Maintaining internal environments

To stay alive and function properly, your body must keep its internal conditions remarkably stable, even when the world around you changes completely. This constant balancing act is called homeostasis.

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What you'll learn

• Why keeping a stable internal environment is vital for metabolic reactions and enzyme activity.
• How the body monitors and adjusts temperature and blood glucose levels.
• How the kidneys regulate water balance to prevent cellular damage.

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1. What is Homeostasis?

Your body is constantly bombarded by changes. You might walk into a freezing room, eat a sugary meal, or go hours without a drink of water. Despite these external changes, your internal environment must remain stable.

Why is homeostasis important?

Almost every chemical reaction in your body is controlled by biological catalysts called enzymes. These reactions are collectively known as your metabolic reactions.

Enzymes require very specific conditions to work efficiently:

• Temperature: If your body temperature rises too high, the chemical bonds holding enzymes together break, changing the shape of their active sites. The enzymes denature and can no longer catalyse metabolic reactions. If the temperature drops too low, reactions slow down to a level that cannot sustain life.
• Water levels and pH: Deviations in water concentration or pH also disrupt enzyme function and can physically damage cells.

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2. Controlling Body Temperature (Separate Biology Only)

If you are taking the separate GCSE Biology course (J247), you need to know how the body regulates its temperature. Your skin plays a massive role in this process, working alongside the thermoregulatory centre in your brain (the hypothalamus).

The hypothalamus acts as your body's thermostat. It receives signals from:

1. Temperature receptors in the skin, which detect changes in the external environment.
2. Temperature receptors inside the hypothalamus itself, which monitor the temperature of the blood flowing through your brain.

When you are too hot

If your core temperature rises, your body needs to transfer more thermal energy to the surroundings. The skin achieves this in several ways:

• Vasodilation: The arterioles (small arteries) supplying the capillaries near the surface of your skin widen (dilate). This allows more blood to flow close to the skin surface, increasing heat loss by radiation.
• Sweating: Sweat glands secrete sweat (mostly water and some salts) onto the surface of the skin. As this sweat evaporates, it takes heat energy away from the body, cooling you down.

When you are too cold

If your temperature drops, your body needs to conserve heat and generate extra warmth:

• Vasoconstriction: The arterioles supplying skin-surface capillaries narrow (constrict). This reduces blood flow near the cold skin surface, keeping warm blood deeper inside the body and reducing heat loss.
• Shivering: Your muscles contract and relax rapidly. This shivering requires rapid muscle respiration, which releases heat energy as a metabolic byproduct, warming your body up.
• Erector muscles contract: Tiny muscles at the base of your skin hairs contract, pulling the hairs upright. This traps a thick layer of insulating air next to your skin.

<commonmistake title="Vasodilation vs. "Moving" Capillaries">

Capillaries themselves do not have muscle walls, so they cannot open, close, or "move" closer to the skin surface. When explaining temperature control, always write that arterioles widen (vasodilation) or narrow (vasoconstriction) to control how much blood flows into the capillaries near the skin surface.

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3. Controlling Blood Glucose

Every cell in your body needs glucose for respiration to release energy. However, having too much or too little glucose in your blood is highly dangerous.

The pancreas is the organ responsible for monitoring and controlling blood glucose concentration. It acts as both a detector and an endocrine gland, releasing two key hormones: insulin and glucagon.

When blood glucose is too high

After you eat a meal rich in carbohydrates, glucose is absorbed into your blood from your small intestine, causing blood glucose levels to rise.

1. The pancreas detects this rise and secretes insulin into the bloodstream.
2. Insulin travels in the blood to target cells, primarily in the liver and muscles.
3. Insulin triggers these cells to absorb glucose from the blood and convert it into an insoluble storage carbohydrate called glycogen.
4. As glucose is removed from the blood to be stored as glycogen, the blood glucose concentration drops back to its normal level.

When blood glucose is too low (Higher Tier Only)

If you exercise or go a long time without eating, your cells use up blood glucose for respiration, causing blood glucose levels to fall.

1. The pancreas detects the drop and secretes glucagon into the bloodstream.
2. Glucagon travels to the liver, where it triggers the conversion of stored glycogen back into soluble glucose.
3. This glucose is released back into the blood, raising blood glucose levels back to normal.

