Like all living things, the human body is made up of cells. Thanks to the cellular structure of the object, its growth, reproduction, restoration of damaged organs and tissues and other forms of activity. The shape and size of the cells are different and correspond to the function performed.
In each cell, two main parts are distinguished: the cytoplasm and the nucleus, in the cytoplasm, in its turn, full-fledged organoids – the smallest cell structures that ensure its vital activity (mitochondria, ribosomes, cell center, etc.). In the nucleus, before the division of the cell, special threadlike bodies, chromosomes, are formed. Outside, the cell is covered with a membrane that separates one cell from the other. The space between the cells is filled with liquid intercellular substance. The main function of the membrane is that it ensures selective entry of various substances into the cell and the removal of metabolic products from it.
The cells of the body consist of a variety of inorganic (water, mineral salts) and organic substances (carbohydrates, fats, proteins and nucleic acids).
Carbohydrates are composed of carbon, hydrogen, and oxygen; Many of them are highly soluble in water and are important resources for providing vital processes.
Fats are formed by the same chemical elements as carbohydrates; They are insoluble in water. Fats are part of the cell membranes and also serve as the most important sources of energy in the body.
Proteins are the main building material of cells. The structure of proteins is complex: the protein molecule has large dimensions and is a chain consisting of tens and hundreds of simpler compounds – amino acids. Many proteins serve as enzymes that accelerate the course of biochemical processes in the cell.
Nucleic acids formed in the cell nucleus consisting of carbon, oxygen, hydrogen, and phosphorus. There are two types of nucleic acids:
1) deoxyribonucleic (DNA) are located in chromosomes and determine the composition of cell proteins and the transfer of hereditary traits and properties from parents to offspring;
2) ribonucleic (RNA) – associated with the formation of the characteristic for this cell proteins.
A living cell has several properties: the ability to metabolize and multiply, irritability, growth, and mobility, on the basis of which the functions of the whole organism are realized.
The cytoplasm and nucleus of the cell consist of substances that enter the body through the digestive organs. In the process of digestion, the chemical decomposition of complex organic substances occurs with the formation of simpler compounds that are brought to the cell with blood. The energy released during chemical decomposition goes to the maintenance of the vital activity of cells. In the process of biosynthesis of simple substances entering the cell, they are processed in it into complex organic compounds. Waste products – carbon dioxide, water and other compounds – the blood carries out from the cell to the kidneys, lungs, and skin, which secrete them to the external environment. As a result of this metabolism, the composition is constantly updated: some substances form in them, others are destroyed.
The cell as an elementary unit of the living system has irritability, ie, the ability to respond to external and internal influences.
Most cells in the human body multiply by indirect division. Before dividing each chromosome is completed by means in the nucleus of substances and becomes double.
The process of indirect division consists of several phases.
   The increase of the core in the volume; Separation of chromosomes of each pair from each other and their dispersal throughout the cell; Education from the cell center of the spindle division.
   Arranging chromosomes against each other in the plane of the equator of the cell and attaching to them the filament spindle threads.Formation of two nuclei from dissociated chromosomes, the appearance of a construction, and the – septa on the body of the cell.
As a result of this division, a precise distribution of chromosomes – carriers of hereditary traits and body properties – is ensured between two daughter cells.
Cells can grow, increasing in volume, and some have the ability to move around.
The tissue is a group of cells and an intercellular substance, united by a common structure, function, and origin. In the human body, there are four main types of tissues: epithelial (integumentary), connective, muscular and nervous.
Epithelial tissue forms a layer of cells, of which the body covers and the mucous membranes of all internal organs and cavities of the body, as well as some glands. Through the epithelial tissue, there is a metabolism between the body and its environment. The cells of the epithelium closely adjoin each other, protecting the body from microbes and harmful influences, and are capable of rapid multiplication, thus ensuring a continuous renewal of the cover material. There are several types of epithelium – skin, intestinal, respiratory, etc., cells of which differ in form and function.
A special feature of connective tissue is the strong development of intercellular substance. Its main functions are nutritious and basic. The connective tissue includes blood, lymph, cartilaginous, bone, fatty tissue.
