A cell is a system as well as a functional unit of life. Some organisms made up of just one cell are called unicellular (e.g. bacteria and protozoans), but animals, including human beings, are multi-cellular. An adult human body is composed of about 100,000,000,000,000 cells! Each cell has basic requirements to sustain it, and the body's organ systems are largely built around providing the many trillions of cells with those basic needs (such as oxygen, food, and waste removal).
There are about 200 different kinds of specialized cells in the human body. When many identical cells are organized together it is called a tissue (such as muscle tissue, nervous tissue, etc). Various tissues organized together for a common purpose are called organs (e.g. the stomach, the skin, the brain, and the uterus).
Ideas about cell structure have changed considerably over the years. Early biologists saw cells as simple membranous sacs containing fluid and a few floating particles. Today's biologists know that cells are inconceivably more complex than this. Therefore, a strong knowledge of the various cellular organelles and their functions is important to every Health Guardian. If a person's cells are healthy, then that person is healthy. All physiological processes, disease, growth and development can be described at the cellular level.
Although there are specialized cells - both in structure and function - within the body, all cells have similarities in their structural organization and metabolic needs (such as maintaining energy levels via conversion of carbohydrate to ATP and using genes to create and maintain proteins). The Health Guardian can leverage these commonalities and needs while addressing chronic illness and pathology prevention.
Some of the different types of specialized cells within the human body include:
- Nerve Cells: Also called neurons, these cells are in the nervous system and function to process and transmit information (it is hypothesized). They are the core components of the brain, spinal cord, and peripheral nerves. They use chemical synapses that can evoke electrical signals, called action potentials, to relay signals throughout the body.
- Epithelial cells: Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability. The epithelium lines both the outside (skin) and the inside cavities and lumen of bodies.
- Exocrine cells: These cells secrete products through ducts, such as mucus, sweat, or digestive enzymes. The products of these cells go directly to the target organ through the ducts. For example, the bile from the gallbladder is carried directly into the duodenum via the bile duct.
- Endocrine cells: These cells are similar to exocrine cells, but secrete their products directly into the bloodstream instead of through a duct. Endocrine cells are found throughout the body but are concentrated in hormone-secreting glands such as the pituitary. The products of the endocrine cells go throughout the body in the bloodstream, but act on specific organs by receptors on the cells of the target organs. For example, the hormone estrogen, acts specifically on the uterus and breasts of females because there are estrogen receptors in the cells of these target organs.
- Red Blood Cells: The main function of red blood cells is to collect oxygen in the lungs and deliver it through the blood to the body tissues. Gas exchange is carried out by simple diffusion.
- White Blood Cells: Are produced in the bone marrow and help the body to fight infectious disease and foreign objects in the immune system. White cells are found in the circulatory system, lymphatic system, spleen, and other body tissues.