Homeostasis

Homeostasis- physiological control systems that maintain a constant internal environment within restricted limits

Factors affecting the internal environment;

- Body temp             - Blood glucose conc.               - Blood salt conc.                  - CO2 conc.

- Water potential of the blood                    - Blood pressure 

Why is homeostasis important?

• Need to mainatain a stable blood glucose concentration- to keep the availability of the resspiratory substrate intact. This is because a low conc. of BGC menas less energy is released due to ess respiration as less glucose is available and hydrolysed for aerobic respiration. ie- less ATP produced.
• Too much glucose = water potential of the blood is too low, so water leaves cell via osmosis and goes intot he blood, so individual becomes dehydrated and blood pressure increases drastically, so you need to maintain a suitable level. 
• Allows organisms to live independantly to external environent- can live in a wider geographic context

Key features of a control mechanism

• Optimal condition 
• Receptor- detect change in condition 
• Co-ordinator- coordinates input from diff receptors 
• Effector- brings about change if required
• Feedback mechanisms- to maintain/stop the change.

How control mechanisms are co-ordinated

• Mostly by negative feedback systems (e.g. blood cglucose conc ad body temp)
• Positive feedback= production of large and rappid response from small stimulus (e.g. A.P)
• control mechanisms have more than one receptor for better monitoring of conditions (e.g. temp receptors in the skin, bladder and cornea.

• Negative feedback (BRING BACK TO NORMAL)- When a change causes a response aiming to eliminate the change, and return levels back to normal conditions (e.g. insulin) brings it back to optimum levels 

e.g. Rise in blood temp = detected by thermoreceptors in the hypothalamus = heat loss in hypothalamus = skin vasodilation = cooler blood temperature. (Blood at original temp turns off corrective mechanism)

positive feedback (ENHANCE)- when a change causes a response that allows the change to continue and keep adding to the change (e.g- action potentials)

Hormones

• Proteins  
• Travel in blood 
• Slow response as it takes a long time to work (widespread but long lasting)
• Effective i low concentration 
• Work by being coplimentary to the receptor 

How hormones work (Adrenaline- flight or fight response) 

• Adrenaline is secreted by the adrenal glands 
• Adrenaline travels to target cells (liver cells) via blood and binds to the receptor causing it to change shape and activate the enzyme adenyl cyclase (AC)
• Activated AC converts ATP into cyclic AMP (c.AMP)- which acts as a second messenger 
• c.AMP changes shape and activates protein kinase enzyme, which catalyses reation of gluconeogenesis and glycogenolysis.

Pancreas

• Contains Islets of Langerhans which produce hormones 
• a cells- produce the hormone glucagon 
• b cells- produce the hormone insulin 

Liver

• Glycogenesis- Turning glucose into glycogen (glucose is taken from the blood, which lowers blood glucose concentration)
• Glycogenolysis- breaking down stored glycogen into glucose (can be released into the blood which increases the blood glucose concentration)
• Gluconeogenesis- synthesis of glucose from other molecules e.g. amino acids (this increases blood glucose concentration)

• Diet- hydrolysis of carbohydrates into glucose inside small intestine. Glucose diffuses into blood plasma

• Diet- type of food eaten and regularity of eating

• Glycogenolysis- hydrolysis of glycogen into glucose inside the liver and muscle cells. Glucose diffuses into blood plasma

• Gluconeogenesis- synthesis of glucose from non-carbohydrate substances such as amino acids and glycerol

• Rates of respiration- dependent on physical activity and body temperature

Insulin- lowers blood glucose 

• Islets of Langerhans stimulate the b cells to produce insulin in the pancreas
• Insulin binds onto the receptors of the liver and muscle cells 
• Causes more transport proteins to fuse with the membrane and become active 
• This allows more glucose to diffuse into the cells 
• Inside the cell- glycogenesis occurs
• stimulates lipid formation
• Increases rate of respiration which uses up more glucose (so it decreases the blood glucose conc)

