Human Body Systems

The human body has many organs organized into several systems that perform essential functions. Investigations confirm that all of them are closely related. For instance, blood performs not only breathing and protective function but also takes an active part in the functioning of other organs and body systems, such as lungs and the immune system. The respiratory system cannot perform its functions without the immune and digestive systems as food enriches the blood and gives the body the necessary minerals necessary for its protection. Thus, all systems in the human body are closely related because they cannot function without additional assistance.


Blood is an essential fluid and watery matter of the human organism. Moreover, it is the most important system of circulation in the body, which has multiple functions. The structure of blood is quite simple as it contains only two main units – plasma and formed elements. Plasma comprises almost half of the blood volume; the formed elements make a smaller part. Plasma consists of water, which comprises the largest part of it (about 92%), plasma proteins (7%) and many dissolved substances, such as nutrients, gasses, hormones and waste products (1%). The formed elements are divided into three groups: white blood cells (leucocytes), red blood cells (erythrocytes) and platelets. Red blood cells make up the biggest part of blood, and white cells account for less than 1%. Blood volume occupied by the red blood cells is known as hematocrit. The nature of the hematocrit is changeable and depends on the level of oxygen in the human body. Therefore, people living in different geographical locations may have a different level of hematocrit.

The main function of blood is transportation. Blood transports not only oxygen but also necessary elements and removes wastes from the human body. Blood performs an important function of delivery of essential materials such as glucose, hormones, and other nutrients to cells and body organs and removes waste products of their activity. Moreover, some elements can be dissolved in blood, forming the plasma of the blood. Thus, substances which can dissolve in blood are glucose, gases such as oxygen, ions, amino acids, hormones, which depend on the person’s sex, and proteins. Apart from useful substances and nutrients, blood can also dissolve some wastes such as carbon dioxide. In addition, there are some elements that do not dissolve in blood. Thus, lipids are insoluble, but they can bind to plasma proteins or may be suspended. Blood proteins perform many useful functions in the human body. The biggest and the most essential of them is the regulation of blood pressure, which impacts the flow of some substances into and out of the capillaries. They also maintain chemical balance, neutralize viruses, forming blood clots, which prevents some losses. These proteins are divided into three groups: albumins, fibrosis and globulins. Blood also regulates the Ph level. In general, blood has a variety of functions due to the number of its components.

Red blood cells are the largest part of the formed elements. These flexible structures have one of the key roles of the fluid, namely transportation of the oxygen. The lack of these elements can cause dangerous diseases. The existence of red cells is quite short, particularly about only 3 months. The old units are removed from blood in the process of blood functioning by liver and spleen. Substitution of old red blood cells with the new ones can be performed only if there is a sufficient amount of iron and vitamins in the body. Red blood cells are replenished every day by the red bone marrow.

Another group of blood cells are white elements or white blood cells. They perform a protective function in the human body. Neutrophils, a special type of white cells, arrive at the place of an injury and neutralize bacteria. The other type of white cells, monocytes clean up the place of the injury from dead cells and microorganisms. Lymphocytes provide immune protection in case of infection. However, problems with the immune system and dysfunction of the blood system can cause serious diseases, such as leukemia.

Platelets form the third most important element of blood. They are needed for repairing blood vessels by means of forming blood clots. The weak functioning of the blood clotting is a factor that causes a threatening genetic disorder, hemophilia. Although blood is a common phenomenon for all human organisms, it does not have a unique structure. Specific glycoprotein determines different groups of the blood. It is possible to distinguish four main blood types. It is necessary to conclude that blood is a separate system of the human body which performs multiple essential functions and is presented in the form of fluid.

Immune System

It is one of many useful systems of the organism that performs a variety of functions, which can be generalized into one general notion – protection. It collects the potential protective possibilities of the organism in order to combat viruses and threatening microorganisms. Moreover, it helps to combat diseases. Formally, the immune system can be separated into two imaginable lines of defense. The first one is the epithelial surface, which protects the body from physical and chemical components. Skin is the first and the most powerful element of the immune system as it provides a strong protection. Although, respiratory and digestive systems have epithelial surfaces as well, their protective possibilities are not as powerful. The epithelial surface produces a variety of protective fluids or chemical matters with sufficient protective function. These liquids contain enzymes which destroy the walls of the bacteria organism and kill them. There are several types of chemical barriers, such as tears, saliva, and others. The second line of defense is located in the organism itself and combats the threat that managed to perpetrate into the body and overcome the first line of protection. The second line is represented by cellular and chemical components. It includes four main types of protective response: inflammation, complement, interferons and pyrogens.

