HOMEOSTASIS

Homeostasis is the maintenance of a stable internal environment.

Homeostasis involves control systems that keep your internal environment roughly constant (within certain limits). Keeping your internal environment stable is vital for cells to function normally and to stop them being damagedIts particularly important to maintain the right core body temperature and blood pH - temp and pH can affect enzyme activity , and enzymes control the rate of metabolic reactions.

Temperature

• If body temp is too high (40c) enzymes become denatured. Molecules vibrate too much, which breaks the hydrogen bonds in the tertiary structure, the active site shape is changed and it no longer works as a catalyst - metabolic reactions are less efficent.
• If body temp is too low, enzyme activity is reduced, slowing down the rate of metabolic reactions
• The highest rate of enzyme activity happens at their optimum temp (37c)

pH

• If blood pH is too high or too low, enzymes become denatured - the hydrogen bonds in the teriary structure are broken, the active site shape changes meaning they no longer work as a catalyst  = metabolic reactions are less efficent
• Highest rate of enzyme activity occurs at the optimum pH of 7 - some enzymes can work at lower pH's e.g. in the stomach

Homeostatic systems detect a change and respond by negative feedback.

• They involve receptors, a communication system and effectors.
• Receptors detect when a level is too high or too low, the infos communicated via the nervous system or the hormonal system, through effectors
• The effectors respond to counteract the change - bringing the level back to normal 
• Negative feedback mechanism restores levels back to normal. Helps keep things around the normal level e.g. body temp is usually kept between 0.5c above or below 37c
• Negative feedback only works within certain limits though - if the change is too big then the effectors may not be able to counteract it

Multiple negative feedback mechanisms give more control. Homeostasis involves multiple negative feedback mechanisms for each thing being controlled - having more than one mechanism gives more control over changes in your internal environment

Having multiple means you can actively increase or decrease a level so it returns to normal. If you only had one you could only turn it on or off.

Only one negative feedback mechanism means a slower response and less control.

Positive feedback mechanisms amplify a change from a normal level - the effectors respond to further increase the level away from the normal level.

Positive feedback is used to rapidly activate something

e.g. blood clot after injury

• platelets become activated and release a chemical, this trigeers more platelets to be activated.
• The platelets form a blood clot at the injury site.
• The process ends with negative feedback once the clot has been formed. 

Positive feedback isnt involved in homeostasis as it doesnt keep your internal environment stable. 

Eating and exercise change the concentration of glucose in your blood. All cells need a constant energy supply to work - blood glucose concentration must be carefully controlled 

The concentration of glucose in the blood is normally 90mg per 100cm3 of blood - its monitored by cells in the pancreas

Blood glucose concentration rises after eating food containing carbohydrate. Blood glucose concentration falls after exercise, as more glucose is used in respiration to release energy. 

Insulin and glucagon control blood glucose concentration - they travel in the blood to their target cells (effectors), theyre both secreted  by clusters of cells in the pancreas called the islets of Langerhans. Beta cells secrete insulin into the blood. Alpha cells secrete glucagon into the blood. 

Insulin lowers blood glucose concentration when its too high 

• Insulin binds to specific receptors on the cell membranes of liver cells and muscle cells - it increases the permeability of the muscle-cell membranes to glucose so the cells take up more glucose. This involves increasing the number of channel proteins in the cell membranes
• Insulin also activates enzymes in liver and muscle cells that convert glucose into glycogen- the cells are able to store glycogen in their cytoplasm as an energy source
• The process of forming glycogen from glucose is called glycogenesis
• Insluin also increases the rate of respiration of glucose, especially in muscle cells. 

Glucagon raises blood glucose concentration when its too low

• Glucagon binds to specific receptors on the cell membranes of liver cells
• Glucagon activates enzymes in liver cells that break down glycogen into glucose - this is glycogenolysis 
• Glucagon also activates enzymes that are involved in the formation of glucose from glycerol ( a component of lipids) and amino acids - the process of fomring glucose from non-carbs is gluconeogenesis
• Glucagon decreases the rate of respiration of glucose in cells

Responses produced by hormones are slower than prodcued by nervous impulses but their effects last for longer. 

Negative feedback mechanisms keep blood glucose concentration normal.

Insulin makes glucose transporters available for facilitated diffusion. Skeletal and cardiac muscles contain a channel protein called GLUT4 - its a glucose transporter 

When insulin levels are low, GLUT4 is stored in vesicles in the cytoplasm of cells. When insulin binds to receptors on the cell-surface membrane it triggers the movement of GLUT4 to the membrane. Glucose can then be transported into the cell through the GLUT4 protein by facilitated diffusion, 

Adrenaline also increase blood glucose concentration

Adrenaline is secreted from adrenal glands, its secreted when theres a low concentration of glucose in your blood, when youre stressed, or when exercising

Adrenaline binds to receptors in the cell membrane of liver cells - it activates glycogenolysis (the breakdown of glycogen to glucose) but inhibits glycogenesis (the synthesis of glycogen from glucose)

Adrenaline gets the body ready for action by making more glucose available for muscles to respire.

