Respiratory system in mammals

1. RESPIRATORY
SYSTEM

2. •Most animals have body systems specialized to obtain
oxygen from the surroundings and convey it to all body
parts, while usually getting rid of waste carbon dioxide at
the same time. These are called respiratory systems.
Typically, they link to a circulatory system. This is based
on a circulating medium, usually a body fluid known as
blood, which conveys the oxygen. Rather than carrying
the oxygen in dissolved form, this fluid may contain
substances specialized to attach to oxygen, termed
respiratory pigments. The medium is kept flowing or
circulating by some form of muscular pump, commonly
called the heart.

3. •External respiration is the interchange of the
gases oxygen and carbon dioxide between the
surroundings and the organism’s body, often
called breathing. Internal respiration is the
interchange of gases taking place inside the
body, between the blood and tissues.
•In simpler aquatic animals the respiratory gases,
oxygen and carbon dioxide, diffuse in and out of
tissues near the surface, and the medium carries
them to and from more distant internal cells.

4. •More complex animals have, as part of the respiratory
system, specialized organs to increase the area of
exposure between the circulating fluid to the external
medium. These are often called gills in aquatic creatures
and lungs in land animals.

5. HUMAN PHYSICAL RESPIRATION (BREATHING)

6. •In humans, as in most other vertebrates, the lungs are
enclosed in the thorax or chest. The ribs support the body wall
of the thorax, which has a domed base formed by the
diaphragm muscle. The ribs slant downwards and forwards and
have intercostal muscles between them. The diaphragm and
intercostal muscles are the primary muscles of breathing or
physical respiration.
•When the ribs are raised by the action of the intercostal
muscles, the volume of the thorax is increased. The volume of
the thorax is also increased by downward contraction of the
muscles of the diaphragm. Within the thorax, the lungs are held
close to the body wall by atmospheric pressure. When the
thorax expands, the lungs also expand and become filled with
air drawn through the upper respiratory passages. This is
breathing in or inhaling. Relaxation of the breathing muscles
allows the springy, elastic tissues of the stretched lungs to
return to their naturally contracted position, forcing air out of
the lungs. This is breathing out or exhaling.

7. •Up to 500 cu cm (30 cu in) of air are usually
inhaled and exhaled with each breath. This is
called tidal volume. About 3,300 cu cm (80 cu in)
of additional air, called inspiratory reserve
volume, can be inhaled on a forced inspiration
and then exhaled; still another 1,000 cu cm,
called expiratory reserve volume air, can be
exhaled on a forced expiration. The sum of
these three quantities is called the vital capacity.
About 1,200 cu cm of air always remains in the
lungs and cannot be exhaled; this volume is
called the residual, or alveolar, air.

8. ORGANS OF RESPIRATION—THE LUNGS

9. •The human lungs are roughly pyramidal in
shape, conforming to the shape of the thorax.
They are not symmetrical. The right lung is
larger and consists of three lobes. The left lung
has two lobes and, near the medial (central)
edge of the base, a scooped-out cardiac notch
into which the heart fits. On the medial side of
each lung is the root, by which the lung is
attached to the mediastinum, or central partition
of the chest. The root consists of pleural folds,
bronchi (main airways), and pulmonary arteries
and veins.

10. •As the main airway or bronchus penetrates the
substance of the lung, it divides and subdivides
repeatedly until it ends in lobules, the structural and
functional units of the lung. Accompanying the bronchus,
the pulmonary arteries and veins divide at the same
points. Nerves from the pulmonary plexus and lymphatic
vessels are also distributed in the same manner. Within
the lobule, the bronchiole divides into terminal bronchi,
each thinner than a hair, which open into groups of
pulmonary atria, or air spaces. Each of the atria opens in
turn into a number of alveolar saccules, the walls of
which are pouched out to form the numerous alveoli, or
air sacs. There are about 350 million alveoli in each lung.
The arterioles and venules of the lobules are connected
by a dense network of capillaries that lie in the walls of
the alveoli. This structure brings air and blood very close
together over a huge surface area so that oxygen can be
absorbed and carbon dioxide given off.

11. •The principal nervous centre for controlling the rate and
depth of physical respiration is in the respiratory section
of the pons and medulla oblongata in the brain stem.
The cells of this nucleus are sensitive to the acidity of the
blood, which reflects the concentration of carbon dioxide
in the blood plasma. When the acidity of the blood is
high, usually caused by an excess of carbon dioxide, the
respiratory centre stimulates the respiratory muscles to
greater activity to “blow off” the excess carbon dioxide
and take in more oxygen. When the carbon dioxide
concentration is low, breathing is depressed (slower and/
or shallower).

12. PHYSICAL RESPIRATION IN MORE COMPLEX
ANIMALS
.

13. •Many aquatic animals carry on external respiration or breathing
by means of gills, over which auxiliary respiratory mechanisms
keep a constant current of fresh water flowing. The gills are
branched to such an extent as to resemble feathers or plumes. In
each branch fine blood vessels are subdivided so that the blood
is separated from the water medium by two layers of cells, one
being the wall of the fine blood vessel, or capillary, and the other
being the epithelium of the gill. Oxygen readily diffuses in and
carbon dioxide passes out. The extended surface produced by
the branching enables large quantities of blood to be oxygenated
in a short time.
•In air-breathing creatures such as the earthworm, external
respiration takes place through the capillaries in the skin.
Amphibians like the frog respire through both skin and lungs.
Insects breathe by means of air tubes, or tracheae, which open
on the outside of the body and branch out through the tissues,
carrying air to internal organs and structures. Reptiles and
mammals respire solely by means of lungs. Birds have auxiliary
air sacs in the body cavity and air spaces within certain bones,
all of which connect with the lungs and act as aids to respiration.

14. •The respiratory and circulatory systems of air-breathing animals
can also adapt and modify for life in low-oxygen environments,
such as high altitudes. For example, people living in the Andes at
altitudes of 3,000 m (10,000 ft) or more have larger lungs, more
highly branched capillary systems, and a faster heartbeat than
people living at lower altitudes. Moreover, the blood of high-
altitude dwellers contains 30 per cent more red cells than the
blood of people living at sea level; they are therefore able to
make efficient use of the available oxygen.
•Aquatic mammals generally have large, complex systems of
veins for the storage of blood. The blood volume of whales and
seals is up to 50 per cent greater per kilogram of body weight
than the blood volume of human beings, which enables them to
supply their tissues with oxygenated blood for a long period
without breathing. Whales may remain submerged from 15
minutes to more than an hour, depending on the species. The
elephant seal may stay underwater for 30 minutes. When a seal
begins an underwater dive, its heartbeat slows from 150 beats
per minute to 10; the oxygen content of the arterial blood is 20
per cent. When the oxygen level drops to nearly 2 per cent, the
seal must surface.