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uitgebreide Samenvatting hoofdstuk 17 silverthorn human physiology
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Fleur sam. Ch 17Lungs have tremendous surface area for gas exchange. This is needed to supply
trillions of cells in the body with adequate amounts of O2.
Aerobic metabolism in cells depends on steady supply of O2 and nutrients from the
environment, coupled with the removal of CO2.
Distance limits diffusion rate, so most multicelled animals require specialized
respiratory organs associated with a circulatory system.
Respiratory organs take a variety of forms, but all possesses a large surface area
compressed into a small space.
Exchange surface must be thin and moist to allow gases to pass from air into
solution, and yet at the same time it must be protected from drying out as a result of
exposure to air.
Solution to prevent dehydration: internalized respiratory epithelium. Human lungs are
enclosed in the chest cavity to control their contact with the outside air.
Internalization creates a humid environment for gas exchange with the blood and
protects the delicate exchange surface from damage.
Internalized lungs challenge: how to move air between the atmosphere and an
exchange surface deep within the body? Air flow requires a muscular pump to create
pressure gradients. More complex respiratory systems therefore consist of 2
separate components: a muscle-driven pump and a thin, moist exchange surface. In
humans, the pump is the musculoskeletal structure of the thorax. The lungs
themselves consist of the exchange epithelium and associated blood vessels.
The 4 primary functions of the respiratory system are:
1. Exchange of gases between the atmosphere and blood. The body brings in O2 for
distribution to the tissues and eliminates CO2 waste produced by metabolism.
2. Homeostatic regulation of body pH. Lungs can alter body pH by selectively retaining
or excreting CO2.
3. Protection from inhaled pathogens and substances. Like all other epithelia that
contact the external environment, the respiratory epithelium is well supplied with
defense mechanisms to trap and destroy potentially harmful substances before they
can enter the body.
4. Vocalization. Air moving across the vocal cords creates vibrations used for speech,
singing, and other forms of communication.
In addition to those functions, the respiratory system is also significant source of
water loss and heat loss from the body. These losses must be balanced using
homeostatic compensations.
Exchange between environment and interior air spaces of the lungs, is the bulk flow
of air. It follows many of the same principles that govern the bulk flow of blood
through the cardiovascular system:
1. Flow takes place from regions of higher pressure to regions of lower pressure.
2. Muscular pump creates pressure gradients
3. Resistance to air flow is influenced primarily by the diameter of the tubes through
which the air is flowing.
Air and blood are both fluids. The primary difference between air flow in the
respiratory system and blood flow in the circulatory system is that air is less viscous,
compressible mixture of gases while blood is a noncompressible liquid.
,17.1 The respiratory system
Cellular respiration refers to the intracellular reaction of oxygen with organic
molecules to produce carbon dioxide, water and energy in the form of ATP.
External respiration is the movement of gases between the environment and the
body’s cell.
External respiration can be subdivided into four integrated processes:
1. The exchange of air between the atmosphere and the lungs: a process known as
ventilation or breathing. Inspiration (inhalation) is the movement of air into the lungs,
expiration (exhalation) is the movement of air out of the lungs. These mechanisms by
which ventilation takes place are collectively called the mechanics of breathing.
2. The exchange of O2 and CO2 between the lungs and the blood
3. The transport of O2 and CO2 by the blood.
4. The exchange of gases between blood and the cells.
External respiration requires coordination between the respiratory and cardiovascular
system. The respiratory system consists of structures involved in ventilation and gas
exchange:
1. Conducting system or passages, or airways, that lead from the external environment
to the exchange surface of the lungs.
2. Alveoli (singular alveolus), a series of interconnected sacs and their associated
pulmonary capillaries. These structures from the exchange surface where oxygen
moves from inhaled air to the blood and CO2 moves from the blood to air that’s about
to be exhaled.
3. Bones and muscles of the thorax (chest cavity) and abdomen that assist in
ventilation.
The respiratory system can be divided into two parts:
- the upper respiratory (mouth, nasal cavity, pharynx and larynx)
- the lower respiratory tract (trachea, 2 primary bronchi, their branches and lungs).
The lower respiratory tract is also known as the thoracic portion of the respiratory
system, bc it’s enclosed in the thorax.
Thorax is bounded by the bones of the spine and rib cage
and their associated muscles. Together, the bones and
muscles are called the thoracic cage. The ribs and spine
(chest wall) form the sides and top of the cage. A dome-
shaped sheet of skeletal muscle, the diaphragm forms the
floor.
Two sets of intercostal muscles (internal and external)
connect the 12 pairs of ribs. Additional muscles, the
sternocleidomastoids and the scalenes run from the head
and neck to the sternum and first two ribs.
Functionally, the thorax is a sealed container filled with three
membranous bags or sacs:
- the pericardial sac, contains the heart.
- the 2 pleural sacs, each surround a lung.
The esophagus and thoracic blood vessels and nerves pass
between the pleural sacs.
, The lungs consist of light, spongy tissue whose volume is mostly air-filled spaces.
These irregular cone-shaped organs nearly fill the thoracic cavity, with their bases
resting on the curved diaphragm. Semi-rigid conducting airways - the bronchi,
connect the lungs to the main airway, the trachea.
Each lung is surrounded by a double-walled pleural sac whose membranes line the
inside of the thorax and cover the outer surface of the lungs.
Each pleural membrane, or pleura, contains several layers of elastic connective
tissue and numerous capillaries.
The opposing layers of pleural membrane are held together by a thin film of pleural
fluid whose total volume is only about 25-30 mL in a 70-kg man.
The result is similar to an air-filled balloon (the lung) surrounded by a water-filled
balloon (the pleural sac).
The pleural fluid serves several purposes:
1. First, it creates a moist, slippery surface so that the opposing membranes can slide
across one another as the lungs move within the thorax.
2. Second, it holds the lungs tight against the thoracic wall.
A similar fluid bond between the 2 pleural membranes makes the lungs ‘’stick’’ to the
thoracic cage and holds them stretched in a partially inflated state, even at rest.
Air enters the upper respiratory tract through the mouth and nose passes into the
pharynx (= a common passageway for food, liquids, air) from pharynx into larynx
into trachea/windpipe primary bronchi bronchioles alveoli
The larynx contains the vocal cords, connective tissue bands that vibrate and
tighten to create sound when air moves past them.
Trachea is a semiflexible tube held open by 15-20 C-shaped cartilage rings. It
extends down into the thorax, where it branches (division 1) into a pair of primary
bronchi, 1 bronchus to each lung. Within the lungs, the bronchi branch repeatedly
(division 2-11) into progressively smaller bronchi.
Like the trachea, the bronchi are semirigid tubes supported by cartilage.
Within the lungs, the smallest bronchi branch to become bronchioles, small
collapsible passageways with walls of smooth muscle.
The bronchioles continue branching (12-23) until the respiratory bronchioles form a
transition between the airways and the exchange epithelium of the lung.
The diameter of the airways become progressively smaller from the trachea to the
bronchioles but as the individual airways get narrower, their numbers increase
geometrically.
As a result, the total cross-sectional area increases with each division of the airways.
The total cross-sectional area is lowest in the upper respiratory tract and greatest in
the bronchioles, analogous to the increase in cross-sectional area that occurs form
the aorta to the capillaries in the circulatory system.
Velocity of air flow is inversely proportional to total cross-sectional area of the
airways. This is similar to the velocity of blood flow through different parts of the
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