Glucagon- increases blood glucose 

• Islets of Langerhans stimulate alpha cells to produce glucagon in the pancreas 
• Glucagon binds to the receptors on the membrane of liver and muscle cells 
• This activates enzymes which allows glycogenolysis and so the glucose diffuses out of the blood
• Gluconeogenesis also occurs which increases blood glucose 

Type 1 (no insulin produced)

• Causes- Insulin not produced due to autoimmune response, where the body attacks beta cells of Islets of Langerhans. Diagnosed in young children. ca be genetic 
• Symptoms/Signs- Blurred vision,Weight loss, Thirsty/Hungry, Urinating a lot, Glucose in urine, Thrush, Tiredness. THESE SIGNS HAPPEN QUICKLY 
• Treatment- Injections of insulin 2-4x a day into bloodstream (cant be taken orally as insulin would get broken down by protease in the stomach and SI). Dose depends on intake of glucose (biosnesors used). Too much inslulin- low blood glucose= unconsciousness. reduced carb diet and controlled excercise.

Type 2 (no response to insulin)

• Glycoprotein receptors dont respond to insulin/ pancreas reduces amount of insulin made. Diagnosed in mid-40's. Genetic but also largely environmental. 
• RIsk factors - obesity, high carb diet, lack of excercise.
• Symptoms/Signs-SAME AS ABOVE, BUT HAPPEN SLOWLY.
• Treatment- Reduced carb diet and more excercise. Can be given insulin injections or drugs that stimulate insulin production or slows rate of absorption in the SI. 

• Outer cortex= renal (Bowman’s) capsule (RBC), glomerulus, proximal and distal tubules, afferent and efferent arterioles, and collecting ducts.
• Inner medulla= lower part of the collecting ducts merge into the renal pelvis. -The Loop of Henle with its surrounding  capillary bed also found there.
• Renal artery brings blood away from the heart, and into the nephron. This artery turns into the afferent arteriole (wider), which then branches off into the dense network of capillaries called the glomerulus.
• Glomerulus- found inside the RBC. Capillaries of the glomerulus merge together into the efferent arteriole (more narrow), which takes blood away from the glomerulus. The efferent arteriole- more narrow than the afferent article, which makes the hydrostatic pressure higher within the glomerulus. This help to force fluid out of the glomerulus and into the  RBC.7
•  RBC= specialised cells called podocytes. Responsible for ultrafiltration of blood to form glomerular filtrate.
• RBC is connected to the proximal convoluted tubule which merges into the descending limb of the Loop of Henle, which then does a sharp U-bend into the ascending limb of the Loop of Henle (which is wider than the ascending limb). This then merges into the distal convoluted tubule.
• PCT role= is to reabsorb glucose and water from the glomerular filtrate.
•  Loop of Henle= to maintain a conc gradient of Na+ ions.
• DCT and collecting duct= to reabsorb water back into the blood.

Describe how urea/amino acids/glucose and H2O is removed from blood ORRR Describe how glomerular filtrate is produced.

• Ultrafiltration from the glomerulus into the renal Bowman's capsule
• This is due to the efferent arteriole being narrower which means there is a high hydrostatic pressure.
• This fres out the small molecules such as glucose, amino acids, minerals, vitamins to be filtered out of the blood. 
• This passes through the basement membrane and through the pores between the endothelium cells of blood capillaries and podocytes of Bowman's capsule.
• Glomerulus filtrate is produced and enters PCT.
• Large molecules such as red blood cells, ad proteins do not leave the blood as they are too large to pass through.

• Many mitochondria are inside the cuboidal cells to produce ATP to provide energy for co-transport of glucose
• Na+ ions are actively transported from the epithelial cells of the PCT and into the blood, so the conc. gradient is maintained.
• Na+ ions diffuse down the conc. gradient from the lumen of PCT, into the epithelial cell of PCT, and brings a glucose molecule at the same time.
• This uses a co-transport arrier protein, and diffuses in via facilitated diffusion from the epithelial cell to the lumen.