Inflammatory response is the first stage of an organism’s resistance. In the place of inflammation, it is possible to notice some red area with swelling. In addition, it is accompanied by high temperatures. Injured tissues released a variety of chemical substances to the place in order to destroy a threatening organism. The chemical elements not only fight bacteria but also serve for treatment of the injured area. Excessive flow of plasma to the zone of injury is necessary for oxygen transportation and causes swelling. It also causes pain reactions with the help of prostaglandin. The system not only tries to overcome the source of the problem but also gives some signals to the brain for additional assistance. In such a way, a person can be aware that something is wrong with the body, and it is necessary to take some action. Therefore, an inflammation is a complex reaction that takes place in the area of a potential problem, which causes redness, pain and swelling and signals about internal resistance to the penetrated threatening microorganisms. In case these two lines of protection cannot cope with the problem, the third line of protection is the immune system.

It is an indistinct system represented by lymphocytes located in blood, lymph and lymphoid organs, such as tonsils, spleen, lymph nodes, which are the main components of the immune system. The immune system starts to act when the first and the second stages of protection are helpless. The beginning of the system’s work is identification of the problem, a possibility of the organism to recognize a threat: bacteria, virus, protozoan, and parasites. As a rule, the immune system can react and identify cancerous cells; however, its possibilities are too weak to stop the disease. Antigens are the substances that cause immune response. The response starts from the stimulation of lymphocytes: T cells and B cells. B cells react to and destroy microorganisms and some viruses. Actually, they react to chemical substances released by threatening organisms. They produce antibodies in response to an attack. By contrast, the process of T cell reaction is quite different. Thus, they attack enemies directly, fighting against viruses, human cells invaded by viruses, parasites and fungi. The response of B cells is also known as primary humoral reaction. The second type of immune reaction is cell-mediated, which is performed by T cells. Despite all protective functions of the immune system, it has some flaws. Thus, it is incapable of providing protection from different diseases. For instance, the immune system cannot properly react in case of such disorders as allergies and AIDS. In other words, allergy is a differentiated, modified and mutated immune response. It involves an unpredictable reaction on some tissues and organs that do not pose a threat to the organism. Allergens are the antigens that occur in case of allergic reaction, which is usually in the form of inflammation. In addition, sometimes the immune system attacks its own cells and tissues. This condition is called an autoimmune disease.

Digestive System

The digestive system is represented by a system of tubes and organs organized into a single unit along the whole organism. The main function of the system is to digest the food and release the wastes out of the organism. It is some kind of a journey of food though the whole body, during which vitamins, minerals and other necessary components are separated. This symbolic journey starts in the mouth. It is the primary stage of digestion. In the mouth cavity, food is digested and split into smaller pieces. It can be performed with the help of sharp teeth, which makes chewing. However, it includes not only some mechanical processes of division and blending but also chemical digestion, which is performed by means of saliva. Saliva is a liquid that contains several chemical components, which perform at least 5 known functions. It moistens food for easy swallowing. It contains enzymes for bacterial neutralization, being a secondary line of human body protection. It also dissolves some substances so that a human can feel some tastes and get pleasure from food. Finally, saliva is necessary for the oral cavity for making it wet and cleaning the teeth, washing them from the pieces of food. The digestive system contains many additional organs that do not perform the main function but assist in the process of digestion. These are the tongue (a muscle helping to swallow food) and teeth (cut food into small pieces and mix them).

After swallowing, the food enters the second phase of digestion. Food bolus gets to epiglottis. The cut elements of food can easily get to the trachea and lungs from there. Therefore, epiglottis moves in a downward position, performing the function of protection of the breathing system from the pieces of food by blocking airflows. The peristaltic process moves bolus easily from the esophagus down to the stomach. When food enters the stomach, the particle sphincter should promptly close in order the acid from the stomach cannot get to the esophagus and a person does not feel an unpleasant irritation known as heartburn. Stomach is a large muscle organ performing the function of storage and digestion of food. Small glands inside the stomach produce stomach juice needed for digestion and liquefaction of food. This juice is highly acidic as it contains a great amount of hydrochloric acid and pepsinogen, which is involved in the processes of churning and mixing with gastric juice. After this stage, food gets to the small intestine. It is a long tube of about 6 meters long for an adult person. Small intestine acquired its name for its small diameter. The process of digestion here is more complex and consists of three levels: duodenum, jejunum and ileum. Large food molecules are divided here into smaller ones, which later get to the lymphatic system and are transported to all parts of the body. In addition, the process of absorption of carbs, proteins and fat occurs in the small intestine.

