Body organisation and homeostasis
The human body is organised from cells to organ systems, all working to keep internal conditions stable.
Levels of organisation
Cells form tissues, groups of similar cells doing a job. The four main tissue types are epithelial (covering and lining), connective (support, such as bone and blood), muscle (movement), and nervous (signalling). Tissues form organs, like the heart, and organs form systems, like the circulatory system.
Homeostasis
Homeostasis is the maintenance of stable internal conditions, such as body temperature near 37 degrees C, blood glucose, water balance, and blood pH near 7.4. Cells work properly only within narrow ranges, so the body constantly monitors and adjusts.
Negative feedback
Most homeostasis uses negative feedback: a change triggers a response that reverses it. When you overheat, sensors signal the brain, which triggers sweating and widens skin blood vessels, cooling you down. When cold, you shiver and skin vessels narrow. It works like a thermostat.
Positive feedback
Positive feedback amplifies a change instead of reversing it, pushing a process to completion. During childbirth, contractions trigger release of the hormone oxytocin, which strengthens contractions until the baby is born. Blood clotting also uses positive feedback.
Blood glucose control
After a meal, rising blood glucose triggers the pancreas to release insulin, which helps cells take up glucose and the liver store it as glycogen. When glucose falls, the pancreas releases glucagon, which tells the liver to release glucose. Type 1 diabetes results from too little insulin; type 2 from cells responding poorly to it.
The integumentary and skeletal systems
Skin is the body's largest organ, protecting against microbes, water loss, and ultraviolet light, and helping control temperature. The skeleton's 206 bones in adults support the body, protect organs, store calcium, and make blood cells in red bone marrow.
The muscular system
Skeletal muscles move bones and work in opposing pairs: the biceps bends the elbow, the triceps straightens it. Smooth muscle lines organs such as the gut and blood vessels; cardiac muscle forms the heart. Muscle contraction uses ATP as filaments slide past each other.
Worked example
On a hot day, body temperature rises to 37.5 degrees C. Temperature sensors detect it; the hypothalamus in the brain acts as the control centre; sweat glands and skin blood vessels are the effectors. Evaporating sweat cools the skin, temperature falls toward 37 degrees C, and the response switches off: negative feedback.
Key takeaways
Cells form tissues, organs, and organ systems. Homeostasis keeps conditions stable, mostly by negative feedback; positive feedback drives processes like childbirth to completion. Insulin and glucagon control blood glucose. Skin protects, bones support and make blood cells, and muscles move the body in opposing pairs.
Flashcard
Group of similar cells doing a job
Check your understanding
Shivering when cold is an example of: