Basic Human Biology and Target Organs PDF

**BASIC HUMAN BIOLOGY AND TARGET ORGANS** **4.1 RESPIRATORY SYSTEM** **4.1.1 Structure of the respiratory system** The respiratory system consists of the airways, the lungs and the respiratory muscles that mediate the movement of air into and out of the body. Inhaled air passes from the nose and...

**BASIC HUMAN BIOLOGY AND TARGET ORGANS** **4.1 RESPIRATORY SYSTEM** **4.1.1 Structure of the respiratory system** The respiratory system consists of the airways, the lungs and the respiratory muscles that mediate the movement of air into and out of the body. Inhaled air passes from the nose and mouth through the trachea and into the branched structures of the lungs called bronchi. Air then travels along the bronchioles to its ending (the terminal bronchiole) which is covered in tiny multi lobed sacs called alveoli where most of the gas exchange occurs. (Source: Tranter 1999 -- *Reproduced with permission*) ***Figure 4.1 -- Respiratory System*** From an occupational perspective, inhalation is usually the main route of absorption of hazardous materials. The lungs have a very large surface area and an excellent blood supply, both of which facilitate absorption. The 23 barrier between inhaled air and the systemic circulation is very thin therefore absorption may be very rapid. In addition to substances being absorbed through the lungs, insoluble particles such as silica and asbestos can lead to lung damage. The respiratory system can be broadly divided into three areas, and materials in different physical states will be absorbed in different regions; - Naso-pharynx or head airways region - Tracheo-bronchiolar region - Alveolar region or deep lung The term 'upper respiratory tract' is often used -- this corresponds to the first two areas above. **The naso-pharynx** region consists of the nose, mouth and pharynx (throat). Nasal hairs can filter out large particles and the nasal cavity is lined with a moist mucous membrane which can also trap foreign particles. Small surface hairs called cilia move the trapped particles towards the nose to be sneezed out. In addition, the mucous membranes moisten and warm the air as it is inhaled. This process is enhanced by the nasal turbinates which divide the main pathways into a number of separate channels thus increasing the area of mucous membrane in contact with the air. It should be noted that breathing through the mouth by-passes the nasal filter and the nasal cavity. If irritant substances penetrate to the throat a coughing reflex is initiated. **The tracheobronchiole** region consists of the trachea, which then branches into the two primary bronchi, which further divide into progressively finer bronchi and bronchioles. With each division the total cross sectional area of the air passages increases and the velocity of the air movement reduces. By the time the air has reached the terminal bronchioles the flow rate has reduced to very low levels meaning that 24 diffusion down concentration gradients becomes the main transport mechanism. This has consequences for particle deposition which will be examined later. ----------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------- ------------ ***Table 4.1 -- showing the cross-sectional area of the airways, and rate of airflow (for a volume flow of 100 millilitre per second)*** Area (cm2) Flow rate (cm / sec) Trachea 2.0 50 Terminal bronchioles 80 1.25 Respiratory bronchioles 280 0.36 Alveoli 10 -- 20 x 105 Negligible ----------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------- ------------