The four basic human body structure units are cells, tissues, organs and systems which, all working together make the human body function.

The cells are the core structure that make up the human body, although there are many different shapes, sizes and types they consist of three main parts, the cell membrane which maintains the shape of the cell and separates and controls the movement of material into and out of the cell, the nucleus which contains the cells DNA and determines its structure and function and the cytoplasm which is the fluid in the cell that contains the organelles which perform its specific functions. There are many cell functions dependant on the cell type but the generalised functions are molecule transport, reproduction and protein synthesis. Molecule transport is the transfer of material through the cell membrane, there are two ways this happens, diffusion and active transport. Diffusion is when molecules such as oxygen, carbon dioxide and water move from areas where there are many of the particles (high concentration) to areas with less (low concentration). Active transport is when substances are at lower concentration and require energy to be transferred. There are two means of active transport, endocytosis which is the movement of molecules from outside to inside the cell and exocytosis which is movement of molecules from inside to outside the cell, both performed via the formation of vesicles which package the molecules then fuse with the cell membrane to transfer them into or out of the cell. Cellular reproduction is broken down into mitosis and meiosis. Mitosis is a cycle of phases that reproduce somatic cells (cells of the body); these phases are interphase, prophase, metaphase, anaphase and telophase. This results in two daughter cells identical to the parent cell formed for growth, repair and replacement of dead cells in the body. Meiosis is a form of sexual reproduction and only occurs in gamates (egg and sperm cells), these cells contain only 23 chromosomes unlike somatic cells which contain 46, this is so that when fertilisation takes place the cell once again as 46 chromosomes. This is vital for the continuation of the human species. Proteins are widely used in cells to serve diverse functions, some are the bodies structural support molecules while others form enzymes or hormones which control chemical reactions within the body. The manufacture of proteins occurs when information of a persons genetic code is taken from DNA in the nucleus and copied to a molecule called mRNA which then moves out of the nucleus to sites of protein synthesis in the cytoplasm.

