The Alveoli and Gas Exchange

As you know friends diffusion is the movement of materials from a higher to a lower concentration. The differences between oxygen and carbon dioxide concentrations are measured by partial pressures.

Respiratory pigments increase the oxygen-carrying capacity of the blood. Humans have the red-colored pigment hemoglobin as their respiratory pigment. Hemoglobin increases the oxygen-carrying capacity of the blood between 65 and 70 times.

bronchioles alveoli air sacs

As you know breathing is an involuntary and automatic process, controlled by pns. It extracts oxygen from the air inspired and expel waste gases with the breath.

The air is inhaled through the nose then it heated and moistened and moves on to the pharynx, larynx and continues to penetrate into the trachea. In the middle of the chest, the trachea is divided into two bronchi which are divided again, again and again, bronchus in secondary, tertiary, and, finally, some 250,000 bronchioles.

At the end of the bronchioles are grouped in clusters of alveoli, small air sacs, where the exchange of gases in the blood. The lungs contain about 300 million alveoli, which deployed occupy an area of 70 square meters, about 40 times the size of the skin

ALVEOLI

Occurring only in the respiratory portion (which their presence distinguishes from the conducting portion), these small (about 200 um in diameter) sacs open into a respiratory bronchiole, an alveolar duct, an atrium, or an alveolar sac. They are separated by thin walls termed interalveolar (or alveolar) septa.

A. Interalveolar Septa: The structural features of these septa, which are specialized for gas exchange, are critical to respiratory function. The septa consist of 2 simple squamous epithelial layers with the interstitium sandwiched between them. The interstitium consists of continuous (nonfenestrated) capillaries embedded in an elastic connective tissue that includes elastic and collagen fibers, ground substance, fibroblasts, mast cells, macrophages, leukocytes, and contractile interstitial cells that contract in response to epinephrine and histamine. This elastic tissue is an important component of the ventilating mechanism. Gas exchange occurs between the air in the alveolar lumen and the blood in the interstitial capillaries.

1. Blood-air barrier. This term refers to the structures that oxygen and CO2, must cross to be exchanged. Varying from 0. 1-1.5 um in thickness, it includes the following layers: a. The film of pulmonary surfactant on the alveolar surface. b. The cytoplasm of the squamous cpithelial (type I alveolar) cells. c. The fused basal laminae sandwiched between the type I alveolar and capillary endothelial cells. d. The cytoplasm of the squamous endothelial cells lining the intcrstitial capillaries.

2. Alveolar pores. Each septum may be interrupted by one or more pores from 10 to 15 um in diameter. These connect adjacent alveoli and may help to equalize pressure and allow collateral air circulation, thus maximizing the use of available alveoli when some small airways are blocked.

GENERAL FEATURES OF THE RESPIRATORY SYSTEM

A. Components and Basic Functions of the Respiratory System: The respiratory system includes the lungs, airways tie, pharnyx, larynx, trachea, bronchi) and associated structures. Specialized for gaseous exchange between blood and air, including the uptake of oxygen and release of carbon dioxide, it is functionally divisible into 2 major parts: the conducting and respiratory portions.

1. Conducting portion. The walls of this system of tubes are specialized to carry air to and from the site of gas exchange without collapsing under the pressures created by the ventilating mechanism. This portion also conditions the air, warming, moistening, and cleaning it to enhance gas exchange. It includes the nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.


2. Respiratory portion. This portion is distinguished by alveoli, small, saccular structures whose thin walls enable the gas exchange between air and blood. Alveoli occur in clusters at the end of the bronchial tree. These clusters extend (like rooms from a hallway) from the walls of respiratory bronchioles, alveolar ducts, and atria and alveolar sacs.


B. Wall Structure: Like the digestive tract, the tubelike respiratory tract has layered walls whose lining epithelium derives from endoderm. The wall layers include an epithelium, a lamina propria that contains mucous glands as well as cartilage that prevent the tract from collapsing under pressure, smooth muscle that regulates the luminal diameter, and an adventitia that contains collagen and elastic fibers. Respiratory epithelium

a. General features. The epithelium lining most of the tract is ciliated pseudostratified columnar with goblet cells; it is generally referred to as respiratory epithelium. As the respiratory tract undergoes branching and its luminal diameter decreases, the epithelium gradually drops in height and loses first goblet cells and then cilia as it approaches the alveoli.

b. Epithelial cell types :

(1) Ciliated columnar cells predominate in the tract. Each has about 300 motile cilia on its apical surface; there are associated basal bodies in the apical cytoplasm.
(2) Mucous goblet cells are the second most numerous type. They secrete the mucus that covers the epithelium and traps and removes bacteria and other particles from inspired air. Cilia projecting from columnar cells sweep the contaminated mucus toward the mouth for disposal. (3) Brush cells. Also columnar, these cells lack cilia; they often have abundant apical microvilli. Two types are present: One resembles an immature cell and apparently serves to replace dead ciliated or goblet cells; the other has nerve endings on its basal surface and appears to be a sensory receptor.
(4) Basal cells. These small round cells lie on the basal lamina but do not reach the lumen. They appear to be stem cells that can replace the other cell types.

c. Metaplasia refers to the change in tissue organization or type undergone by epithelia in response to changes in the physical or chemical environment. For example, a smoker's respiratory epithelium typically develops more goblet cells in response to high pollutant levels and fewer ciliated cells in response to carbon monoxide. These changes, which are reversible, frequently cause congestion of the smaller airways.