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The composition of air in the atmosphere and in the alveoli differs. In both cases, the relative concentration of gases is nitrogen>oxygen>water vapor>carbon dioxide. The amount of water vapor present in alveolar air is greater than that in atmospheric air ( [link] ). Recall that the respiratory system works to humidify incoming air, thereby causing the air present in the alveoli to have a greater amount of water vapor than atmospheric air. In addition, alveolar air contains a greater amount of carbon dioxide and less oxygen than atmospheric air. This is no surprise, as gas exchange removes oxygen from and adds carbon dioxide to alveolar air. Both deep and forced breathing cause the alveolar air composition to be changed more rapidly than during quiet breathing. As a result, the partial pressures of oxygen and carbon dioxide change, affecting the diffusion process that moves these materials across the membrane. This will cause oxygen to enter and carbon dioxide to leave the blood more quickly.

Composition and Partial Pressures of Alveolar Air
Gas Percent of total composition Partial pressure
(mm Hg)
Nitrogen (N 2 ) 74.9 569
Oxygen (O 2 ) 13.7 104
Water (H 2 O) 6.2 40
Carbon dioxide (CO 2 ) 5.2 47
Total composition/total alveolar pressure 100% 760.0

Ventilation and perfusion

Two important aspects of gas exchange in the lung are ventilation and perfusion. Ventilation is the movement of air into and out of the lungs, and perfusion is the flow of blood in the pulmonary capillaries. For gas exchange to be efficient, the volumes involved in ventilation and perfusion should be compatible. However, factors such as regional gravity effects on blood, blocked alveolar ducts, or disease can cause ventilation and perfusion to be imbalanced.

The partial pressure of oxygen in alveolar air is about 104 mm Hg, whereas the partial pressure of the oxygenated pulmonary venous blood is about 100 mm Hg. When ventilation is sufficient, oxygen enters the alveoli at a high rate, and the partial pressure of oxygen in the alveoli remains high. In contrast, when ventilation is insufficient, the partial pressure of oxygen in the alveoli drops. Without the large difference in partial pressure between the alveoli and the blood, oxygen does not diffuse efficiently across the respiratory membrane. The body has mechanisms that counteract this problem. In cases when ventilation is not sufficient for an alveolus, the body redirects blood flow to alveoli that are receiving sufficient ventilation. This is achieved by constricting the pulmonary arterioles that serves the dysfunctional alveolus, which redirects blood to other alveoli that have sufficient ventilation. At the same time, the pulmonary arterioles that serve alveoli receiving sufficient ventilation vasodilate, which brings in greater blood flow. Factors such as carbon dioxide, oxygen, and pH levels can all serve as stimuli for adjusting blood flow in the capillary networks associated with the alveoli.

Ventilation is regulated by the diameter of the airways, whereas perfusion is regulated by the diameter of the blood vessels. The diameter of the bronchioles is sensitive to the partial pressure of carbon dioxide in the alveoli. A greater partial pressure of carbon dioxide in the alveoli causes the bronchioles to increase their diameter as will a decreased level of oxygen in the blood supply, allowing carbon dioxide to be exhaled from the body at a greater rate. As mentioned above, a greater partial pressure of oxygen in the alveoli causes the pulmonary arterioles to dilate, increasing blood flow.

Questions & Answers

the diagram of cell membrane
Teboh Reply
***bit.ly/3kfrdfV Link to cell membrane diagram and more info... Remember, you can search the A&P book.
Favour
describe diffusion as used in body fluid movement
brenda Reply
how was the movement?
Jenelyn
classify joint function with example
James Reply
cytoskeleton diagram
Miriam Reply
two types of cells biological name
Miriam
eukaryotic and prokaryotic
Emraan
Difference between extracellular and intracellular
Eli Reply
extra (exit) outside the cell, intra inside the cell
Amy
extracellular is 1/3 of total body water...intracellular 2/3 of total body water
Favour
what is culture?
Nepi
is the way of life of people
Senior
He meant in the context of biology...as in "tissue culture".
Favour
what does depolarization mean...in terms of the cardiovascular system?
Tracy
depolarization means contraction
Abhishek
Depolarization, in the context of nerve and muscle tissue, is the decrease in membrane potential (the separation of charge between the extracellular and intracellular compartments). Basically, the charge in the cell becomes more positive when depolarization occurs.
Favour
In fact, the word 'polarize' used to mean dividing into extremes or opposite sides. The opposite is true for 'depolarize'
Favour
intracellular fluid is the fluid compartment of the body consisting of all water present within the cell...while extracellular fluid is water present out of the cell
brenda
Why apoptosis is important?
Duchess Reply
name the body organ which helps in the oxygenation of body.
Eli Reply
respiratory system (lungs) brings in oxygen...cardiovascular system (blood) circulate it
Favour
Lungs
Emil
the heart
Mpho
Mpho explain
Favour
cells... tissues...organs
Angel Reply
...organ system...organism...species...population...
Favour
connection between respiratory and circulatory system
Philip Reply
what's respiratory system
Philip
what is the site of CHON synthesis?
World
send them boi
alex
dont send via email...use google drive
Favour
Which major organ lies deep to the right hypochondriac region?
Biswajit Reply
deep as in most posterior kidney, or deep as in inferior small intestine?
Amy
liver
Emil
what is the weakest muscle in the body?
zharla Reply
stapedius
Isaac
Connective tissues composed with the blood
EDIGAR Reply
blood cell
nancy
Why is left bronchi narrow?
nancy
what is the most widely distributed connective tissue in the body?
Aila Reply
hai
SURENDRA
why most we study anatomy
Xavier Reply
hello
Aila
Anatomy it is a Latin word which mean to dissect mean to cut apart the study of body structure is called anatomy internel anatomy to study of internel structure of oganisms e.g frog dissection external anatomy/ Grass anatomy the study of external stucture of the organi the study of body
Ikram
Anatomy it is a Latin word which mean to dissect mean to cut apart the study of body structure is called anatomy internel anatomy to study of internel structure of oganisms e.g frog dissection external anatomy/ Grass anatomy the study of external stucture of the organi the study of body
Ikram
Anatomy it is a Latin word which mean to dissect mean to cut apart the study of body structure is called anatomy internel anatomy to study of internel structure of oganisms e.g frog dissection external anatomy/ Grass anatomy the study of external stucture of the orga the study of body
Ikram
hi
Ikram
hi Ikram
Favour
analyse the structure and function of the brainstem
Clinton Reply

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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