B4 - Homeostasis What you should know….

What is homeostasis? 1 minute - write a definition of homeostasis

Homeostasis means keeping a constant internal environment

Why do we need it? If there was not a constant internal environment, our enzymes would not work properly. That would mean that nothing would operate correctly and we would die.

What needs to be controlled? 1. Carbon dioxide Extra carbon dioxide must be removed, otherwise the body becomes too acidic. It is lost mainly in the air we breathe out, but a small amount is lost in the urine. 2. Urea This is the waste chemical produced when we digest amino acids in the liver. It is poisonous and so must be removed. This is done mainly through the urine, although some is lost through our sweat.

3. Ions If the right balance of ions is not kept, our cells can become shriveled, swollen or even burst! Important ions include sodium, potassium, hydrogen and phosphate. These are controlled through our urine and the amount of water we drink. We also lose some, like sodium ions, through our faeces and our sweat.

4. Sugar Having enough glucose for respiration plus adequate stores of glycogen is critical. If the blood glucose level falls too low we will die. 5. Water Seventy percent of our body mass is water. Without keeping the right amount of water we would die. The kidney is the key means of this control 6. Temperature The enzymes that control every chemical reaction in our body work best at their optimum temperature of 37 degrees Centigrade. If our body cells get too hot or too cold they would die. So would we!

A way to remember these 6 things is by learning this... When (Water) Shall (Sugar) I (Ions) Clean (CO2) The (Temperature) Utensils (Urea)

Which organs are involved? The hypothalamus (in the brain) monitors water, temperature and carbon dioxide content of blood. The pituitary gland secretes a number of hormones, a key one is ADH (anti-diuretic hormone) which is important in regulating the water content of the body. The liver helps to control glucose content of the body by storing it as glycogen. It is also involved in temperature regulation, acting as the body's furnace by increasing the rate of respiration when we are cold.

The lungs are involved by getting rid of carbon dioxide from the body. The pancreas is involved in maintaining a constant amount of glucose in the body through the actions of glucagon and insulin. The muscles of the body can help to maintain a stable body temperature as muscular activity and shivering help to generate heat.

The kidneys are involved in controlling the amount of water in the body. The skin is the largest organ and has a central role in maintaining a constant temperature

What if we get too hot? The hypothalamus sends impulses to the skin which cause 3 things to happen: 1. Our hairs lie flat: so letting more heat out. 2. We sweat: the evaporation of this cools us down. 3. More blood goes through the skin: this acts like a radiator to radiate out heat.

What if we get too cold? If we get too cold the hypothalamus sends other impulses so that the reverse happens: 1. Our hairs stand up: this traps a layer of air which acts like an insulator. 2. We stop sweating: this stops the heat loss by evaporation. 3. Less blood goes through the skin: the skin will appear paler and colder.

The kidney - what does it do? One job that they are involved in is reabsorbing excess water so that we don't dry out. Blood enters the kidney through the renal artery. It is filtered and the 'clean' blood leaves via the renal vein. Any waste material leaves through the ureter, then to the bladder and the world outside!

If nothing else happens then the materials, such as water and urea, will end up going all the way through the nephron, down the ureter, through the bladder and into the toilet! However, sometimes the body needs to grab back chemicals such as water and glucose which are still useful. This happens when they move out of the fluid in the nephron back into the capillary network that twists around the nephron. This process is called reabsorption.

If there isn’t enough water in the body? When there is too little water in the body the following sequence of events occurs: Flowchart

If there’s too much water in the body? Draw a flow chart similar to the previous one for if there was too much water in the body.

Negative feedback These are examples of negative feedback. Receptors in the body identify a change. The body then carries out any necessary processes to return the body to its normal state.

What happens if the kidneys don’t work properly? DIALYSIS Blood is taken out of a vein and pumped through a machine that cleans it. This cleaning is done by getting the waste materials like urea to diffuse across a selectively permeable membrane into a plasma-like fluid.The 'clean' blood is then returned to the patient.

A better way is to have a kidney transplant A better way is to have a kidney transplant. Here, a healthy kidney is taken from a donor, usually someone who has sadly been killed in an accident or illness.The kidney is then sewn into place with all of the blood vessels and tubes connected. The success rate is about 80% if the tissue types are found to be the same between donor and patient.However the patient must take antibiotics and anti-rejection drugs for the rest of their life! Quite a daunting prospect but at least they can lead a normal life otherwise.

Revision key points: What is homeostasis? Why do we need it? What has to be controlled? Which organs are involved? Controlling temperature Controlling body water Dealing with kidney failure

Diffusion Movement of particle from an area of high conc to a low conc.

Osmosis Movement of water particles from an area of high conc to a low conc, through a partially permeable membrane

Active Transport Molecules that are too big Molecules moving from low to high concs. Involves a carrier protein in the cell membrane