Biology: Homeostasis

15.1 Homeostasis

15.1 Homeostasis

What is homeostasis?

Homeostasis is the maintenance of a stable internal environment within restricted limits in organisms.

This ensures that cells function normally despite changes in the external environment.

Why homeostasis is important:

1. It keeps the internal environment constant for metabolic reactions.

2. It ensures cells function properly and avoid damage.

3. It helps organisms respond and adapt to external changes.

Control mechanisms in homeostasis

Homeostasis is coordinated by several different control mechanisms, consisting of receptors, coordinators, and effectors throughout the body.

The roles of receptors, coordinators, and effectors in homeostasis:

• Receptors - These sensory receptors detect stimuli and send signals to the brain about changes in the internal environment, like changes in blood pH and temperature.

• Coordinator - This receives and interprets information from receptors and sends instructions to an appropriate effector.

• Effectors - These are muscles or glands that act on signals from the brain and cause responses to reverse changes and regain equilibrium, such as sweating to reduce high temperature.

These control mechanisms aim to maintain conditions around an optimum point: the point at which the system operates best.

Negative feedback systems

Negative feedback systems involve coordination between receptors and effectors to control conditions around set optimum points, where the system works best. A derivation from the optimum point leads to changes that bring the system back to the optimum point.

Negative feedback works as follows:

1. Receptors detect a change in one direction, like rising blood glucose.

2. Signals trigger effectors to produce responses that reverse the initial change, like releasing insulin to lower blood glucose.

3. Conditions return to their set range.

Examples of negative feedback systems

There are many examples of negative feedback mechanisms in the body, but there are a few that you need to know about.

Maintaining blood glucose concentration

• Why it is important - Glucose is needed for respiration, but too much glucose can affect water potential in blood and cells.

• How it is achieved - Insulin and glucagon adjust blood glucose concentration to maintain a healthy supply of glucose.

Maintaining blood pH

• Why it is important - Changes in pH can impair enzyme action.

• How it is achieved - Adjustments are made to the acid-base balance in the blood to maintain the optimum pH.

Maintaining temperature

• Why it is important - Changes in temperature can impair enzyme action.

• How it is achieved - Adjustments are made, for instance by sweating or shivering, to maintain the optimum temperature.

Water regulation

• Why it is important - Too much or too little water in the blood and cells can cause cells to burst or shrink due to osmosis. 

• How it is achieved - Water is removed or reabsorbed from blood or tissue fluid to maintain the optimum water potential.

Positive feedback systems

Positive feedback, in contrast to negative feedback, amplifies changes rather than reversing them. In other words, a deviation from an optimum causes changes that result in an even greater deviation from the optimum point.

Positive feedback works as follows:

1. An initial change occurs, like the release of clotting factors after a blood vessel injury.

2. Effectors are stimulated and enhance the change, like more clotting factors being released.

3. The change continues until an endpoint is met, like a clot being fully formed.

Examples of positive feedback systems

Positive feedback is less common than negative feedback in homeostasis, as uncontrolled responses can disrupt the body's equilibrium. Tight regulation is essential to prevent harm when changes intensify in these systems.

But, there are some examples of positive feedback mechanisms in the body that are useful to know.

Examples of positive feedback:

• Blood clotting - Clotting factors activate further clotting.

• Childbirth - Oxytocin stimulates more uterine contractions.

Cell signalling

Cell signalling is the process by which cells communicate. It can occur between adjacent cells, like when neurones release neurotransmitters to stimulate nearby nerve cells or muscle cells, or between very distant cells.

How cell signalling occurs between distant cells:

1. Cells can communicate by releasing hormones.

2. These hormones travel in the blood and signal to target cells that may be far away.

3. Cell-surface receptors enable cells to recognise and respond to these hormones.

15.2 Thermoregulation

15.2 Thermoregulation

What is thermoregulation

Thermoregulation is the process of maintaining a relatively constant core body temperature. This is important to maintain optimum enzyme activity.

Ectotherms and Endotherms +

Mechanism of thermoregulation in mammals +

To reduce body temperature when it is too high:

• Increased sweating - Effector sweat glands produce more sweat to promote evaporative cooling.

• Flattening hair - Effector erector pili muscles relax, flattening hairs and reducing insulation.

• Vasodilation - Effector arterioles near the skin dilate, increasing blood flow to the skin and heat radiation from the skin surface.

To increase body temperature when it is too low:

• Shivering - Effector skeletal muscles contract to generate heat through increased cellular respiration, an exothermic reaction.

• Minimising sweating - Effector sweat glands produce less sweat, which helps to conserve body heat.

• Erecting hair - Effector erector pili muscles contract, raising hairs, trapping a layer of warm air, and increasing insulation.

