# Anatomy and Physiology of Animals/Respiratory System

!original image by Zofia
P cc
by{width="400"}

## Objectives

After completing this section, you should know:

-   why animals need energy and how they make it in cells
-   why animals require oxygen and need to get rid of carbon dioxide
-   what the term gas exchange means
-   the structure of alveoli and how oxygen and carbon dioxide pass
    across their walls
-   how oxygen and carbon dioxide are carried in the blood
-   the route air takes in the respiratory system (i.e. the nose,
    pharynx, larynx, trachea, bronchus,

:   bronchioles, alveoli)

-   the movements of the ribs and diaphragm to bring about inspiration
-   what tidal volume, minute volume and vital capacity are
-   how the rate of breathing is controlled and how this helps regulate
    the acid-base balance of the blood

## Overview

!**Diagram 9.1**: Alveoli with blood
supply
Animals require a supply of energy to survive. This energy is needed to
build large molecules like proteins and glycogen, make the structures in
cells, move chemicals through membranes and around cells, contract
muscles, transmit nerve impulses and keep the body warm. Animals get
their energy from the large molecules that they eat as food. Glucose is
often the energy source but it may also come from other carbohydrates,
as well as fats and protein. The energy is made by the biochemical
process known as **cellular respiration** that takes place in the
**mitochondria** inside every living cell.

The overall reaction can be summarised by the word equation given below.

**Carbohydrate Food (glucose) + Oxygen = Carbon Dioxide + Water +
energy**

As you can see from this equation, the cells need to be supplied with
**oxygen** and **glucose** and the waste product, **carbon dioxide**,
which is poisonous to cells, needs to be removed. The way the digestive
system provides the glucose for cellular respiration will be described
in Chapter 11 (\"The Gut and Digestion\"), but here we are only
concerned with the two gases, oxygen and carbon dioxide, that are
involved in cellular respiration. These gases are carried in the blood
to and from the tissues where they are required or produced.

Oxygen enters the body from the air (or water in fish)and carbon dioxide
is usually eliminated from the same part of the body. This process is
called **gas exchange**. In fish gas exchange occurs in the gills, in
land dwelling vertebrates lungs are the gas exchange organs and frogs
use gills when they are tadpoles and lungs, the mouth and the skin when
adults.

Mammals (and birds) are active and have relatively high body
temperatures so they require large amounts of oxygen to provide
sufficient energy through cellular respiration. In order to take in
enough oxygen and release all the carbon dioxide produced they need a
very large surface area over which gas exchange can take place. The many
minute air sacs or **alveoli** of the lungs provide this. When you look
at these under the microscope they appear rather like bunches of grapes
covered with a dense network of fine capillaries (see diagram 9.1). A
thin layer of water covers the inner surface of each alveolus. There is
only a very small distance -just 2 layers of thin cells - between the
air in the alveoli and the blood in the capillaries. The gases pass
across this gap by **diffusion**. 

## Diffusion And Transport Of Oxygen

!**Diagram 9.2**: Cross section of an
alveolus
The air in the alveoli is rich in oxygen while the blood in the
capillaries around the alveoli is deoxygenated. This is because the
haemoglobin in the red blood cells has released all the oxygen it has
been carrying to the cells of the body. Oxygen diffuses from high
concentration to low concentration. It therefore crosses the narrow
barrier between the alveoli and the capillaries to enter the blood and
combine with the haemoglobin in the red blood cells to form
**oxyhaemoglobin**.

The narrow diameter of the capillaries around the alveoli means that the
blood flow is slowed down and that the red cells are squeezed against
the capillary walls. Both of these factors help the oxygen diffuse into
the blood (see diagram 9.2).

When the blood reaches the capillaries of the tissues the oxygen splits
from the haemoglobin molecule. It then diffuses into the tissue fluid
and then into the cells. 

## Diffusion And Transport Of Carbon Dioxide

Blood entering the lung capillaries is full of carbon dioxide that it
has collected from the tissues. Most of the carbon dioxide is dissolved
in the plasma either in the form of **sodium bicarbonate** or **carbonic
acid**. A little is transported by the red blood cells. As the blood
enters the lungs the carbon dioxide gas diffuses through the capillary
and alveoli walls into the water film and then into the alveoli. Finally
it is removed from the lungs during breathing out (see diagram 9.2).
(See chapter 8 for more information about how oxygen and carbon dioxide
are carried in the blood).

## The Air Passages

When air is breathed in it passes from the nose to the alveoli of the
lungs down a series of tubes (see diagram 9.3). After entering the nose
the air passes through the **nasal cavity**, which is lined with a moist
membrane that adds warmth and moisture to the air as it passes. The air
then flows through the **pharynx** or throat, a passage that carries
both food and air, to the **larynx** where the voice-box is located.
Here the passages for food and air separate again. Food must pass into
the oesophagus and the air into the windpipe or **trachea**. To prevent
food entering this, a small flap of tissue called the **epiglottis**
closes the opening during swallowing (see chapter 11). A reflex that
inhibits breathing during swallowing also (usually) prevents choking on
food.

