PULMONARY VENTILATION

The process of gas exchange in the body, called respiration, has three basic steps :

1. Pulmonary ventilation (pulmon- = lung), or breathing, is the inhalation (inflow) and exhalation (outflow) of air between the atmosphore and the alveoli of the lungs.
2. External (pulmonary) respiration is the exchange of gases between the alveoli of the lungs and the blood in pulmonary capillaries across the respiratory membrane. In the process, pulmonary capillary blood gains O₂ and loses CO_2­.
3. Internal (tissue) respiration is the exchange of gases between blood in systemic capillaries and tissue cells. In this step the blood loses O₂ and give off CO₂ during the production of ATP are termed cellular respiration .

In pulmonary ventilation, air flows between the atmosphere and the alveoli of the lungs because of alternating pressure differences created by contraction and relaxation of respiratory muscles. The rate of airflow and the amount of effort needed for breathing is also influenced by alveolar surface tension compliance of the lungs, and airway resistance.

Blood Supply to the Lungs

The lungs receive blood via two sets of arteries : pulmonary arteries and bronchial arteries. Deoxygenated blood passed through the pulmonary trunk, which divides into a left pulmonary artery that enters the left lung and a right pulmonary artery that enters the right lung. (The pulmonary arteries are the only arteries in the body that carry deoxygenated blood). Return of the oxygenated blood to the heart occurs by way of the four pulmonary veins, which drain into the left atrium.

A unique feature of pulmonary blood vesels is their constriction in response to localized hypoxia (low O₂ level). In all other body tissues, hypoxia diverts pulmonary blood from poorly ventilated areas of the lungs to well ventilated regions. This phenomenon is known as ventilation perfusion coupling because the perfusion (blood flow) to each area of the lungs matches the extent of ventilation (airflow) to alveoli in that area.

Bronchial arteries, which branch from the aorta, deliver oxygenated blood to the lungs. This blood mainly perfuses the walls of the bronchi and bronchioles. Connections exist between branches of the bronchial arteries and branches of the pulmonary arteries, however, and most blood returns to the heart via pulmonary veins. Some blood, however, drains into bronchial veins, branches of the azygos system, and returns to the heart via the superior vena cava.

Alveoli

Around the circumference of the alveolar ducts are numerous avlveoli and alveolar sacs. Aan alveolus (al-VĒ-ō-lus) is a cup shaped outpouching lined by simple squamos epithelium and support by a thin basement membrance; an alveolar sac consist of two or more alveoli that share a common opening . The walls of alveoli consist of two types of alveolar epithelial cells .

The more numerous type I alveolar cells are simple squamous epithelial cells that form a nearly continuous lining of the alveolar wall. Type II alveolar cells, also called septall cells, are fewer in number and are found between type I alveolar cells. Then thin type I alveolar cells are the main sites of gas exchange. Type II alveolar cells, rounded or cuboidal epithelial cells with free surfaces containing microvilli, secrete alveolar fluid, which keeps the surface between the cells and their air moist. Included in the alveolar fluid is surfactant (sur-FAK-tant), a complex mixture of phospholipids and lipoproteins. Surfactant lowers the surface tension of alveolar fluid, which reduces the tendency of alveoli to collapse (described later).

Asscciated with the alveolar wall are alveolar macrophages (dust cells), phagocytes that remove fine dust particles and other debris from the alveolar spaces. Also present are fibroblasts that produce reticular and elastic fibers. Underlying the layer of type I alveolar cells is an elastic basement membrane. On the outer surface of the alveoli, the lobule’s arteriole and venule disperse into a network of blood capillaries (see Figure 23.11a) that consist of a single layer of endothelial cells and basement membrane.

The exchange of O₂ and CO₂ between the air spaces in the lungs and the blood takes place by diffusion across the alveolar and capillary walls,which together form the respiratory membrane. Extending from the alveolar air space to blood plasma, the respiratory membrane consists of four layers :

1. A layer of type I and II alveolar cells and associated alveolar macrophages that constitutes the alveolar wall.
2. An epithelial basement membrane underlying the alveolar wall.
3. A capillary basement membrane that is often fused to the epithelial basement membrane.
4. The capillary endothelium

Despite having several layers, the respiratory membrane is very thin- only 0.5 µm thick, about one-sixteenth the diameter of a red blood cell – to allow rapid diffusion of gases. It has been estimated that the lungs contain 300 million alveoli, providing an immense surface area of 70m­­²(750ft²) – about the size of a racquetball court – for gas exchange.

Google Set To Change Ranking Algorithm

By Jason Lee Miller - Tue, 04/21/2009

Blackhat SEO spammers force Google’s hand

Google is set to make changes to its search ranking algorithm to combat the spate of links leading to malicious web pages appearing at the top of Google’s search results, according to an inside source.

Over the past few months, cybercriminals have been using blackhat SEO techniques to manipulate search rankings. When it first began, they were marginally successful at following Google Trends to find buzzy search queries and elevating a newly created targeted webpage.

But after a short period of time, these same gangs appear to have become disturbingly effective. Last week, when researching a news story, I found the top five results all led to fake scareware pages.

Obviously if Google fails to do something about this manipulation, users will lose trust and the good ole days of Google will be over fast. A Googler speaking on condition of anonymity told WebProNews a ranking change is pending that tackles spam of this kind. Once the change goes live, users shouldn’t see it “nearly as often.”

A report from security company PandaLabs identified over a million links targeting malicious webpages ranking for auto part searches. Google noted that many of the phrases mentioned in the report were rare. A phrase like [1989 Nissan Pickup Truck Engine Check Light Troubleshooting], for example, only appears on attack sites set up by spammers, which explains why Google brought back so many attack sites in response to it and similar queries.

Google's response seems also an admission of how difficult it is to provide fresh, timely search results while simultaneously combating spammers. Part of the appeal of Twitter to many people is the platform’s ability to provide real-time information; the live Web works remarkably well there so far because Twitter’s set up isn’t very conducive to spam (yet). At least Twitter has to some extent control over accounts.

Google, on the other hand, cannot control for content appearing on the Web at large, and historically its famous algorithm performed better than any other at weeding out spammy webpages and malicious results. Unfortunately, that was a version of the Web that was more static. The live Web presents entirely new challenges manifesting as the first major weakness the search engine has faced.

The company naturally didn’t have a comment on the recently pondered “link velocity” ranking factor. Search engine optimization experts have identified the speed at which organic links appear as a possible important influence.

Link velocity therefore aids in explaining how blackhatters were able to manipulate search results by dropping enormous amounts of link spam into comment and discussion areas of social sites. The freshness or buzzy nature of a query also aided in this pursuit, and cybercriminals merely have to follow Google Trends and Google News to know which keywords and phrases to target.

source : www.webpronews.com