Humans have billions of cells, tissues, organs, and systems. Cells make up all living things, from single-celled plants to multibillion-celled mammals. One fertilised cell becomes the human body. An adult body has 100 trillion cells. Organs are specialised tissue groups. An organ system performs complex physiological activities. Muscular, neurological, endocrine, cardiovascular, lymphatic, respiratory, digestive, urinary, and reproductive systems. Physiological and psychological body functions. Homeostasis, a stable internal environment, is vital to survival. Reproduction, growth, differentiation, respiration, digesting, and excretion are human life processes. Life requires oxygen, food, heat, and pressure. Cell vitality depends on extracellular and intracellular chemicals being separate. Cell membrane transport involves diffusion, osmosis, filtration, active transport, endocytosis, and exocytosis. Cell division creates growth, repair, and replacement cells. Mitosis replicates soma
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