Cells, tissues and organs

Principles of Human Physiology

Cells, tissues and organ systems

All life on earth is based on the activity of cells. Cells are the smallest functional units of the human body and here we are concerned about what they are and how they form tissues and organs. The primary tissues include:

epithelial tissue made of five different types of epithelial cells

nervous tissue made of various types of neurons

muscle tissue made of three types of muscle cells

connective tissue everything else, e.g. made of blood cells, bone cells, fat cells, glial cells, fibroblast cells and all extracellular fluid compartments

The primary tissues are defined by the type of cells with related function. Organs are functional units of the body that are made of the four primary tissues in various proportions. The organs are part of organ systems, which are:

integumentary system the skin for protection, touch, temperature control

nervous system
the brain, spinal cord, and peripheral nerves for control and senses

endocrine system the pituitary, thyroid and adrenal glands and many local small gland tissues and individual cells for hormones

cardiac system the heart, blood vessels and blood for transport of nutrients, hormones, immune cells, proteins and waste

respiratory system the lung for breathing and gas exchange

muscoskeletal system
skeletal muscles and bones

immune system
bone marrow, spleen, thymus, lymp vessels and white blood cells for immune protection, recycling of red blood cells, and prevention of tumors

digestive system
the gut, liver and pancreas for food uptake and waste release

urinary system
the kidney and urinary bladder for fluid, salt and nitrogenous waste removal

reproductive system testis, ovaries for production of gametes sperm and eggs

Organ systems are made of several organs and in some cases individual cells located with in other organ systems or through out the body, as is the case for the immune and endocrine system. All organ systems, for instance harbor immune cells. The digestive tracts has endocrine cells et cetera.

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