Biology Edexcel Topic 7 (Homeostasis)

7.9 to 7.17

What is homeostasis

The maintenance of a stable internal environment in the body (despite fluctuations in internal and external conditions)

Why is homeostasis important

It ensures the optimum conditions for enzymes and cellular process in our body to work well

State three conditions within the body that must be controlled by homeostasis

Temperature, blood glucose concentration and water levels

What is thermoregulation

The maintenance of our core body temperature

Why must body temperature be controlled

Enzymes have an optimum temperature which they work the best at so deviating from this temperature will decrease the rate of enzyme-controlled reactions

What is the optimum temperature for enzymes in the human body

37^C

What does the maintenance of an ideal body temperature depend on

A negative feedback system that involves receptors, the hypothalamus, and effectors

Where are the temperature sensitive receptors located

On the skin in the epidermis (outer layer) and the dermis (layer below the epidermis) and in the hypothalamus

What are the functions of receptors in the skin and in the hypothalamus

They detect changes in blood temperature and send information to the hypothalamus

What is the function of the hypothalamus

It coordinates information from the receptors and sends instructions to the effectors

What is the function of the effectors

They produce a response to counteract the change in blood temperature in order to return it to a set point

Outline the responses of a body to an increase in temperature above 37^C

Vasodilation, sweating, and hair erector muscles relax

What structure produces sweat

Sweat glands in the dermis

How is sweat released from the skin

Sweat from the sweat glands get released from pores in the epidermis onto the skin's surface

How does sweating help reduce body temperature

Heat energy is used to evaporate sweat so increased heat transfers from the skin to the environment due to evaporation and this decreases body temperature

What is vasodilation

Dilation of arteries leading to the capillaries near the skin surface so more warm blood flows closer to the skin surface which causes a greater heat loss to the surroundings

How does relaxing the hair effector muscles help reduce body temperature

Relaxing the hair effector muscles makes the hairs lie flat and not trap a layer of hot air next to your skin

Outline the responses of the body to a decrease in temperature below 37^C

Vasoconstriction, shivering, hair erector muscles contract and little sweat is produced

How does shivering help to increase body temperature

Rapid contraction of muscles generates heat energy due to the respiration

How does the contraction of hair erector muscles help increase body temperature

Hairs stand on end which traps pockets of warm air between hairs and forms a layer of insulation

What is vasoconstriction

Constriction of arteries leading to the capillaries near skin surface so less warm blood flows closer to the skin surface which causes less heat to be lost to the surroundings

Which organ is responsible for the maintenance of blood glucose concentrations

Pancreas

How are blood glucose concentrations controlled

They are controlled by the hormones insulin and glucagon which are both secreted by the pancreas

Insulin and glucagon are antagonistic hormones - what does this mean

They have opposite effects which counteract one another

Describe the role of insulin in the regulation of blood sugar levels

It causes the liver and muscle cells to increase their uptake of glucose from the blood - this glucose is then converted into glycogen (a storage molecule)

Describe the role of glucagon in the regulation of blood sugar levels

It causes the breakdown of glycogen into glucose in the liver and so glucose is released into the blood

What is the control of blood glucose concentration an example of

Negative feedback

Describe what happens when blood glucose concentrations become too high

Pancreas secretes insulin and stops producing glucagon, liver and muscle cells increase the uptake of glucose which is converted into glycogen and stored - some glucose may be stored as lipid in tissues - this decreases the blood glucose concentration, returning it to a normal level

Describe what happens when blood glucose concentrations become too low

Pancreas secretes glucagon and stops producing insulin, liver cells convert glycogen into glucose, which is released into the blood - this increases the blood glucose concentration, returning it to a normal level

What is diabetes

A condition where the homeostatic control of blood glucose levels stops working

What is the cause of type 1 diabetes

Pancreas does not produce enough insulin

How is type 1 diabetes treated

Daily insulin injections at meal times - limiting the intake of refined sugars and regular exercise

What is the cause of type 2 diabetes

Person develops insulin resistance or doesn't produce enough insulin (often due to obesity)

How is type 2 diabetes treated

Eating a balanced diet, exercise and medication or insulin injections (medication and insulin injections are less effective than other methods)

How does exercise help to control diabetes

Exercise increases respiration in muscle cells - excess glucose is removed from the blood to produce energy in the form of ATP

Why are type 2 diabetics advised to replace simple carbohydrates with more complex ones

Simple carbohydrates are broken down quickly so can raise blood glucose levels rapidly - complex ones take longer to break down so have a reduced effect on blood glucose levels

What is BMI

A value based on height and mass used to categorise an individual as underweight, normal weight, overweight or obese

How is BMI calculated

Mass (kg)/height (m)^2

What BMI values indicate obesity and an increased risk of type 2 diabetes

BMI value larger than 30

How is an individual's waist to hip ratio calculated

Waist circumference (cm)/hip circumference (cm)

What does a waist to hip ratio higher than 1 in males or 0.85 in females indicate

Abdominal obesity and an increased risk of developing type 2 diabetes

What is osmoregulation

The maintenance of constant water levels in the body fluids of an organism

Why is osmoregulation important

It prevents cells from bursting or shrinking when too much water enters or leaves by osmosis