Muscle tissue is formed by muscle fibers. In their cytoplasm are the finest threads, capable of contraction. There is smooth and striated (skeletal and cardiac) muscle tissue. Due to the smooth muscles that form part of the walls of the stomach, intestines, bladder, blood vessels, there is a reduction in internal organs and a change in the diameter of the blood vessels. Due to the reduction of skeletal muscles, it becomes possible to move the body in space; A special structure of cardiac muscle tissue provides a simultaneous reduction of large areas of the heart muscle.
The structural unit of neural tissue is a neural cell – a neuron consisting of an oval, star-shaped or polygonal body and outgrowths from it. Most neurons have one long and thin process with branches off from it (excitation is transmitted from one neuron to other neurons or cells of other tissues) and several short, thick, branching cells close to the body of the body, contacting other cells and providing perception and Carrying out neural influences to the neuron. Long processes of neurons form nerve fibers. The main property of the neuron is its ability to be excited and conduct this excitation through nerve fibers. Excitation propagates along the neuron and can be transmitted to the processes connected to it by other neurons or the executive organs (muscle, gland).
The human body consists of organs. An organ is a part of an organism that has only its peculiar form and structure and performs a certain function.
Usually, the organ consists of several types of tissues, one of which plays a primary role.
The organs, united by a definite physiological function, constitute the physiological system. Distinguish the following physiological systems: integumentary, a system of support and movement, digestive, circulatory, respiratory, excretory, sexual, endocrine, nervous.
The integumentary system includes the skin and mucous membranes, which protect the body from external influences.
The system of support and movement is represented by a large number of bones forming the skeleton and attached to them by the muscles. They give the body a certain shape, protect the internal organs, provide support and movement.
The digestive system includes organs of the oral cavity (tongue, teeth, salivary glands), pharynx, esophagus, stomach, intestines, liver, pancreas; Their joint work ensures the ingestion of food and its processing. The formed nutrients necessary for the normal vital activity of cells and tissues are delivered to them with blood.
The circulatory system includes the heart and blood vessels; Their work provides the process of blood circulation, as a result of which a constant supply of oxygen and the necessary substances to cells and tissues is carried out and their release from the products of metabolism.
The respiratory system, which includes the nasal cavity, nasopharynx, larynx, trachea, and lungs, is involved in providing the body with oxygen and in releasing it from carbon dioxide.
The excretory system includes the kidneys, ureters, bladder, urethra. It performs the function of removing from the body the final products of metabolism, excess water, salts, organic compounds and toxic substances.
The reproductive system provides the function of reproduction. The organs of the male sexual system include the testicles, scrotum, prostate gland, penis. To the organs of the female reproductive system – ovaries, uterus, vagina, external female genital.
To regulate physiological processes in accordance with the needs of the organism and changes in the environment, there are two mechanisms: humoral and nervous.
Humoral regulation of physiological processes is carried out with the help of chemicals that come from various organs and tissues of the body into the blood and are carried by it throughout the body.
Nervous regulation of physiological processes is possible due to the interaction of the organs of the body with the nervous system. Unlike humoral regulation, nerve influences always target specific organs and tissues and spread many times faster.
The nervous and humoral ways of regulating functions are closely related. On the one hand, the activity of the nervous system is constantly influenced by chemicals bought with the blood flow, on the other – the formation of most chemical substances and their release into the blood is under the constant control of the nervous system. Therefore, the regulation of physiological functions in the body is always provided by a single neurohumoral mechanism.
In addition, separate organs and systems of organs mutually influence each other, thanks to which self-regulation of all physiological processes of the organism is achieved.
The nervous system is divided into two parts: central and peripheral.
The central nervous system includes the brain and spinal cord, consisting of a gray (congestion of the bodies of neurons) and a white (accumulation of processes of neurons) matter.
The peripheral nervous system is formed by nerve nodes – the bodies of nerve cells lying near the internal organs or in their walls, and nerves – bundles of long processes of neurons that go beyond the central nervous system and penetrate all organs.
By function, all nerve cells are divided into three types: sensitive (transmitting nerve impulses from the senses and internal organs to the brain), executive (forming the response nerve impulses and transmitting them to the appropriate organs) and intercalary (making the connection between the sensory and executive neurons).