Explain how the loop of Henle maintains the gradient of ions which allows water to be reabsorbed from the filtrate in the collecting duct

• A counter current multiplier is achieved as the DL and the AL have filtrate flowing in opposite directions.
•  AL actively transports Na+ out against the conc. gradient into the interstitial fluid which lowers the water potential gradient, allowing water to move from the descending limb into the interstitial fluid by osmosis,
• This then diffuses into the blood capillaries to maintain the lower water potential in the interstitial fluid. 
• Process continues so that an ion concentration in the medulla occurs- the deeper you go, the higher the ion concentration.
• So as water passes down the collecting duct, the water potential gradient becomes greater so more water is reabsorbed by osmosis, making the urine more concentrated and = countercurrent multiplier.

• Al-  Na+ ions are actively transported out of the filtrate and into the interstitial fluid. 
• Creates a water potential gradient as there is a higher ion conc. in the interstitial fluid. So water moves out of the descending limb by osmosis down the water potential gradient. 
• Al is impermeable to water so water cannot be reabsorbed by it once water enters the interstitial fluid from the descending limb. 
• Sodium ions diffuse into the DL which helps increase the ion concentration at the hairpin.
• The deeper into the medulla you go, the lower the water potential, and the higher the ion concentration in the interstitial fluid.
• Causes water to leave the DCT and CD by osmosis, down water potential gradient.

Role of DCT

• Allows reabsorption of any more water that the body needs before it is passed out as urine
• Allows the maintenance of the acid-base balance by sereting excess H+ ions
• Allows the trnasfer of all the waste products such as urea, toxins, and waste into the collecting duct to be passed out. 

Role of ADH

• When water potential of the blood is too low
• IT is detected by recptors in the hypothalamus 
• so more ADH is secreted from the posterior pituitary gland 
• which makes the DCT and the collecting duct more permeable due to the number of aquaporins 
• so MORE water is reabsorbed- so it leaves the nephron and enters the blood
• via osmosis, down the water potential gradient 
• a smaller volume of more concentrated volume is produced 
• VICE VERSA FOR HIGH WATER POTENTIAL OF BLOOD 

Role of DCT

• Allows reabsorption of any more water that the body needs before it is passed out as urine
• Allows the maintenance of the acid-base balance by sereting excess H+ ions
• Allows the trnasfer of all the waste products such as urea, toxins, and waste into the collecting duct to be passed out. 

Role of collecting duct- for water to move out by osmosis and create a concentrated filtrate 

Role of ADH

• When water potential of the blood is too low
• IT is detected by recptors in the hypothalamus 
• so more ADH is secreted from the posterior pituitary gland 
• which makes the DCT and the collecting duct more permeable due to the number of aquaporins 
• so MORE water is reabsorbed- so it leaves the nephron and enters the blood
• via osmosis, down the water potential gradient 
• a smaller volume of more concentrated volume is produced 
• VICE VERSA FOR HIGH WATER POTENTIAL OF BLOOD 

Non diabetic

• Glucose is selectively reabsorbed bck into PCT so none of it comes out as urine
• Uses active transport 

Diabetic 

• Very high concntration of glucose in filtrate 
• during selective reabsorption- the protein channels become staurated.
• This means that not all of he glucose can be reabsorbed into blood and therefore some glucose ends up in the urine 

• LoH- AL actively transports Na+ out of the LoH and into the interstitial fluid, which creates a water potential gradient, and higher ion concentration gradient the deeper you go into the medulla.
• This means more water will be reabsorbed by DL, the DCT and collecting duct by osmosis down the water potential gradient.
• So the longer the LoH, the more water is reabsorbed, and the lower the volume of urine will be produced.
• ADH- binds onto receptors on cells of the DCT and collecting duct, and causes vesicles containing aquaporins to fuse with the cell surface membranes.
• This makes them more permeable to water so again more water is reabsorbed and the lower the volume of urine is produced.