1.1 Describe the four basic human body structure units and their functions
The way in which physical processes and functions take place in the body can be looked at on four main levels, moving progressively from the detailed to the general. At the most detailed level are the cells – the most basic of the physical structure units. Similar cells, when combined and held together, form tissue, the next main structure unit. Organs are collections of tissues that are combined to perform a particular function in the body. Organs do not work in isolation, however, and combine with other organs to create a biological system.
Cells
The cell is the most basic structural unit in the body. There are around 300 different cell types in the human body. While they differ in function, there are certain fundamental components or features that they generally have in common, such as a nucleus (with the exception of red blood cells), cytoplasm and a cell or plasma membrane.
A cell membrane is composed of 60% protein, and 40% lipid, or fat. This membrane is a phospholipid bilayer which contains embedded protein molecules. It is a living boundary separating the living contents of the cell from its environment. Its function is to regulate the passage of molecules into and out of the cell. Within the membrane, the cell consists of cytoplasm, a gel-like substance containing organelles - small structures each with their own function.
Common to all human cells except red blood cells, the nucleus is responsible for the storage of genetic information and the synthesis of DNA and RNA. Rough endoplasmic reticulum and the ribosomes within it are responsible for protein synthesis, while the smooth endoplasmic reticulum has various functions including the synthesis of lipids. The golgi apparatus takes care of processing, packaging and distribution of proteins and lipids. Vacuoles and vesicles store substances and have various functions, including cellular respiration, metabolic tasks and intercellular digestion. The cell has a cytoskeleton composed of microtubules, actin filaments, and intermediate filaments. The cytoskeleton gives the cell shape and allows it and its organelles to move.

All cells have three main functions that ensure the cell’s own survival. These are transport of molecules, including nutrients; reproduction/replication of itself; and metabolism. In addition, different cell types themselves have their own unique functions in the body. These include secretory cells (secreting sweat, saliva, milk etc.); keratinizing (making hair and nails); barrier epithelial cells (e.g. of the cornea, tongue, oral cavity, urethra and bladder lining etc.); nerve cells of many different types; metabolism and storage cells (e.g. in the liver); barrier function cells (in the lungs, gut, exocrine glands and urogenital tract); extracellular matrix cells (e.g. in teeth, bone marrow and cartilage); contractile cells (in muscles and iris); blood and immune system cells; germ cells (in sperm); nurse cells (in ovarian follicle, testis and thymus); and interstitial cells (in the kidney).
Tissue
Tissue is formed by the grouping together of similar cells to perform a specific function. There are four types of tissue in the human body: connective, muscle, epithelial and nervous tissue.
Connective tissue cells are distributed throughout a non-living, extracellular fluid or matrix. This matrix is composed of the ground substance – a clear, colourless, viscous fluid consisting of glycosaminoglycans, proteoglycans and glycoproteins – and the fibres within the ground substance. Some, but not all, types of connective tissue are fibrous. Examples of the non-fibrous types are blood and adipose tissue. One of the functions of the latter is to provide mechanical cushioning for the body. Adipose cells are held together by collagen fibres and sheets. The matrix of blood, on the other hand, is plasma.
Connective tissue, as the name implies, connects or separates tissues or organs in the body. It is generally made of collagen. The deep fascia, or fascia of muscles, is a dense, fibrous connective tissue which interpenetrates and surrounds the muscles, nerves, bones and blood vessels. Its function is to reduce friction to minimise the reduction of muscular force. Connective tissue around organs gives shape to them as well as holding them in place. Blood and bone are also examples of special connective tissue, supporting and binding other tissues. Other types of special connective tissue are cartilage, adipose tissue (body fat) and reticular tissue (found around the liver, kidneys, spleen and lymph nodes, as well as in bone marrow).
Generally speaking, connective tissue serves various functions. These are: storage of energy; protection and holding in place of organs; the provision of a structural framework for the body; the connection of body tissues, and the connection of epithelial tissues to muscle tissues.
Muscle tissue is active contractile tissue which serves to produce force and cause motion, either for locomotion of the body or motion required for the functioning of internal organs. There are three categories of muscle tissue: visceral or smooth muscle, found in the inner linings of organs; skeletal muscle, which is attached to bone and enables locomotion and gross movement; and cardiac muscle, which causes the heart to contract and thus pump blood through the body.
Epithelial tissue is a protective lining or covering on the surface of organs, the inside of the digestive tract, the respiratory tract, the reproductive tract and the surface of the skin. The function of this tissue lies in protecting the organs from microorganisms, fluid loss and injury. Certain epithelial tissues can also assist in absorption of water and nutrients and in secretion of waste products.