Tissue is made up of cells that have much the same structure and perform the same function. Spaces between the cells that form tissue are filled with an intercellular matrix which can contain substances unique to the specific tissue. There are four main tissue types, these are epithelial, connective, muscle and nervous. Epithelial tissue forms the skin as well as covering most organs and lining hollow cavities. The epithelial cells may be squarmous, cuboidal or columnar and are arranged in single or multiple layers dependant on their place and function in the body. The tissue is made up of cells closely packed together with almost no intercellular matrix and performs several functions such as protecting underlying tissue from harmful chemicals, bacteria and excessive water loss, secretion of chemical substances such as enzymes and hormones in glands and excretion of waste products from the body through the kidneys. It also performs absorsion, sensory reception, diffusion and filtration functions. Connective tissue occurs throughout the body providing a framework to support the body and organs as well as connecting all types of tissue, repairing tissue damage, storing fat, transporting substances and protecting against disease, it is made primarily of intercellular matrix with very few cells. There are six types of connective tissue found in the body these are; Loose connective tissue which bind epithelia to underlying tissue and hold organs in place. Adipose tissue which stores fat, it expands and shrinks dependant on the amount of food stored. It is also used for padding and insulating organs. Fibrous connective tissue which is made of high-strength, slightly stretchy fibres that make up tendons and ligaments to surround and support the bones and organs. Cartilage tissue is stiff but flexible and found in many parts of the body, it lines the bones in joints assisting in their smooth movement, connects tendons and ligaments to bones, protects joints from shocks and defines and maintains the shape of the area in which it is present. Bone tissue consists of two types, compact and spongy. Compact tissue is made up of compact cylindrical layers that surround and protect the blood vessels and nerves. It forms the outer shell and provides the strength of the bone. Spongy bone is found beneath compact bone and as a meshwork structure which contains the bodies bone marrow, the two working together form the bones and therefore the skeleton providing support to softer tissue, protecting organs, assisting movement, storing minerals, producing blood cells and storing chemical energy. Blood tissue consists of cells suspended in an intercellular fluid called blood plasma and is a vital part of the body. It functions in the maintenance of the growth, health and life of the human body by transporting oxygen, carbon dioxide, nutrients, waste and hormones to and from the bodies’ cells. Muscle tissue is made up of cells which have the ability to contract and relax to assist movement of body parts and is found in three main types, skeletal, smooth and cardiac. Skeletal is most abundant in the body, they are attached to and aide movement of the bones and are under voluntary control. Smooth is found in walls of hollow organs and are involuntary in their control, they work in organs such as the intestines by contracting to push food through the body or the bladder by contracting to release urine from the body. Cardiac muscle tissue makes up the heart and is also involuntary but unlike other muscle tissue can contract repeatedly without becoming tired. It works by contracting and relaxing, causing the rhythmic beating of the heart which circulates the blood and its contents throughout the body. Nervous tissue controls many body activities, it is found in the brain, spinal cord and nerves that branch throughout the body. The cells that make up the tissue are called neurons and have three main parts, the cell body, the dendrites and the axon. The cell body carries out the general functions whilst the dendrites are extensions of the cytoplasm that carry impulses from other neurons to the cell body and the axon is an extension that carries impulses away from the cell body to other tissue. The neurons also have a support cell called glial cells which bind and insulate neurons, protect against bacteria and provide nutrients enabling the human body to be aware of and adapt to its internal and external environment emotionally, physically and mentally.
An organ is made up of several different types of tissue working together to perform the specialized function of the organ. The major organs of the body are the brain which is the control centre of the body handling the processes of the central nervous system, the eyes which detect light and turn it into vision by electrical signals, the heart which keeps blood pumping through the body, the lungs which intake air, help with the oxygenation of the blood and stop contaminants entering the bloodstream, the stomach which maintains an acidic environment to break down food to be processed by the intestines which remove usable nutrition from food and expels waste. The liver and kidneys function as filters to remove waste, the spleen breaks down old blood cells, stores blood and helps produce fresh blood and the pancreas produces enzymes and hormones to help the body run smoothly. It’s these major organs along with others in the body that keep us alive and functioning.

Systems are the most complex of the four main body units, made up of varying numbers and kinds of organs. There are ten major systems of the human body skeletal, muscular, nervous, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary and reproductive. The skeletal system provides the framework for the human body and protects the internal organs from damage. The muscular system is made up of specialized cells called muscle fibres, its main function is to aide the movement of the body as well as helping with posture, joint stability and heat production. The nervous system includes the spinal cord, brain, nerves, skin, eyes, ears, tongue and nose. Its main function is the control of the body activities and the reaction to stimuli helping to keep us in touch with our internal and external environments. The endocrine system is made up of a group of glands which produce hormones that control body functions and influence the growth, development and metabolic activities of the body. The cardiovascular system is the bodies transport system, it is made up of the heart which pumps the blood and the arteries and veins which transport the blood around the body. When the blood enters the smallest vessels, called capillaries, it gives nutrients and oxygen to the cells and takes in carbon dioxide, water and waste products, the blood then goes back into the veins and is carried back to the heart which pumps it to the lungs to pick up oxygen and eliminate waste carbon dioxide. The lymphatic system is the bodies defence system, filtering organisms that cause disease, producing white blood cells and generating disease-fighting antibodies. The system is made up of a network of vessels which carry excess fluid away from spaces between tissues and organs and return it to the bloodstream. The respiratory systems function is to bring oxygen into the body and remove carbon dioxide. Air enters the nose or mouth and moves down the trachea and through a series of passages which lead to the lungs, oxygen follows this path and enters the bloodstream, at the same time carbon dioxide passes into the lungs and is exhaled. The digestive system is the system of the body that breaks food down into protein, vitamins, minerals, carbohydrates and fats which the body uses for energy, growth and repair. The urinary systems function is to eliminate waste from the body, in the form of urine, this is done when the kidneys remove waste from the blood which combines with water and travels to the bladder which when full removes the urine. The reproductive systems function is to allow humans to produce children and maintain the survival of the human species.