Adrenaline and glucagon act via a second messenger - both can activate glycogenolysis inside a cell even though they bind to receptors on the outside of the cell

• The receptors for adrenaline and glucagon have specific tertiary structures that make them complimentary in shape to their respective hormones - they bind to their receptors and activate an enzyme called adenylate cyclase 
• Activated adenylate cyclase converts ATP into a chemical signal called a 'second messenger' - this is known as cyclic AMP
• cAMP activates an enzyme called protein kinase A - this activates a cascade that breaks down glycogen into glucose (glucogenolysis)

Diabetes occurs when blood glucose concentration is not controlled

TYPE 1 - the immune system attacks beta cells so they cant produce any insulin. Some have a genetic predisposition to developing type 1, it may also be triggered by a viral infection. After eating the blood glucose level rises and stays high (hyperglycaemia) this can result in death. The kidenys cant reabsorb all of this glucose, so some is excreted in the urine. Its treated with insulin therapy - most need regualr insulin injections - this has to be carefully controlled because too much insulin can produce a dangerous drop in glucose levels (hypoglycaemia). Eating regularly and controlling simple carb intake helps to avoid a sudden rise in glucose.

TYPE 2 - usually acquired later on in life - often linked with obesity, more likely in people with a family history of the condition. Other risk factors include lack of exercise, age and poor diet. It occurs when body cells dont respond properly to insulin - inslulin receptors on their membranes dont work properly so the cells dont take up enough glucose - blood glucose concentration is higher than normal. Cna be treated by changes to diet and exercise, eating a healthy balanced diet, loosing weight and regular exercise. Glucose lowering medication can be taken if diet and exercise cant control it. Eventually, insulin injections may be needed. 

Type 2 diabetes is a growing health problem, it can cause additonal health problems including visual impairment and kidney failure.

To reduce the risk of developing type 2, helath advisors reccomned that;

• people eat a diet thats low in fat, sugar and salt with plenty of whole grains, fruits and veg
• get regular exercise
• loose weight if applicable

Change4Life aims to educate people on how to have a healthier diet and lifestyle and so reduce the risk of developing type 2

In response to criticisms, some food companies have attempted to make their products more healthy by

• using sugar alternatives to sweeten foods/drinks 
• reducing the sugar, salt, fat contents of products.

However, there is pressure on compaines to increase profits, they say that the industry will only respond fully in the long term as public perception about healthy eating changes. 

Colourimetry is used to determine the concentration of glucose in a solution 

Higher concentrations of glucose in urine may indicate diabetes. Heres how to determine the glucose concentration in a 'urine' sample;

• Quantitative Benedicts reagent is different to normal benedicts. When heated with glucose, the inital blue colour is lost but a brick red ppt is not produced.
• You can use a colorimeter to measure the light absorbance of the solution after the quantitative benedicts test has been carried out
• The higher the conc of glucose, the more blue colour will be lost (paler solution), this decreases the absorbance of the solution. 

The kidneys excrete waste and regulate blood water potential

One of the main functions of the kidneys is to excrete waste products, such as urea. The kidneys also regulate the water potential of the blood.

As blood passes through the capillaries in the cortex of the kidneys, substances are filtered out of the blood and into long tubules that surround the capillaires this is known as ultrafiltration. 

Useful substances, such as glucose and the right amount of water are then reabsored back into the blood - this is known as selective reabsorption.

The remaining unwanted substances pass along to the bladder and are excreted as urine.

Blood is filtered at the start of the nephrons - these are long tubules along with the bundle of capillaries where the blood is filtered.

• blood from the renal artery enters smaller arterioles in the cortex of the kidney
• each arteriole splits into a structure called a glomerelus which is a bundle of capillaries looped inside a hollow ball called a bowmans capsule.
• this is where ultrafiltration takes place
• the afferent arteriole takes blood into each glomerelus
• the efferent arteriole takes the filtered blood away from the glomerelus
• the efferent is smaller in diameter, so the blood in the glomerelus is under high pressure
• this high pressure forces liquid and small molecules in the blood out of the capillary and into the bowmans capsule
• the liquid and small molecules pass through three layers to get into the bowmans capsule and enter the nephron tubules - a capillary wall, the basement membrane and the epithelium of the bowmans capsule
• larger molecules like proteins and blood cells cant pass through, so stay in the blood. the substances that enter the bowmans capsule are known as the glomerular filtrate.
• The golmerular filtrate passes along the rest of the nephron and useful substances are reabsored along the way.
• Finally the filtrate flows through the collecting duct and passes out of the kidney along the ureter.

