Coordination, excretion and homeostasis
How the body removes waste, responds to stimuli with nerves and hormones, keeps conditions constant, and how plants respond with tropisms.
📌 Key points
- The lungs excrete carbon dioxide; the kidneys excrete urea, excess water and excess ions in urine. Urea is made in the liver by deamination: removal of the nitrogen-containing part of amino acids.
- In the kidney, blood is filtered in the glomerulus; all the glucose, some ions and much of the water are reabsorbed in the tubule. Urine passes down the ureter to the bladder and out through the urethra.
- The nervous system is the central nervous system (brain and spinal cord) and peripheral nerves. Impulses are electrical signals carried by sensory, relay and motor neurones.
- In a reflex arc: stimulus → receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector (muscle or gland) → response. Reflexes are fast and automatic and protect the body.
- At a synapse, a neurotransmitter diffuses across the gap and binds to receptors on the next neurone; this makes impulses travel in one direction only.
- In bright light the circular muscles of the iris contract and the radial muscles relax, making the pupil smaller. To focus on a near object the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes fatter.
- A hormone is a chemical substance made by a gland, carried by the blood, which alters the activity of target organs. Adrenaline increases heart rate, breathing rate, pupil diameter and blood glucose. Hormones act more slowly than nerves but their effects last longer.
- Homeostasis is the maintenance of a constant internal environment by negative feedback. When blood glucose rises, the pancreas releases insulin and the liver converts glucose to glycogen; when it falls, glucagon is released. In type 1 diabetes the pancreas makes no insulin.
- When too hot: sweating and vasodilation (arterioles near the skin widen). When too cold: shivering and vasoconstriction (arterioles narrow).
- Shoots grow towards light (positive phototropism) and roots grow towards gravity (positive gravitropism). Auxin made in the shoot tip moves to the shaded side and stimulates cell elongation there. Antibiotics kill bacteria but do not affect viruses.
📖 Key terms
- Excretion
- Removal of waste products of metabolism, toxic materials and substances in excess.
- Deamination
- Removal of the nitrogen-containing part of amino acids to form urea.
- Reflex action
- A fast, automatic response to a stimulus.
- Synapse
- A junction between two neurones.
- Hormone
- A chemical made by a gland, carried by the blood, that alters the activity of target organs.
- Homeostasis
- The maintenance of a constant internal environment.
- Negative feedback
- A change from the normal level causes a response that returns it to normal.
- Tropism
- A growth response of a plant to a directional stimulus.
✏️ Worked example
Describe the pathway of the reflex action when your hand touches a hot object.
- The stimulus (heat) is detected by receptors in the skin.
- An impulse travels along a sensory neurone to the spinal cord.
- It passes across a synapse to a relay neurone, then across another synapse to a motor neurone.
- The motor neurone carries the impulse to the effector, a muscle in the arm.
- Response: the muscle contracts and the hand is pulled away.
In vasodilation the arterioles widen – blood vessels do not "move closer to the skin". Also, sweat cools you because the water evaporates and takes thermal energy from the skin.
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