exchange);

covered with SURFACTANT that keep them from

collapsing

(air in

alveolus)

(extracellular

fluid)

alveolar

wall

surfactant

fluid

red

blood

cells

hemoglobin

O2

O2

O2 capillary

walls

(plasma) cells of

body tissues

respiratory

membrane

Oxygen Transport

CO2

CO2

CO2

CO2

CO2

CO2 CO2

CO2

CO2

CO2

+ H2O

H2O

+

H+

H+ HCO3–

1

2

3

4

5 HCO3–

HCO3–

Carbon Dioxide

Transport

Pneumocytes

Type I Pneumocytes – Form the lining of 90% of the alveolar surface

– Gas exchange

Type II Pneumocytes – Pulmonary Surfactant – is the fluid secreted that

spreads over the alveolar surface

– Moist surface inside the alveoli to prevent the

sides of the alveolus adhering to each other by

reducing surface tension

Alveolar Adaptations

Alveoli are structurally adapted for gas

exchange:

1. Covered by dense network of capillaries

with high CO2 and low O2 concentrations

– Creates the concentration gradient necessary for

diffusion

2. Thin walls (one cell thick) ensure gases

only diffuse a short distance into capillaries

3. Moist inner surface allows gases to

dissolve and prevents alveoli from

sticking together.

4. Millions of alveoli provide adequate

surface area for exchange.

Alveolar Adaptations

Diffusion of gases

Concentration gradient & pressure drives

movement of gases into & out of blood at both

lungs & body tissue

blood lungs

CO2

O2

CO2

O2

blood body

CO2

O2

CO2

O2

capillaries in lungs capillaries in muscle

Gas Exchange and Partial

Pressure

• Diffusion of gas

always occurs down

the partial pressure

gradient

• O2 tends to move

toward tissues and

CO2 tends to move

toward air.

PO2

(mm Hg)

PCO2

(mm Hg)

Air 152 3

Lungs 100 40

Blood 40 46

Tissues <40 >46

Gas Exchange

and Partial

Pressure

• In Lungs: – Lung PO2

> blood PO2, so

O2 diffuses into blood

• In Tissues: – Tissue PO2

< blood PO2,

so O2 diffuses into tissue

• Process is reversed for CO2

Animation

Breathing rate is controlled by the respiratory center of

the brain

– Breathing rate can be 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