Homeostasis

Define homeostasis.

Homeostasis is the maintenance of a constant internal environment, keeping conditions such as temperature, blood pressure, water concentration and glucose concentration within set limits.

Why is homeostasis important for the body's cells?

It keeps internal conditions within set limits so that reactions in body cells can function properly, allowing the organism as a whole to live.

What happens if an internal condition deviates far from normal and is not brought back within set limits?

The body will not function properly, and without medical intervention the eventual consequence is death.

What is negative feedback?

A process where conditions changing from the ideal or set point are returned to that set point: if a level rises, systems reduce it; if it falls, systems raise it.

How do negative feedback mechanisms keep levels within a narrow range?

They work as a continuous cycle of bringing levels down and then back up, so overall the level stays within a narrow range of what is considered normal.

What is thermoregulation and what temperature does the human body maintain?

Thermoregulation is the control of body temperature. The body maintains a temperature at which enzymes work best, around 37°C.

Which part of the brain monitors and controls body temperature?

The thermoregulatory centre in the hypothalamus, which contains receptors sensitive to the temperature of the blood.

How does the skin contribute to temperature monitoring?

The skin contains temperature receptors within the epidermal layer which send nerve impulses to the thermoregulatory centre.

What changes occur in the skin when body temperature is too high?

Hair erector muscles relax, blood vessels dilate (vasodilation) and sweat is produced from the sweat glands, transferring energy from the skin to the environment to cool the body down.

What changes occur in the skin when body temperature is too low?

Hair erector muscles contract, blood vessels constrict (vasoconstriction), sweating stops and skeletal muscles contract (shiver), reducing heat loss and increasing heat released in the body.

What is vasodilation and how does it help control body temperature?

The muscles in the arteriole walls relax, causing arterioles near the skin to dilate so more blood flows through skin capillaries, increasing heat loss to the environment by radiation.

What is vasoconstriction and how does it help control body temperature?

The muscles in the arteriole walls contract, causing arterioles near the skin to constrict so less blood flows through skin capillaries, reducing heat loss to the environment by radiation.

Why is it the arterioles, not the capillaries, that dilate or constrict?

Arterioles have a layer of muscle in their walls that can relax or contract; capillary walls are only one cell thick and contain no muscle capable of changing diameter.

During vasoconstriction, where is blood diverted and why?

Blood is diverted through shunt vessels, which are further down in the skin and therefore do not lose heat to the environment.

Why is vasoconstriction not strictly a 'warming' mechanism?

It does not raise the temperature of the blood; it reduces heat loss from the blood as it flows through the skin.

Define osmoregulation.

Osmoregulation is the process of maintaining water and salt concentrations (osmotic balance) across membranes within the body.

What happens to body cells if there is too much water in the blood?

Cells swell as water moves into them by osmosis; this has a diluting effect and can lead to cell lysis (bursting).

What happens to body cells if there is too little water in the blood?

Cells lose water by osmosis, which has a dehydrating effect and can lead to cell death.

What are the two sources of water in the body?

Water produced as a result of aerobic respiration, and water in the diet.

In what ways is water lost from the body?

Via the lungs during exhalation, from the skin as sweat (alongside mineral ions and urea), and in urine.

Which water loss cannot be controlled, and which can?

Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys.

What are the two key functions of the urinary system?

To filter waste products from the blood and expel them as urine, and to control the water levels of the body (osmoregulation).

Name the main structures of the urinary system.

Two kidneys joined to the bladder by two ureters; the urethra carries urine from the bladder to outside the body.

What is the difference between the ureter and the urethra?

The ureter is the tube joining a kidney to the bladder; the urethra is the tube that carries urine from the bladder to outside the body.

Which blood vessels connect each kidney to the main circulation?

The renal artery, which comes from the aorta and delivers oxygenated blood to the kidney, and the renal vein, which delivers deoxygenated blood from the kidney to the vena cava.

What are the two important functions of the kidneys?

They regulate the water content of the blood (vital for maintaining blood pressure) and excrete the toxic waste products of metabolism (such as urea) and substances in excess of requirements (such as salts).

Name the three regions of the kidney.

Cortex (outermost region), medulla (inner section) and renal pelvis (the tube linking the kidney to the ureter).

