 Now, tissues, tissues joined together, different types of tissues joined together to make organs. Now we look at what are the organs. Organs are combination of different types of tissues, working together. All of these tissues which makes an organ have a particular function which joined together to make or to perform the combined function of the organ. For example, heart, heart is an organ. It consists of various types of tissues, for example, the muscle tissues which contract and relax inside the heart. The blood is present, blood comes from the body and goes back towards the body and blood itself is a liquid tissue. For example, in our stomach, there are different types of tissues which are combined and working together. As you know that when we take food, it goes inside the stomach and stomach have to grind this food. For that stomach have certain strong muscle cells which contract and relax to grind the food and move it towards intestine. The lining of the stomach also have few different types of cells. These cells are glandular in nature, that is they are like glands, they produce secretions of various kinds, we call them gastric juices. And these juices have various enzymes and various like for example, acid HCl which help in digestion of different types of foods, particularly the proteins. So, stomach consists of muscular tissue and the glandular tissue, but both have some similar objective that is grinding the food and digesting the food. Major purpose is digesting the food, for that it is ground by the contraction and relaxation of the muscle cells and for digesting food, there are glandular cell which produce their secretions. So, their function is combined that is digestion by grinding muscle cells and by enzymes, the glandular tissues. There are various types of organs which are also present in our body like our brain, like our testis and ovaries, like pancreas which produces insulin for us and it also produces glucagon for us. Pancreas is also an important organ which consists of many types of cells, alpha cells which produces glucagon which lowers the glucose level in the body, glucagon is a hormone and the beta cells which produces insulin and insulin is also a hormone and it lowers the level of glucose in the body. Insulin and glucagon act counter regulatory, insulin decrease blood glucose, glucagon increase blood glucose both are produced by tissues present inside the pancreas. Now, organs makes organ systems, organs various organs join together to make a system, there are different types of organs and there are different types of organ systems. The organs which perform a similar function join together to make the organ systems for example, the digestive system, digestive system consists of an oral cavity that is our mouth, then an esophagus along canal runs through our neck to our stomach, then comes the stomach which is a storage organ which stores food for temporary time and also digested by using enzymes and by grinding, then comes the intestines, the next organ the small intestine in which food is further digested and absorbed, then comes the large intestine in which the semi digested food or undigested food moves down towards the rectum and it is collected in the rectum and then released by the anus. So, digestive system consists of various organs like the oral cavity, the esophagus, the stomach, the small intestine, large intestine and it is last part the rectum. All of these organs have their particular function like function of the oral cavity, the mouth is to break down the food into smaller parts and by the function of its enzyme breaking down of certain foods inside the mouth even and then function of the esophagus is to move the food down towards the stomach. So, there is a slow movement by contraction of different muscles of the esophagus, leads the food towards the stomach, function of the stomach, the third organ is to grind the food and is to digest the proteins and for temporary time store that food. There are few organisms which eat lot much grasses because these organisms like for example the buffaloes, the cows which are also called reuminants have a highly modified stomach because these were the animals of prey and in jungle they have to deal with their predators like lions. So, what do they do that they go in the fields, they quickly eat up the grasses and then they sit in the shades in the hiding places and they slightly churn and grind their foods later and then get it in the other part of stomach. So, their stomach is modified for initial storage for long periods and then when the food goes back in the mouth and it is broken down into smaller pieces then it goes towards the next section of the stomach, the next part of the stomach and then go into the next and then go into another part. So, it means the stomach storage organ which is modified according to the food of the particular organism. Just like that we have a cardio vascular system, the system for the transmission of blood for distributing foods, for distributing oxygen, for taking away carbon dioxide and other gases and all other hormones and other ingredients which are required by the body to move from one part to another part. The cardio vascular system consists of a heart, heart is pumping organ, heart pumps blood from when it when the blood comes inside the heart it pumps it away and then it receives the blood which is coming back from the body and convert it into oxygenated blood through passing it through the lungs and then return that blood back to the body. So, heart is an organ and the vessels which takes blood away from the heart towards the body, these vessels are called arteries, they makes one different organs and then those vessels which takes the deoxygenated blood from the body and return it to the heart, these are called veins, these are a different type of organs and their structures are also different because their function is different, their structure is different. In arteries for example, the pressure of blood is very high because heart pumped