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4. Diabetes

Diabetes is a non-communicable disease where the body cannot successfully regulate blood glucose levels. There are two distinct types:

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5. Osmotic Balance and Cells (Separate Biology Only)

The water potential of your blood and tissue fluid must be kept at a steady level. If the concentration of water in your body fluids changes, it has a direct, physical effect on your cells due to osmosis.

• If body fluids are too dilute (high water potential): Water will move into your cells by osmosis down a water potential gradient (from high water potential outside to lower water potential inside). Because animal cells do not have a cell wall, they will swell up and burst. This process is called lysis.
• If body fluids are too concentrated (low water potential): Water will leave your cells by osmosis. This causes the cells to lose volume, shrivel, and shrink, which prevents them from carrying out normal metabolic functions.

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6. The Kidneys and Water Balance (Separate Biology Only)

Your kidneys are the main organs of excretion and osmotic balance. They have two major roles:

1. Excretion: Filtering waste products (such as urea, which is produced in the liver from excess amino acids) out of the blood.
2. Osmoregulation: Controlling the water and ion balance of your blood by varying the volume and concentration of urine excreted.

Gross Structure of the Urinary System

Blood enters the kidneys through the renal arteries. The kidneys filter this blood, producing urine. This urine travels down tubes called ureters to the bladder, where it is stored before being excreted from the body via the urethra.

Structure of the Kidney Tubule (The Nephron)

Each kidney contains around one million microscopic filtering units called nephrons (or kidney tubules).

The nephron works through a multi-step process:

1. Bowman's capsule (Ultrafiltration): Blood arrives under high pressure at a capillary knot (the glomerulus) nestled inside the Bowman's capsule. Small molecules—such as water, urea, glucose, and ions—are squeezed out of the blood into the capsule to form a filtrate. Large molecules like proteins and red blood cells are too big to pass through and remain in the blood.
2. Proximal Convoluted Tubule (Selective Reabsorption): As the filtrate moves along, all the glucose and some useful ions are pumped back into the blood by active transport. This is highly selective because the body cannot afford to lose glucose.
3. Loop of Henle: This U-shaped loop helps to establish a concentration gradient in the kidney tissue, allowing water to be reabsorbed back into the blood.
4. Collecting Duct: The remaining waste fluid (now called urine) passes down the collecting duct. This is where final adjustments to water levels are made, depending on how hydrated the body is.

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7. ADH and Osmotic Challenges (Separate & Higher Tier Only)

The permeability of the collecting duct walls to water is controlled by a hormone called ADH (Anti-Diuretic Hormone), which is released by the pituitary gland in the brain. This is another negative feedback system.

How the ADH feedback loop works

                       [ High water concentration in blood ]
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                                       ▼
                         Pituitary releases LESS ADH
                                       │
                                       ▼
                       Collecting duct becomes LESS permeable
                                       │
                                       ▼
                         Less water is reabsorbed
                                       │
                                       ▼
                       Large volume of DILUTE urine produced

                        [ Low water concentration in blood ]
                                       │
                                       ▼
                         Pituitary releases MORE ADH
                                       │
                                       ▼
                       Collecting duct becomes MORE permeable
                                       │
                                       ▼
                         More water is reabsorbed
                                       │
                                       ▼
                       Small volume of CONCENTRATED urine produced

Dealing with Osmotic Challenges

You may be asked to explain how the body responds to specific physical challenges:

• High sweating / dehydration: Sweating causes a significant loss of water from the body. Blood water potential drops. Osmoreceptors in the brain detect this and trigger thirst, encouraging you to drink. Simultaneously, the pituitary gland releases more ADH. This makes the collecting ducts highly permeable, so most of the water is reabsorbed into the blood. You produce a very small volume of dark, concentrated urine.
• Excess water intake: If you drink large volumes of water, the blood water potential rises. The brain detects this and releases less ADH. The collecting ducts become less permeable to water, meaning less water is reabsorbed. You produce a large volume of pale, dilute urine.
• High salt intake: Eating salty food lowers your blood water potential. This triggers the release of more ADH to conserve water, while also stimulating thirst to dilute the salt in your blood.

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Worked Example: Calculating Changes in Urine Output

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