Cells to systems

Cells to systems
The living structure of the human body is built from different types of cells. Cells mass together in clumps or layers to form tissues, which are specialized for different roles within the body. The cell is fundamental unit of life. We all come into existence as a single fertilized cell that rapidly multiplies. By the time we reach adulthood, however, the cells that make up the body number approximately 75 trilion. Cells also contain DNA – the genetic material that is responsible for our development and our individual characteristics. Some cells last a lifetime; others wear out after a day or so, or fall victim to damage or disease. Cells act together in an organized way in order to function effectively – they communicate with one another via chemical messages.
The variety of cells and cell function
Around 200 different types of cell have been identified in the human body. Cells begin to differentiate, becoming specialized for particular roles, at a very early stage in our development. Cells may differ widely in their external appearance and in their activities, but almost all of them share the same internal components. They also have many basic functions in common.
Cells are busy factories, carrying out several thousand different tasks in an orderly, integrated way. Each type of cell has a specialized role, but most cells are involved in the breakdown of glucose, creating energy to drive their respective activities. Small structures called organelles within each cell carry out the cell’s vital activities, coordinated by the cell’s control center, the nucleus. One important task of organelles is the production of proteins, which are needed to carry out vital biochemical reactions in the body as well as for development and growth. Some organelles are involved in digestion processes while others can destroy dangerous chemicals that may potentially harm the cell.
Types of tissues
Collection of similar cells, and the substances around them, form tissue. There are five man types of tissue: epithelial, connective, skeletal, nervous and blood. Epithelial tissue lines and protects body organs. It is fairly closely packed and occurs in sheets that may be several layers thick. Connective tissue is the supporting tissue of the body, and includes bone, cartilage, and fat. It is more loosely packed than epithelial tissue. The protein collagen is very important in giving connective tissue its toughness. There are three types of muscle tissue. Skeletal muscle is attached to the bone and allows us to make voluntary movements; smooth muscle tissue is involved in involuntary movements; and cardiac muscle is specialized tissue allowing the coordinated beating of the heart. Nervous tissue consists of nerve cells (also known as neurons) that conduct electrical impulses, and supporting cells, called glial cells, which supply the working nerve cells with nutrients and oxygen.
Organs of the body
An organ is a working part of the body, consisting of two or more types of tissue. Human organs include the skin (the largest organ in the body), the heart, the lungs, the stomach, the liver, the kidneys and the intestines. The tissues that make up organs are specialized to perform specific function. The stomach for example needs to be able to churn food and secrete substances that aid digestion. The exterior surface of the stomach’s outer layer, the serosa, consists of a type of epithelial tissue made up of squamous (flattened) cells that protect the stomach against friction during churning. The serosa’s interior layer is connective tissue that supports the epithelial tissue and nourishes the surrounding structures. Three layers of smooth muscle tissue lie inside the connective tissue layer. Each layer of muscle tissue runs at a different angle, providing maximum directional movement for churning. Simple columnar epithelial tissue forms the inner lining of the stomach, called the mucosa. This type of epithelial tissue is made up of tall columnar cells, so that the tissue can lie in rugae (folds) when the stomach is empty and flatten to accommodate incoming food when necessary. The epithelial tissue in the lining of the stomach also houses glands that secrete enzymes and acid to break down food as well as cells that produce protective mucus.
Body systems
Systems of the body are made up of a group of tissue and organs that work together to carry out a specific function or set of functions. For example, the musculoskeletal system consists of bon, muscle, cartilage and tendons, which together provide support for the body and enable us to move. The main functions of each body system are listed in the table below and their components are described on the next page. Systems cannot work alone: each system is dependent on the others to function. For example, all systems in the body are reliant on the cardiovascular system to bring them nutrient – and oxygen-rich blood that provides them with the energy they require to function. The nervous system and the endocrine system are the body control’s systems: they continuously monitor body activities and adjust them appropriately.
BODY SYSTEMS AND THEIR FUNCTIONS
Musculoskeletal:
Provides the framework on which the body is built and facilitates movement. Integumentary:
Provides protection from the environment through the skin, hair and nails.
Respiratory:
Through breathing, supplies fresh oxygen to body tissues and expels carbon dioxide. Lymphatic and immune:
Defend and protect the body from infection and some cancers.
Cardiovascular:
Circulates blood to deliver nutrients and oxygen to all body tissues. Nervous:
Senses the environment; through nerve impulses, monitors and controls body activities.
Digestive:
Fuels the body by food breakdown and processing of nutrients; also removes waste. Endocrine:
Controls the body through the action of hormones secreted by glands and tissues.
Urinary:
Forms urine to rid the body of waste and help maintain its chemical balance. Reproductive:
Makes new bodies through the production of hormones, sperms and eggs.

Source: HUMAN (DK, SMITHSONIAN INSTITUTION – Robert Winston, Dr. Don E. Wilson, 2006.)