Useful substances are reabsorbed along the nephron tubules

• Selective reabsorption takes place as the glomerular filtrate flows along the proximol convoluted tubule (PCT), though the loop of henle and along the distal convoluted tubule (DCT)
• Useful substances leave the tubules of the nephrons and enter the capillary network thats wrapped around them.
• the epithelium of the wall of the PCT has microvilli to provide a large surface area for the reabsorption of useful materials from the glomerular filtrate (in the tubules) into the blood (in the capillaries)
• Useful solutes, like glucose, are reabsorbed along the PCT by active transport and facillitated diffusion.
• Water enters the blood by osmosis - the water potential of the blood is lower than that of the filtrate.
• Water is reabsorbed from the PCT, loop of henle, DCT, and collecting duct.
• The filtrate that remains is urine, which passes along the ureter to the bladder.

Urine is usually made up of; water and dissolved salts, urea, other substances such as hormones and excess vitamins.

It doesnt normally contain; proteins and blood cells - theyre too big to be filtered out, glucose - its actively reabsorbed back into the blood.

Water is essential to kepp the body functioning , so the amount of water in the blood needs to be kept constant

Mammals excrete urea in solution, which means water is lost during excretion. Water is also lost in sweat. The kidneys regulate the water potential of the blood (and urine) so the body has just  the right amount of water - this is osmoregulation.

If the water potential is too low (the body is dehydrated), more water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more concentrated, so less water is lost during excretion.

If the water potential of the blood is too high (the body is too hydrated), less water is reabsorbed by osmosis into the blood from the tubules of the nephrons. This means the urine is more dilute, so more water is lost during excretion

Water is reabsorbed into the blood along almost all of the nephron, but regulation of water potential mainly takes place in the loop of henle, DCT and collecting duct. The volume of water reabsorbed by the DCT is controlled by hormones.

The loop of henle maintains a sodium ion gradient. Its located in the medulla (inner layer) of the kidneys. Its made up of two limbs - the ascending limb and the descending limb. The limbs control the movement of sodium ions so that water can be reabsorbed by the blood.

• Near the top of the ascending limb, Na+ ions are pumped out into the medulla using active transport. The ascending limb is impermeable to water, so the water stays inside the tubule. This creates a low water potential in the medulla, becuase theres a high conc of ions.
• Because of the low WP in the medulla than in the descending limb, water moves out of the descending limb (which is permeable) into the medulla by osmosis. This makes the filtrate more concentrated (the ions cant diffuse out - descending limb isnt permeable to them). The water in the medulla is reabsorbed into the blood through the capillary network.
• Near the bottom of the ascending limb Na+ diffuse out into the medulla, further lowering the water potential in the medulla. The ascending limb is impermeable to water so it stays in the tubule.
• Water moves out of the DCT by osmosis and is reabsorbed into the blood,
• The first three stages massively increase the ion con in the medulla, which lowers the water potential. This causes water to move out of the collecting duct by osmosis. As before, the water in the medulla is reabsorbed into the blood through the capillary network. 

Water reabsorbtion is controlled by hormones. 

Water potential of the blood is monitored by cells called osmoreceptors in the hypothalamus.

When the water potential of the blood decreases, water will move out of the osmoreceptors by osmosis - causing the cells to decrease in volume. This sends a signal to other cells in the hypothalamus, which sends a signal to the posterior pituitary gland - this then releases hormones called ADH (antidiuretic hormone) into the blood.

ADH makes the walls of the DCT and collecting duct more permeable to water, this means more water is reabsorbed from these tubules into the medulla and into the blood by osmosis. A small amount of conc urine is produced which means less water is lost by the body.

Heres how ADH changes the water content of the blood when its too low or too high. 

Blood ADH level rises when youre dehydrated

• The water content of the blood drops, so its water potential drops
• This is detected by osmoreceptors in the hypothalamus
• The posterior pituitary gland is stimulated to release more ADH into the blood.
• More ADH means that the DCT and collecting duct becomes more permeable, so more water is reabsorbed into the blood by osmosis
• A small amount of highly conc urineis produced and less water is lost

Blood ADH levels fall when youre hydrated

• The water content of the blood rises, so its water potential rises.
• This is detected by the osmoreceptors in the hypothalamus.
• The posterior pituitary gland releases less ADH into the blood.
• Less ADH means that the DCT and collecting duct become less permeable, so less water is reabsorbed into the blood by osmosis.
• A large amount of dilute urine is produced and more water is lost.