The trachea is the tube that ducts the air down the throat. Incomplete
rings of cartilage in its walls help keep it open even when the neck is
bent and head turned. The fact that acrobats and people that tie
themselves in knots doing yoga still keep breathing during the most
contorted manoeuvres shows how effective this arrangement is. The air
passage now divides into the two **bronchi** that take the air to the
right and left lungs before dividing into smaller and smaller
**bronchioles** that spread throughout the lungs to carry air to the
alveoli. Smooth muscles in the walls of the bronchi and bronchioles
adjust the diameter of the air passages.

The tissue lining the respiratory passages produces**mucus** and is
covered with miniature hairs or **cilia.** Any dust that is breathed
into the respiratory system immediately gets entangled in the mucous and
the cilia move it towards the mouth or nose where it can be coughed up
or blown out.

## The Lungs And The Pleural Cavities

!**Diagram 9.3**: The respiratory
system
The lungs fill most of the chest or **thoracic cavity**, which is
completely separated from the abdominal cavity by the **diaphragm**. The
lungs and the spaces in which they lie (called the **pleural cavities)**
are covered with membranes called the **pleura**. There is a thin film
of fluid between the two membranes. This lubricates them as they move
over each other during breathing movements.

### Collapsed Lungs

The pleural cavities are completely airtight with no connection with the
outside and if they are punctured by accident (a broken rib will often
do this), air rushes in and the lung collapses. Separating the two lungs
is a region of tissue that contains the oesophagus, trachea, aorta, vena
cava and lymph nodes. This is called the **mediastinum**. In humans and
sheep it separates the cavity completely so that puncturing one pleural
cavity leads to the collapse of only one lung. In dogs, however, this
separation is incomplete so a puncture results in a complete collapse of
both lungs. 

## Breathing

!**Diagram 9.4a**: Inspiration; **Diagram 9.4b**:
Expiration
The process of breathing moves air in and out of the lungs. Sometimes
this process is called **respiration** but it is important not to
confuse it with the chemical process, **cellular respiration**, that
takes place in the mitochondria of cells. Breathing is brought about by
the movement of the diaphragm and the ribs.

### Inspiration

The diaphragm is a thin sheet of muscle that completely separates the
abdominal and thoracic cavities. When at rest it domes up into the
thoracic cavity but during breathing in or **inspiration** it flattens.
At the same time special muscles in the chest wall (external intercostal
muscles) move the ribs forwards and outwards. These movements of both
the diaphragm and the ribs cause the volume of the thorax to increase.
Because the pleural cavities are airtight, the lungs expand to fill this
increased space and air is drawn down the trachea into the lungs (see
diagram 9.4a).

### Expiration

**Expiration** or breathing out consists of the opposite movements. The
ribs move down and in and the diaphragm resumes its domed shape so the
air is expelled (see diagram 9.4b). Expiration is usually passive and no
energy is required (unless you are blowing up a balloon).

### Lung Volumes

!**Diagram 9.5**: Lung
volumes
As you sit here reading this just pay attention to your breathing.
Notice that your in and out breaths are really quite small and gentle
(unless you have just rushed here from somewhere else!). Only a small
amount of the total volume that your lungs hold is breathed in and out
with each breath. This kind of gentle "at rest" breathing is called
**tidal breathing** and the volume breathed in or out (they should be
the same) is the **tidal volume** (see diagram 9.5). Sometimes people
want to measure the volume of air inspired or expired during a minute of
this normal breathing. This is called the **minute volume**. It could be
estimated by measuring the volume of one tidal breath and then
multiplying that by the number of breaths in a minute. Of course it is
possible to take a deep breath and breathe in as far as you can and then
expire as far as possible. The volume of the air expired when a maximum
expiration follows a maximum inspiration is called the **vital
capacity** (see diagram 9.5).

### Composition Of Air

The air animals breathe in consists of 21% oxygen and 0.04% carbon
dioxide. Expelled air consists of 16% oxygen and 4.4% carbon dioxide.
This means that the lungs remove only a quarter of the oxygen contained
in the air. This is why it is possible to give someone (or an animal)
artificial respiration by blowing expired air into their mouth.

Breathing is usually an unconscious activity that takes place whether
you are awake or asleep, although, humans at least, can also control it
consciously. Two regions in the hindbrain called the **medulla
oblongata** and **pons** control the rate of breathing. These are called
**respiratory centres**. They respond to the concentration of carbon
dioxide in the blood. When this concentration rises during a bout of
activity, for example, nerve impulses are automatically sent to the
diaphragm and rib muscles that increase the rate and the depth of
breathing. Increasing the rate of breathing also increases the amount of
oxygen in the blood to meet the needs of this increased activity.