the blood and this blood have to go till the last part of the body. So in arteries blood is pressurized, arteries have thicker walls to deal with such pressures. In the veins however blood is because coming from the body towards the heart pressure is low and for that reason veins are comparatively thinner in their structure and they are comparatively wider in their structure, they have a wider lumen. So according to the nature of the function organ have a particular structure. So structure and function in organs, in organ systems, in tissues, in cells are always related to each other. If we say that this particular organ is performing this function or this particular tissue is performing this function, that tissue or organ must have the same structure which is required to make that particular tissue or the organ. The structure and function are always related. We can say that function is based upon structure. Now organism, organ systems, many organ systems makes organisms. For example an organism like a frog have different organ systems. It have a digestive system, it have a cardiovascular system, it have a reproductive system, it have a nervous system, brain, it's a spinal cord, it have a muscular system, it have a skeletal system which makes its skeletal framework. If we look at the human beings, they have the most complicated form of life. They have a maximum number of organ systems. We also have a nervous system, very complicated nervous system. We do have a cardiovascular system, we do have a digestive system, we also have a reproductive system, we have a respiratory system. If we look at other related organisms, they also have different type of organ systems. But the function of all of these organ systems is actually work in a coordinated manner to act as an organism. For example you see that when a person is exercising, for example if I am exercising, for exercise my muscles have to move and my bones also have to move. My muscles have to move my bones. But when I will exercise, as you know if you exercise sometimes or you walk fast, you know that not only your muscles are moving but you feel that your heart is pounding, your heart is moving faster. You also observe that if you are exercising for a long time, your respiratory rate is also high, your ventilation is increased, it means that why this respiration is increased and why this heartbeat is increased. Reason is this that when muscles are working fast, they need more energy, they need more oxygen, they need nutrients, they need to remove their waste. With the result, your heart pump faster because heart have to pump the blood towards the muscles and other organs. Because muscles and the blood carries away oxygen, it carries nutrients and it have to carry away from the muscles, their carbon dioxide, it means that and when the muscles are using more oxygen, they need more of further oxygen. So your ventilation rate, that is your respiratory rate increased because when your respiratory rate is increased, you are getting in more oxygen from the environment and with the result, your muscles are getting maximum for oxygen and more of the nutrients. You feel that when you exercise after that, maybe you need to get more food or maybe you are asked to get more food and water because when your muscles are working, they need more water and they need more food. So all the functions of an organisms are coordinated and these are coordinated, they are all organ systems have to coordinate with each other to perform same function. For example, for just exercise, just one function, your muscular system is working, your respiratory system is working, your cardiovascular system is working, all of these are joining their hands, they are combining their functions and ultimately they are performing same function that is the exercise. Just like that, the same thing happen everywhere, organisms, they have to coordinate their functions. You see, various organs and the organ systems are working together in just one function, the exercise, cardiovascular system, muscular system, the skeletal system, the respiratory system, but somehow their actions should be coordinated. In human beings and in all the major animals, there is a system called nervous system which is there to master all the activities. Nervous system is the system of coordination. This system is connected to various systems, various other systems like to the digestive system, to the cardiovascular system, to the respiratory system and this system coordinates the activities of the different systems. For example, if muscles need more energy or if muscles need more oxygen, they will send signal to the nervous system and nervous system will activate the cardiovascular system, the respiratory system or other related system to perform a function which is required to send oxygen, send more oxygen to the muscles and for example, if they need more nutrients then the brain can order, we can say order other organ system to send more nutrients towards the muscles and reduce the content of nutrients from some other part of body. This is why that we usually says that if you have just taken your food, don't go for a bath, why? Because if you just have taken your food, your muscles of the digestive tract are moving, are contracting and relaxing again and again to digest the food. If you just go for a bath or if you start exercising after just taking your heavy meal, what will be the result? More energy, oxygen and everything will be directed towards your exercising muscles or handling with the cold maybe which is coming with the water and the result is that your digestion process which needs, whose muscles are contracting and relaxing and they need more energy, energy which is directing towards these muscles, muscles of the digestive tract will be reduced and energy will go towards your exercising muscles that is your skeletal muscles in your arms, in your legs and the result will be, your digestive rate will be slow and you may experience a undigestion or semi-digestion type situation.