Describe what happens to an animal cell if it is placed into a solution with a higher water concentration

Water molecules move down their water concentration gradient into the cell by osmosis, increasing the pressure inside the cell, causing the cell to burst (lysis)

Describe what happens to an animal cell if it is place into a solution with a lower water concentration

Water molecules move down their water concentration gradient out of the cell by osmosis, decreases pressure inside the cell, causing the cell to shrink (crenation)

Describe the function of the kidneys

They remove toxic waste substances from the body and alter blood water and ion levels

What is urine

A waste product of the kidney that contains urea, excess water, and excess ions

How is urea produced

In the liver, urea is produced from the breakdown of excess amino acids

Identify the structures of the urinary system labelled in the diagram

A - kidney
B - renal vein
C - renal artery
D - ureter
E - urethra
F - bladder

What is the function of the renal artery

Supplies blood to the kidneys to be filtered

What is the function of the renal vein

Drains filtered blood from the kidneys

What is the function of the ureter

Takes urine to the bladder from where it is formed in the kidneys

What is the function of the urethra

Releases urine from the bladder, out of the body

What is a nephron

A functional unit of the kidney where filtration and selective reabsorbtion takes place

Identify the structures of a nephron labelled in the diagram

A - glomerulus
B - Bowman's capsule
C - first (proximal) convoluted tube
D - collecting duct
E and F not needed
G - loop of Henle
H - second (distal) convoluted tube

What does the glomerulus do

The glomerulus is a wound up ball of small capillaries, which filters the blood - filtering out the small molecules

Describe ultrafiltration in the kidneys

Blood flows through the glomerulus under high pressure, small molecules (urea, glucose, water, and ions) are filtered out of the blood as there is a large surface area for them to filter out of and into the Bowman's capsule of the nephron

Why do large molecules (e.g. red blood cells and proteins) remain in the blood

They are too large to fit through the pores in the capillary walls (in the glomerulus)

What does the Bowman's capsule do

It collects the small molecules that were filtered out of the blood in the glomerulus and carries them into the first (proximal) convoluted tube

What does the first (proximal) convoluted tube do

Selective reabsorption of all of the glucose and some ions back into the blood

Which substances are selectively reabsorbed from the nephron tubule

All sugars, some water and some ions

What happens to the molecules not selectively reabsorbed

They travel down the collecting duct as urine and are transported to the bladder via the ureter - they are stored here and eventually excreted

How is the first (proximal) convoluted tube specialised for its purpose

The cells in the tube have lots of mitochondria which releases lots of energy for active transport - the cell membrane also has lots of microvilli which increases the surface area for absorption

What does the loop of Henle do

Some water is reabsorbed back into the bloodstream here through osmosis to create a concentration gradient for more water to be reabsorbed if needed in the collecting duct

What does the second (distal) convoluted tube do

Nothing (for GCSEs)

What does the collecting duct do

Osmoregulation - it allows some water to be reabsorbed back into the bloodstream depending on the amount of ADH released by the pituitary gland

How is the concentration and volume of urine controlled

By the secretion of ADH (anti-diuretic hormone) from the pituitary gland

Describe how ADH affects the reabsorption of water from the collecting ducts

ADH increases the permeability of the collecting ducts, enabling more water to be reabsorbed into the blood

What is required to maintain the ideal water content in the blood

A negative feedback system involving receptors in the hypothalamus, the hypothalamus, and an effector (e.g. the pituitary gland)

Describe the negative feedback loop which occurs when low blood water concentration is detected

Receptors send information to the hypothalamus which coordinates the information and sends instructions to the pituitary gland to release more ADH which increases the collecting duct membrane permeability so more water is reabsorbed - this increases the blood water content and produces more concentrated urine

Describe the negative feedback loop which occurs when high blood water concentration is detected

Receptors send information to the hypothalamus which coordinates the information and sends instructions to the pituitary gland to release less ADH which decreases the collecting duct membrane permeability so less water is reabsorbed - this decreases the blood water content and produces more dilute urine

How can kidney failure be treated

Kidney dialysis or a kidney transplant

What is kidney dialysis

Kidney dialysis is the use of a specialist machine to carry out the function of kidneys and artificially filter a person's blood

How does kidney dialysis work

A selectively permeable barrier separates the blood from the dialysis fluid - materials are diffused across the barrier and urea, excess ions and water move out of the blood into the dialysis fluid - large cells and proteins remain in the blood like in a kidney

Describe the composition of dialysis fluid

Same concentration of glucose, ions and water as in normal blood (no urea)

What does a kidney transplant involve

Taking a kidney from a living donor or someone recently deceased and implanting it into the patient

What is the risk associated with kidney transplants

There is a risk of the body rejecting the transplanted kidney

What precautions are taken to minimise the risk of rejection

Tissue typing ensures that the transplanted organ has matching tissue types with the recipient so reduces the risk of the organ being rejected - immunosuppressant drugs are also given to the patient for the rest of their lives to prevent the immune system from rejecting the organ