• Vasoconstriction - Effector arterioles near the skin constrict, reducing blood flow to the skin and heat radiation from the skin surface.

• Releasing adrenaline and thyroxine - Effector glands release these hormones to speed up cellular metabolism, which produces more heat.

The role of the hypothalamus in controlling body temperature

The hypothalamus is the thermostat of the brain, and is crucial in coordinating thermoregulation in mammals.

It does this as follows:

1. The hypothalamus collects information about core body temperature from temperature receptors in the hypothalamus and about surface temperature from peripheral receptors in the skin.

2. This information is processed in the hypothalamus to detect deviations from normal levels in core and surface body temperature.

3. The hypothalamus then sends signals to effectors like muscles and sweat glands.

4. These effectors implement mechanisms to restore the ideal temperature.

This homeostatic process lets mammals maintain a stable internal temperature, even when external temperatures fluctuate.

Heat loss and heat gain centres in hypothalamus

The two control centres involved in thermoregulation are the heat loss centre and the heat gain centre.

When blood temperature increases:

1. Impulses are sent to the heat loss centre in the hypothalamus.

2. This sends impulses to the effector organs to increase heat loss.

3. The body temperature returns to the optimum point.

When blood temperature decreases:

1. Impulses are sent to the heat gain centre in the hypothalamus.

2. This sends impulses to effector organs to reduce heat loss.

3. The body temperature returns to the optimum point.

15.3 Excretion, Homeostasis and The Liver

15.3 Excretion, Homeostasis and The Liver

What is excretion

The liver plays a crucial role in metabolism, which involves many chemical reactions. These reactions generate waste products, including CO₂ and nitrogenous substances, which can harm cells if they accumulate.

Excretion is the process of removing metabolic waste from cells. This is essential for maintaining normal metabolism and homeostasis. For instance, CO₂ is excreted by cells following respiration and is then removed from the body by the lungs.

Many metabolic waste products, like urea, are metabolised in and excreted from the liver cells.

Function of the liver in detoxification

The liver breaks down toxic substances such as alcohol, medications, hormones, and excess amino acids. This detoxification process converts these substances into less harmful compounds that cells can excrete.

The liver breaks down amino acids through these steps:

1. Amine groups are removed from amino acids by deamination, producing toxic ammonia and organic acids.

2. Organic acids are either used for ATP production or stored as glycogen.

3. Ammonia combines with CO₂ to form urea via the ornithine cycle, occurring partially in the mitochondria of liver cells.

4. Urea is then excreted from liver cells, enters the bloodstream, and is filtered out of the body via the kidneys as a part of urine.

Other substances detoxified by the liver include:

• Alcohol - The enzyme alcohol dehydrogenase breaks down ethanol to ethanal, which is then converted to ethanoate to prevent damage to cells.

• Hydrogen peroxide - The enzyme catalase splits hydrogen peroxide into oxygen and water to prevent cell damage.

• Paracetamol - This is broken down to prevent toxicity to the liver and kidneys.

• Insulin - This is metabolised to help regulate blood glucose concentration.

Function of liver in regulating blood glucose

The liver also plays other crucial roles in managing blood glucose levels.

Functions of the liver in blood glucose regulation:

• Converting excess glucose into glycogen, a storage molecule.

• Storing glycogen granules within its cells.

• Releasing glucose into the bloodstream by breaking down glycogen when blood glucose levels fall.

Function of liver in breaking down RBCs

Haemoglobin from old red blood cells is broken down in hepatocytes (liver cells) into bile pigments.

These are then excreted from liver cells and transported by the bile duct to the gallbladder where they are stored before their removal from the body.

Structure of the liver

The liver's structure is complex, featuring several vital blood vessels and ducts.

The key large parts of the liver structure you need to know:

1. The hepatic artery supplies oxygenated blood.

2. The hepatic vein carries away deoxygenated blood towards the heart.

3. The hepatic portal vein brings nutrient-rich blood from the intestines.

4. The bile duct transports bile to the gallbladder.

Structure of liver lobules +

The liver is composed of numerous lobules, that mostly consist of liver cells called hepatocytes. Hepatocytes have large nuclei, a prominent Golgi apparatus, and several mitochondria that help them carry out their many functions.

The key cells and tissues of the liver you need to know in each liver lobule:

1. Hepatocytes are arranged along channels called sinusoids.

2. The sinusoids are where oxygen-rich blood from the hepatic artery mixes with blood rich in the products of digestion from the hepatic portal vein.

3. A branch of the hepatic vein is located in the centre of each lobule to remove deoxygenated blood.

4. Kupffer cells ingest pathogens and other foreign particles, helping to protect against disease.

5. A channel separate from the sinusoids, called the bile canaliculus, links to a branch of the bile duct.