Respiration

The resspiratory system

The Alveoli

The lungs have about 300 million alveoli, with a total crosssec onal area of 50–70 m2.. Each alveolar sac is surrounded by blood capillaries. The walls of the sac is squamous epithelium. Gas exchange occurs between air in the alveoli and blood in the capillaries. Oxygen diffuses across the alveolar wall and enters the bloodstream,

and carbon dioxide diffuses from the blood across the alveolar wall to enter the alveoli The alveoli of human lungs are lined with a surfactant,a fi lm of lipoprotein that lowers the surface tension of water and prevents the alveoli from closing. The lungs collapse in some newborn babies—especially premature infants—who

lack this fi lm. The condition, called infant respiratory distress syndrome,

Inspiration

Inspiration is the active phase of ventilation because this is the phase in which the diaphragm and the external intercostal muscles contract. In its relaxed state,the diaphragm is dome-shaped. During inspiration, it contracts and becomes a fl attened sheet of muscle. Also, the

external intercostal muscles contract, causing the rib cage to move upward and outward. Following contraction of the diaphragm and the external intercostal muscles, the volume of the thoracic cavity is larger

than it was before. As the thoracic volume increases, the lungs increase in volume as well because the lung adheres to the wall of the thoracic cavity. As the lung volume increases, the air pressure within the alveoli decreases, creating a partial vacuum. In other words, alveolar pressure is now less than atmospheric pressure (air pressure outside the lungs).

Air will naturally fl ow from outside the body into the respiratory passages and into the alveoli, because a continuous column of air reaches into the lungs

Expiration

Usually, expiration is the passive phase of breathing, and no effort is required to bring it about. During expiration, the diaphragm and external intercostal muscles relax. The rib cage returns to its resting position, moving down and inward As the volume of the bellows decreases, the air pressure inside increases. Now air fl ows out

. The elastic properties of the thoracic wall and lung tissue help them to recoil.

Volumes of Air Exchanged During Ventilation

Tidal Volume Normally, when we are relaxed, only a small

amount of air moves in and out with each breath, similar,. This amount of air, called the dal volume, is only about 500 mL.

Inspiratory and Expiratory Reserve Volume As noted previously,we can increase inspiration by expanding the chest and also by lowering the diaphragm to the maximum extent possible. Forced inspiration (inspiratory reserve volume) usually adds another 2,900 mL of inhal

We can increase expiration by contracting the abdominal and thoracic muscles. This so-called expiratory reserve volume is usually about 1,400 mL of air. The maximum volume of air that can be moved in plus the maximum amount that can be moved out during a single breath is called the vital capacity . Residual Volume It is a curious fact that some of the inhaled air never reaches the lungs; instead, it fi lls the nasal cavities, trachea, bronchi, and bronchioles.

Nervous Control of Breathing

Normally, adults have a breathing rate of 12 to 20 ven la ons per minute. The rhythm of ventilation is controlled by a respiratory control center located in the medulla oblongata of the brain. The respiratory control center automatically sends out nerve signals to the diaphragm and the external intercostal muscles of the rib cage, causing inspiration to occur.When the respiratory center stops sending nerve signals to the diaphragm and the rib cage, the muscles relax and expiration occurs. Sudden infant death syndrome (SIDS),. An infant under one year of age is put to bed seemingly healthy, and sometime while sleeping, the child stops breathing. the cause is that respiratory center stops sending out the impluse that cause breath

Chemical Control of Breathing

working cells produce carbon dioxide, which enters the blood. There, carbon dioxide combines with water, forming an acid, which breaks down and gives off hydrogen ions (H+). These hydrogen ions can change the pH of the blood. Chemoreceptors are sensory receptors in the body that are sensitive to chemical composition of body fl uids. Two sets of

chemoreceptors sensitive to pH can cause breathing to speed up. A centrally placed set is located in the medulla oblongata of the brain stem, and a peripherally placed set is in the circulatory system. Carotid bodies, located in the carotid arteries, and aortic bodies, located in the aorta, are sensitive to blood pH. These chemoreceptors are stimulated when the carbon dioxide entering the blood is sufficient to change blood pH.

Gas Exchange

there are two types of air exchange in the body

1- External respiration refers to the exchange of gases between

air in the alveoli and blood in the pulmonary capillaries

Blood in the pulmonary capillaries has a higher PCO2 than atmospheric air. Therefore, CO2 diffuses out of the plasma into the lungs. Most of the CO2 is carried in plasma as bicarbonate ions (HCO3 −). The enzyme carbonic anhydrase speeds the breakdown of carbonic acid (H2CO3) in red blood cells

Blood in the pulmonary capillaries is low in oxygen, and alveolar air contains a higher partial pressure of oxygen. Therefore, O2 diffuses into plasma and then into red blood cells in the lungs.

Internal Respiration 2-

Internal respiration refers to the exchange of gases between

the blood in systemic capillaries and the tissue cells.

Blood entering systemic capillaries is a bright red color

because red blood cells contain oxyhemoglobin. The temperature

in the tissues is higher and the pH is slightly lower (more acidic), so oxyhemoglobin naturally gives up oxygen.After oxyhemoglobin gives up O2 , it diffuses out of the blood into the tissues Oxygen diffuses out of the blood into the ssues because the PO2 of tissue fl uid is lower than that of blood Carbon dioxide diffuses into the blood from the tissues because the PCO2 of ssue fl uid is higher than that of blood.