Course 18.3Lesson 3 of 3
The nervous system and the endocrine system together coordinate the body. The hypothalamus joins them. Negative feedback keeps a level steady around a set point, and positive feedback drives a process to its end, as in childbirth. Hormones often work in opposite pairs, such as insulin and glucagon.
Try the code and the simulations beside the lesson.
When you run for a bus, your legs move by signals along nerves. At the same time, your heart beats faster, your breathing quickens and your blood sugar rises, and no nerve told your liver to release glucose. A hormone did. The neural system and the endocrine system together coordinate and integrate all the functions of the body.
The hypothalamus joins the two systems. It is a part of the brain, and its neurosecretory cells make the hormones that control the pituitary. In this way the brain controls the glands.
Most hormones are controlled by negative feedback. The result of the action comes back and switches the source down. The level then stays near a set point.
These are examples of homeostasis, the keeping of a steady internal state.
In positive feedback, the result makes the process go faster. A good example is childbirth. Signals from the foetus and the placenta cause mild contractions of the uterus. These trigger the release of oxytocin from the pituitary of the mother. Oxytocin makes the uterus contract more strongly. The stronger contraction releases more oxytocin. The loop goes on, with stronger and stronger contractions, until the baby is born. The birth then ends the loop.
Many hormones work in pairs with opposite effects. Together they hold a level steady.
After a meal. The glucose of the blood rises. The beta cells release insulin. The glucose falls. Hours later, the glucose is low, the alpha cells release glucagon, and the glucose rises again. In an emergency, adrenaline raises it further.
After a long walk in the heat. The body loses water. ADH is released from the posterior pituitary, so the distal tubules take back more water. Aldosterone keeps sodium. The urine becomes small in volume and concentrated.
In a fright. Nerves tell the adrenal medulla to release adrenaline within seconds. If the stress goes on, the hypothalamus and the pituitary release ACTH, which makes the cortex release cortisol. The hormones of the cortex act for hours.
A caution about the vocabulary. NCERT does not use the words negative feedback and positive feedback. A-Level and IB do.
You should find that the level grows slowly at first and faster later, because each cycle multiplies the level and does not add a fixed amount. A larger rise in each cycle shortens the time to the birth. This is a teaching model of the idea of positive feedback, not a measurement.
The program compares a negative loop that settles at a set point with a positive loop that keeps growing.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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