What are nephrons and where are they found?

Each kidney contains around a million tiny structures called nephrons (kidney tubules). They start in the cortex, loop down into the medulla and back up to the cortex.

Name the sections of the nephron.

Bowman's capsule, proximal convoluted tubule, loop of Henlé, distal convoluted tubule and collecting duct.

What is ultrafiltration?

Blood is filtered as it passes through the glomerulus in the Bowman's capsule; high pressure forces smaller molecules out of the capillaries to form the filtrate.

Which substances are forced out of the capillaries during ultrafiltration?

Glucose, water, urea and salts.

Where and how is glucose reabsorbed from the filtrate?

Glucose is the first substance reabsorbed, at the proximal convoluted tubule, by active transport.

How is the nephron adapted for the active transport of glucose?

It has many mitochondria to provide the energy needed for active transport of glucose molecules.

Why can glucose only be reabsorbed at the proximal convoluted tubule?

The gates that facilitate the active transport of glucose are only found in the proximal convoluted tubule.

Why does glucose appear in the urine of people with diabetes?

Their blood glucose levels are often very high, so not all of the glucose filtered out can be reabsorbed in the proximal convoluted tubule; it continues in the filtrate and ends up in the urine.

How are water and salts reabsorbed in the nephron?

Necessary salts are reabsorbed back into the blood by diffusion as the filtrate drips through the loop of Henlé; as salts are reabsorbed, water follows by osmosis. Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs.

What does urine produced by the kidneys contain?

A mixture of urea, excess mineral ions and excess water.

Why is a large quantity of urine usually pale yellow?

It contains a lot of water, so the urea is less concentrated.

Why is a small quantity of urine usually darker yellow or orange?

It contains little water, so the urea is more concentrated.

How does water intake affect the concentration of urine?

The more fluids drunk, the more water is removed from the body, so a large quantity of pale yellow, dilute urine is produced.

How do temperature and exercise affect the concentration of urine?

The higher the temperature or the greater the level of exercise, the more water is lost in sweat, so less appears in the urine, meaning a smaller quantity of dark yellow, concentrated urine is produced.

Which gland releases ADH and what does ADH affect?

The pituitary gland in the brain constantly releases ADH; it affects the permeability of the tubules to water.

What happens when the water content of the blood is too high?

The pituitary gland releases less ADH, so less water is reabsorbed in the tubules of the kidney (they become less permeable to water) and a large volume of dilute urine is produced.

What happens when the water content of the blood is too low?

The pituitary gland releases more ADH, so more water is reabsorbed in the tubules of the kidney (they become more permeable to water) and a small volume of concentrated urine is produced.

What is the usual treatment for someone with kidney failure?

Dialysis, an artificial method of filtering the blood to remove toxins and excess substances.

How does a dialysis machine work?

Unfiltered blood is taken from an artery in the arm, pumped into the machine and returned to a vein. Inside, blood and dialysis fluid are separated by a partially permeable membrane and flow in opposite directions, allowing exchange where a concentration gradient exists.

What does dialysis fluid contain?

A glucose concentration similar to a normal level in blood, a concentration of salts similar to a normal level in blood, and no urea.

Why does urea diffuse out of the blood during dialysis?

The dialysis fluid has no urea in it, so there is a large concentration gradient and urea diffuses across the partially permeable membrane from the blood to the dialysis fluid.

Why does glucose not move across the membrane during dialysis?

The dialysis fluid contains a glucose concentration equal to a normal blood sugar level, so no concentration gradient exists and there is no net movement of glucose.

Why is the fluid in a dialysis machine continually refreshed?

To maintain the concentration gradients between the dialysis fluid and the blood.

Why is an anticoagulant added to blood before it runs through a dialysis machine?

To prevent the blood from clotting and slowing the flow.

Give two disadvantages of kidney transplants.

Donors won't have the same antigens on cell surfaces, so there is some immune response and risk of rejection; immunosuppressant drugs must be taken for life, which can have long term side effects and leave the patient vulnerable to infections. There are also not enough donors to cope with demand.

Give three benefits of a kidney transplant over dialysis.