Some organs perform additional function in the process of digestion by producing enzymes. Pancreas is the biggest source of enzymes. After the main processes of digestion in the small intestine, the rest of food and the wastes get to the large intestine. The large intestine is not only the place of the waste release but also a place of the vitamin K production and water absorption. It is also necessary to admit the function of the liver in the process of digestion. Thus, the liver produces bile, which is stored and purred in the gallbladder. The large intestine or colon is only about 5-6 feet long. Its function is to protect the organism from harmful bacteria. Finally, the last stage of digestion is a release of the wastes through anus. It consists of two muscles – internal and external, which control the stool and comfort feeling for a human.

Respiratory System

It is one of the most responsible systems in the human organism, performing the function of breathing. It consists of a set of organs that takes an active part in breathing, taking oxygen from the air and expelling carbon dioxide. The primary and the most significant organ in the respiratory system is lungs. Certainly, lungs are responsible for making chemical changes of gasses inside the body. The lungs are a pair of organs that collect oxygen inside. Red blood cells take oxygen from lungs and transport it to different parts of the body. In case of a decreased amount of oxygen, the organism can suffer from hypoxia. A complete lack of oxygen is known as anoxia, and death can occur in a few minutes as after four minutes without oxygen brain cells start dying.

Apart from the main respiratory organs, the system includes some additional organs performing secondary functions. These are the nose, mouth, and sinuses. The nose and mouth help the air get to the body. The nose also performs the cleaning function, preventing microorganisms, parasites, dust and other pollutants from penetrating into the respiratory system. After that, air gets to sinuses, small organs in the skull that collect the air for temperature regulation as well as humidity. Then the air enters the trachea, or the windpipe. It is a pipe or tube which cleans and filtrates oxygen. It is separated into two additional tubes, known as bronchi. These organs provide a connection with the lungs and perform an additional function of filtration. The process of filtration is performed with the help of tiny hairs called cilia. They move mucus, a special liquid, which collects dust. This liquid is possible to notice when an ill person is sneezing or coughing. Bronchi are connected with the lobes of the lungs. The right lung has three lobes and the left one has two lobes. The reason for this difference is the location of the heart since a large left lung could prevent the heart function. The lobes are full of small cells of the lungs – alveoli, where the main process of exchange is performed. The alveoli get oxygen and release carbon dioxide. The smallest capillaries known as pulmonary capillaries are provided for the alveoli processes. They serve for the blood transportation. The arteries carry oxygen to the heart and different organs, which cannot perform their functions without oxygen. It is necessary to recognize the function of the diaphragm, which separates the chest from the abdominal cavity. Certainly, it does not have a separate function but only additional. However, it helps to make the process of breathing. It includes a muscle that controls how a human inhales air enriched with oxygen and exhales air full of carbon dioxide. Although bones do not refer to the respiratory system, they are also very necessary for the respiratory function. Thus, ribs protect the lungs from the injuries and harm.

In addition, the respiratory system performs the function of voice pronunciation. Actually, this role depends on the larynx. The voice sounds appear there with the help of breathing. Thus, the air that moves in this organ produces such an effect, and a human can pronounce some separate words and sounds. The walls of pharynx contain tonsils, which are a part of the lymphatic system and take part in protection of the organism. However, in case of their infection, they can be removed. Adenoids are also a part of the lymphatic system. However, in some cases they cause problems, not allowing the normal process of breathing or speaking. Therefore, they also can be removed. Thus, the respiratory system contains respiratory organs and additional organs that take an active part in the process of breathing.


Human body has many essential organs, all of which are organized into systems performing a set of important functions. However, it is necessary to conclude that all the systems are closely connected and cannot perform their activity when at least one part fails to perform its function. Blood is a liquid that performs many useful functions, the main of which is transportation of oxygen produced by the respiratory organs. The respiratory organs produce the effect of breathing.

However, breathing is useless if capillaries and the smallest vessels could not transport it to the rest of the body. The respiratory system provides oxygen for the whole body, but it is impossible if the immune system cannot provide necessary protection of the air with the help of filtration by the second and third line of immune protection. Similar to the immune system, the digestive system helps the body to support the necessary balance and be in a form for further life functions as it provides digestion and a sufficient vitamin balance. All the essential matters, liquids and elements get to the body through the process of digestion and enrich blood with nutrients. Thus, it is necessary to conclude that all the systems in the body form some kind of a cycle with interconnected processes.