 Now, we are going to talk about respiratory system and human being, the most complicated organism, human being, us. We have a complicated respiratory system. We have two conduct metabolic reactions all the time. In every second, many molecules are in the process of making and many are in the process of breaking. We need all the time oxygen and we have to remove our carbon dioxide. For this, we have a very efficient respiratory system. Now, we look at various parts of respiratory system. What parts actually makes our respiratory system? We have mainly lungs. Human beings are mainly the land vertebrates, the mammals and human beings have lungs as their major respiratory surface and carbon. The human respiratory system consists of lungs and air passageways. Look at the slide. We have lungs as our major respiratory surface. We have air passageways which allows movement of air from the outside through nostrils till lungs. We simply name them. Air passageways are in human beings, starts from nostrils, then comes the nasal cavity, then comes a tube, a muscular tube larynx, then comes another small tube larynx which leads into trachea, trachea leads into bronchi, bronchi leads into bronchioles and then comes the alveoli. The alveoli are the smallest parts of the respiratory tract which are present inside the lung. From bronchi, the passageway enters the lung. Now, we look at a diagram to observe various parts. Look at the diagram in front of you. It shows various parts of a human respiratory system. First of all, the air have to enter two nostrils in the nose and there are two nostrils. Air enters two nostrils into the nasal passage. Then there is a muscular tube just behind the throat which is called pharynx. Pharynx leads into a small box which is also a sort of a tube is called larynx. We know that larynx is also called a voice box because our voices which are generated by us are generated by the vibration of certain parts of this voice box or the larynx. But this also act as a passageway for the air coming through the nasal cavity in pharynx and lead towards the trachea. After larynx, trachea comes. As you can see that trachea have ring like structures. These rings are actually cartilaginous C shaped structures and these help the trachea to not to collapse. Then trachea is going down to the thoracic cavity or the chest cavity and then we can see the trachea branch into two. These two branches are called the bronchi. Now closely look at the diagram both of the bronchi are entering in the lungs. We have two lungs we can say a pair of lungs one on the right and one on the left side of the chest cavity trachea divides into two bronchi and one bronchi each enters one lung one on the right one on the left. Then we can see that each bronchus one of the lung is shown from surface on the right and on the left on the other side the lung is shown in a cuts in a cross section which shows that each bronchus is divided into further so many branches like that of branches of a complex or a very big tree. These branches are called bronchioles bronchioles then ends into sacs spongy sacs these sacs are called air sacs air sacs are the functional unit of the lung and air sacs consist of more small structure microscopic structures single wall structures called alveoli. So this is about a generalized structure of human air passageways and the lungs now we look at the structure and function of each part of this air passageway and lungs one by one. So now we are going to talk about structures and functions of various parts the passageways in human beings and then the lungs the nose and the nasal cavity as we know that air has to directly enter inside the nasal cavity to nostrils air may have many dust particles may be some microorganisms may be different types of a particular matter nasal cavity is structured to handle all these problems the nasal cavity have a ciliated apithelium the apithelial lining of the nasal cavity have cilia on its surface. Secondly this lining is mucoid the cells of the apithelium it creates mucus this mucus covers the whole membrane is the surface of this nasal cavity the result is this that air when passes through the nasal cavity it is filtered it is all the dust particles some microbes even and some other tiny particles they are trapped in the ciliated apithelium which is also mucoid. So the dirt particles particulate matter some microbes remains attached to the surface of the nasal cavity due to ciliated apithelium and the mucus. So air which passes through the nasal cavity towards the next part which is called the pharynx is warm filtered and moist then comes the pharynx pharynx is a muscular tube it is a muscular passage it is also lined with a mucus membrane the mucus membrane help to we can say filter and clear up the air a more a lot more air moves down the pharynx towards the larynx larynx is the next larynx provides for the respiratory system passageway from which the air passes down towards the trachea the larynx have a ciliated opening its opening is also ciliated and mucoid so air is filtered more larynx have some other property as well larynx is cartilaginous it have cartilaginous fibrous materials