### The Acidity Of The Blood And Breathing

The degree of acidity of the blood (the **acid-base balance)** is
critical for normal functioning of cells and the body as a whole. For
example, blood that is too acidic or alkaline can seriously affect nerve
function causing a coma, muscle spasms, convulsions and even death.
Carbon dioxide carried in the blood makes the blood acidic and the
higher the concentration of carbon dioxide the more acidic it is. This
is obviously dangerous so there are various mechanisms in the body that
bring the acid-base balance back within the normal range. Breathing is
one of these homeostatic mechanisms. By increasing the rate of breathing
the animal increases the amount of dissolved carbon dioxide that is
expelled from the blood. This reduces the acidity of the blood.

### Breathing In Birds

Birds have a unique respiratory system that enables them to respire at
the very high rates necessary for flight. The lungs are relatively solid
structures that do not change shape and size in the same way as
mammalian lungs do. Tubes run through them and connect with a series of
air sacs situated in the thoracic and abdominal body cavities and some
of the bones. Movements of the ribs and breastbone or sternum expand and
compress these air sacs so they act rather like bellows and pump air
through the lungs. The evolution of this extremely efficient system of
breathing has enabled birds to migrate vast distances and fly at
altitudes higher than the summit of Everest.

## Summary

-   Animals need to breathe to supply the cells with **oxygen** and
    remove the waste product **carbon dioxide**.
-   The lungs are situated in the **pleural cavities** of the
    **thorax**.
-   **Gas exchange** occurs in the **alveoli** of the lungs that provide
    a large surface area. Here oxygen diffuses from the alveoli into the
    red blood cells in the capillaries that surround the alveoli. Carbon
    dioxide, at high concentration in the blood, diffuses into the
    alveoli to be breathed out.
-   **Inspiration** occurs when muscle contraction causes the ribs to
    move up and out and the diaphragm to flatten. These movements
    increase the volume of the pleural cavity and draw air down the
    respiratory system into the lungs.
-   The air enters the nasal cavity and passes to the **pharynx** and
    **larynx** where the **epiglottis** closes the opening to the lungs
    during swallowing. the air passes down the trachea kept open by
    rings of cartilage to the **bronchi** and **bronchioles** and then
    to the alveoli.
-   **Expiration** is a passive process requiring no energy as it relies
    on the relaxation of the muscles and recoil of the elastic tissue of
    the lungs.
-   The rate of breathing is determined by the concentration of carbon
    dioxide in the blood. As carbon dioxide makes blood acidic, the rate
    of breathing helps control the **acid/base balance** of the blood.
-   The cells lining the respiratory passages produce mucus which traps
    dust particles, which are wafted into the nose by cilia.

## Worksheet

Work through the Respiratory System
Worksheet to
learn the main structures of the respiratory system and how they
contribute to inspiration and gas exchange.

## Test Yourself

Then use the Test Yourself below to see how much you remember and
understand.

1\. What is meant by the phrase "gas exchange"?

2\. Where does gas exchange take place?

3\. What is the process by which oxygen moves from the alveoli into the
blood?

4\. Why does this process occur?

5\. How does the structure of the alveoli make gas exchange efficient?

6\. How is oxygen carried in the blood?

7\. List the structures that air passes on its way from the nose to the
alveoli:

8\. What is the function of the mucus and cilia lining the respiratory
passages?

9\. How do movements of the ribs and diaphragm bring about inspiration?
Circle the correct statement below.

:   a\) The diaphragm domes up into the thorax and ribs move in and down
:   b\) The diaphragm flattens and ribs move up and out
:   c\) The diaphragm domes up into the thorax and the ribs move up and
    out.
:   d\) The diaphragm flattens and the ribs move in and down

10\. What is the function of the epiglottis?

11\. What controls the rate of breathing?

/Test Yourself Answers/

## Websites

-   <http://www.biotopics.co.uk/humans/resyst.html> Bio topics

A good interactive explanation of breathing and gas exchange in humans
with diagrams to label, animations to watch and questions to answer.

-   <http://www.schoolscience.co.uk/content/4/biology/abpi/asthma/asth3.html>
    School Science

Although this is of the human respiratory system there is a good diagram
that gives the functions of the various parts as you move your mouse
over it. Also an animation of gas exchange and a quiz to test your
understanding of it.

-   <http://en.wikipedia.org/wiki/Lung> Wikipedia

Wikipedia on the lungs. Lots of good information on the human
respiratory system with all sorts of links if you are interested.

## Glossary

-   Link to
    Glossary