Nervous or neural tissue forms the cells in the peripheral and central nervous systems. These include the brain and spinal cord in the central nervous system, and the cranial and spinal nerves, including the motor neurons, in the peripheral nervous system. The purpose of the nervous system is the transmission of sensory information, integration of this information, and the control of muscles and glands. The nervous system also plays a vital role in homeostasis, as well as in mental activity. Organs
Organs are formed by groups of tissues which together form a discrete structural unit to perform a specific function in the body. There are two categories of tissue in an organ: the parenchyma and the stroma. The parenchyma are the tissues which are unique to the particular organ they are found in, and are central to that organ’s functioning. Examples of this include the nephrons in the kidneys, the myocardium in the heart, hepatocyte tissue in the liver, and neurons and glial cells in the brain.
The stroma in an organ are the tissues which form the structural framework of the organ, along with the connective tissue surrounding it.
The five vital organs in the human body are the brain, heart, lungs, liver and kidneys. With the exception of the heart, these are solid organs, but there are other organs in the body which are hollow (the visceral organs): the stomach, small and large intestine, and bladder. The skin is also an organ which serves the vital functions of protection against pathogens and excessive water loss. Its other functions are thermoregulation, homeostasis, sensation, synthesis of vitamin D, and protection of vitamin B folates.
Whilst all organs have their own unique functions, they are components of biological systems and operate within these, affecting or being affected by other organs, glands etc. to perform the tasks which are fundamental to physical existence.
Biological systems
The human body is a highly complex organisation of a number of systems, each of which works to fulfil its own particular role or roles using groups of organs.
The main human biological systems are as follows:
The circulatory system uses the heart, blood and blood vessels to pump and circulate blood around the body and to and from the lungs.
The digestive system consists of the salivary glands, oesophagus, stomach, intestines, liver, gallbladder, pancreas, rectum and anus, and is concerned with the intake, digestion and processing of food, along with excretion of waste products.
The urinary system maintains fluid balance in the body, as well as electrolyte balance, and is also responsible for the excretion of waste products via urine. It consists of the kidneys, ureters, bladder and urethra.
The respiratory system is responsible for gas exchange between the external environment and the body’s internal environment, and thus the oxygenation of the blood and removal of carbon dioxide and other gaseous waste products. It consists of the pharynx, larynx, bronchi, lungs and diaphragm.
The skeletal system is made up of the structural elements that support and protect the body – the bones, cartilage, ligaments and tendons.
The reproductive system consists of the ovaries, fallopian tubes, uterus, vagina and mammary glands in the female; and the testes, vas deferens, seminal vesicles and prostate in the male.
The endocrine system uses hormones secreted by endocrine glands for communicating with organs in the body to perform certain functions. The endocrine glands are the hypothalamus, pituitary gland, pancreatic gland, pineal gland, thyroid, parathyroids and adrenal glands, as well as the ovaries and the testes.
The lymphatic system plays a major role in the body’s immune system. Its main functions are the removal of interstitial fluid from tissues; transportation of fats and fatty acids from the digestive system; the transportation of white blood cells to and from the lymph nodes to the bones; and the transportation of Antigen Presenting Cells to the lymph nodes, which initiates an immune response. The lymphatic system is composed of lymph tissues, -nodes and -vessels.
The muscular system involves the skeletal muscles for locomotion, posture and heat generation.
The nervous system involves the brain, spinal cord and the peripheral nervous system in receiving stimuli, transmitting messages and processing information.
The integumentary system is composed of the skin, hair, fat and nails and is concerned with protection and cushioning, waterproofing and prevention of water loss, sensory detection and vitamin D synthesis (in exposure to sunlight).
Bibliography
Martini, Frederic H and Timmons, Michael J: Human Anatomy, Prentice Hall, New Jersey 1995 http://en.wikipedia.org/wiki/Tissue_(biology) http://en.wikipedia.org/wiki/Connective_tissue http://en.wikipedia.org/wiki/Muscle_tissue http://en.wikipedia.org/wiki/Nervous_tissue http://en.wikipedia.org/wiki/Ground_substance http://en.wikipedia.org/wiki/Organs http://en.wikipedia.org/wiki/Parenchyma http://en.wikipedia.org/wiki/Myocyte http://en.wikipedia.org/wiki/Stroma_(animal_tissue) http://en.wikipedia.org/wiki/Human_skin http://en.wikipedia.org/wiki/Biological_system http://en.wikipedia.org/wiki/Respiratory_system
http://en.wikipedia.org/wiki/Lymphatic_system