The patient has much more freedom as they are not tied to having dialysis several times a week in one place; their diets can be much less restrictive; and the cost of using dialysis machines is removed. A transplant is also a long-term solution whereas dialysis only works for a limited time.

Define excretion.

Excretion is the removal of the waste substances of metabolic reactions, toxic materials and substances in excess of requirements.

Why must carbon dioxide be excreted?

Too much carbon dioxide is toxic; it dissolves in water to form an acidic solution which can lower the pH of cells, reducing the activity of enzymes essential for controlling the rate of metabolic reactions.

Which organs are involved in excretion and what does each excrete?

The lungs excrete carbon dioxide, the liver forms urea from excess amino acids, and the kidneys excrete water, excess salts and urea.

What is deamination and where does it occur?

The breakdown of excess amino acids that are not needed to make proteins, carried out by enzymes in the liver.

What happens to the parts of an amino acid molecule during deamination?

The part containing carbon is turned into glycogen and stored; the part containing nitrogen is turned into ammonia, which is highly toxic and is immediately converted into urea, which is less toxic.

What happens to the urea formed in the liver?

It dissolves in the blood and is taken to the kidney to be excreted; a small amount is also excreted in sweat.

What is the difference between excretion and egestion?

Excretion is the removal from the body of waste products of metabolic reactions, toxic substances and substances in excess of requirements; egestion is the expulsion of undigested food waste from the anus.

Which organ monitors and controls blood glucose concentration?

The pancreas, which is an endocrine gland that makes and secretes hormones into the bloodstream (and also plays a separate role in digestion).

What does the pancreas do when blood glucose concentration is too high?

It produces the hormone insulin to bring the level back down.

Why is too high a level of glucose in the blood dangerous?

It can lead to cells of the body losing water by osmosis, which can be dangerous.

How does insulin restore blood glucose to normal levels?

Insulin stimulates cells to take in glucose from the bloodstream (particularly liver and muscle cells); in liver and muscle cells excess glucose is converted into glycogen for storage.

What is glycogen?

A polymer of glucose, the form in which excess glucose is stored in liver and muscle cells.

What does the pancreas do when blood glucose concentration is too low?

It produces the hormone glucagon, which causes glycogen to be converted into glucose and released into the blood.

How do insulin and glucagon interact to control blood glucose?

They act as a negative feedback cycle: insulin is produced when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen, reducing blood glucose; glucagon is produced when blood glucose falls too low and stimulates those cells to convert stored glycogen into glucose, increasing blood glucose.

What is the difference between glucagon and glycogen?

Glucagon is the hormone produced by the pancreas; glycogen is the polysaccharide that glucose is stored as.

What causes type 1 diabetes?

The pancreas fails to produce sufficient insulin to control blood glucose levels; scientists think this is a result of a person's own immune system destroying the cells of the pancreas that make insulin during development.

How is type 1 diabetes characterised and normally treated?

It is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.

What causes type 2 diabetes?

The body cells no longer respond to insulin produced by the pancreas; the person still makes insulin but their cells are resistant to it and don't respond as well as they should, leading to uncontrolled high blood glucose levels.

How is type 2 diabetes commonly treated?

With a carbohydrate-controlled diet and an exercise regime.

Why is obesity a major risk factor for type 2 diabetes?

A person who is obese is likely to consume a diet rich in carbohydrates, which causes an over-production of insulin resulting in the development of insulin resistance.

What BMI value classifies an individual as obese?

A BMI greater than 30.

How is BMI calculated?

BMI = mass (kg) ÷ height (m)²

How is waist-to-hip ratio calculated and what values indicate increased risk of type 2 diabetes?

Waist-to-hip ratio = waist circumference (cm) ÷ hip circumference (cm). A ratio above 1.0 for men and 0.85 for women is associated with increased risk of developing type 2 diabetes.

Why does where fat is stored matter for type 2 diabetes risk?

Individuals with a lot of excess fat stored around the abdomen area have an increased risk of developing type 2 diabetes.

What are the components of a control system in the body?

Receptors, which detect stimuli (changes in the environment); coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors; and effectors, muscles or glands, which bring about responses that restore optimum levels.

What is the difference between a receptor and an effector?

A receptor detects a stimulus (a change in the environment); an effector is a muscle or gland that brings about a response to restore optimum levels.