crossing across from one part to another part which are also called vocal cords these vocal cords when they are vibrated by air help us in producing our voices. So when we speak out actually these vocal cord help us to produce different types of sounds by vibration of these cartilaginous vocal cord but for respiration larynx is ciliated mucoid pathway so you can observe generally that the air is filtered and body the passageways the function of the passageways is to filter the air keep it moist and make it clean as clean as possible because the next part lungs in which air has to go are so delicate and thin and no dirty particles no larger particles should go inside the lung. So the passageways are ciliated and mucoid and they clear up and filter air and make it moist from larynx air enters into the trachea a long tube which is actually supported by cartilaginous rings we should more precisely say that these are c shaped cartilaginous structures which line almost all the part of the trachea and they actually help trachea to maintain its shape and not to collapse when air is moving up moving down this the trachea is a long tube it goes from larynx it starts from larynx and ends into a branching into into two branches one towards right and one towards left and these branches are called bronchi the trachea divides to form bronchi bronchi enters into the lung and these bronchi also have a cartilaginous ring like structures around they still need to protect themselves from collapsing because air is always going out coming in breathing process as we know is a continuous process always goes on then bronchi enters inside the lungs and they are divided further into smaller branches these smaller branches inside the lungs when the bronchi enters inside the lungs they they divide into smaller branches then these smaller branches reaches a diameter of about 1 mm 1 millimeter then these are called bronchiose bronchiose further divides into air sacs air sacs don't have any cartilaginous material to support they are spongy just like that the bronchiose themselves have irregular cartilaginous plates actually when the trachea divides into bronchus and bronchus divide into further bronchi the cartilaginous rings then are they become irregularly shaped and ultimately they makes irregularly placed cartilaginous plates at the ends of the bronchiose and the bronchiose and then the air sacs don't have this cartilaginous material lung have lung or the lungs have millions of air sacs in the alveoli each air sac have actually extensions of its membrane which are only single layer these are called alveoli the air sacs are the are called the functional units of the lung and alveoli are microscopic structures these are very small and these are seen under the microscope due to presence of these millions of alveoli and the air sacs the lungs are spongy in nature as we know that trachea is hard bronchi are hard because of the presence of the cartilaginous rings but the lungs consist of so many air sacs and bronchiose and bronchiose the lungs generally are spongy in nature and not the hard structure soft structure look at a diagram this diagram shows one bronchus which is supported by a cartilaginous ring though but you can see that at at the end of the bronchus these cartilaginous rings are more irregular and they become some irregularly placed plates when the bronchus is divided into bronchiol you can observe that there are no cartilaginous rings or plates and then when the bronchiol is further divided into air sac and air sacs extensions the alveoli as you can see in the top left diagram there are no cartilaginous rings these alveoli air sac and bronchiose give the lung its characteristic spongy appearance then observe the next thing from the right side of the bronchiol a bronchus a vein is coming down this is actually bringing this is the pulmonary arteriole and this is bringing deoxygenated blood to the lung from heart and this is making a capillary network around the whole of the air sac and then entering into an pulmonary venule this pulmonary venule is going up and giving oxygenated blood back to the heart now as we know that air enters through the nasal cavity nostrils to the nasal cavity to pharynx to larynx to trachea to bronchi to bronchioles and to the air sacs so air sacs are filled with air and this these air sacs are surrounded by lots of a big network of the capillaries now as I previously described that capillaries have a single layered structure capillaries are a single layered structure they have just an endothelium and the alveoli are also single layered structure so there are just two direct epithelia in contact exchange of gases at this surface is very easy to take place and these alveoli in the air sacs actually gives a very large surface area for the exchange of gases and as we know that every alveoli is surrounded by a capillary network a very rich capillary network so it the exchange the area the respiratory surface for the exchange of gases is very high so we have a thin epithelium in contact two thin epithelia actually in contact and we have very large surface area so lung is an ideal surface for exchange of gases