6.4. Homeostasis

Homeostasis

physiological control systems that maintain the internal environment within restricted limits e.g. temperature, glucose concentration, water

what is the importance of maintaining temperature

* Low temp = less kinetic energy so there are less successful collisions, so less likely for a enzyme-substrate complex to form
* high temp = denatures enzymes

importance of maintaining blood pH

stable environment for enzyme-controlled reaction

acidic pH = H+ ions interact with H bonds and ionic bonds in tertiary structure on enzymes = shapes changes = no ES complexes

what is the importance of maintaining water potential

* May cause cells to shrink/burst as a result to the change in water potential
* Maintenance of blood glucose concentration is essential in maintain a water potential
* Ensure a reliable source of glucose for the respiration in cells

what internal conditions are controlled

1. Body temperature – nervous system
2. Water content
3. Blood glucose levels – hormone system

Autonomic control systems

Nervous response →

Chemical response →

nervous system

hormone system

all control systems have…

receptors, coordinators, effectors

Endotherms

organisms which generate their own heat via metabolic processes so can maintain their internal body temperature

e.g. mammals, birds

Exotherms

depend mainly on external heat sources, and their body temperature changes with the temperature of the environment

e.g. fish, reptiles

regulating body temp: warming up

* Vasoconstriction
* Blood vessels which supply blood to the skin constrict so that less blood flows on the surface of the skin (causes skin to go white when cold)
* Nerve impulses from the hypothalamus to the smooth muscles in the arterioles cause them to contract
* Blood is diverted away from the capillaries near the skin so less heat is lost by radiation
* Decreased sweating
* No ‘cooling effect’ as less heat energy is lost
* Piloerection
* Nerve impulses cause erector pili muscles to contracts, causing hairs on the body surface to be pulled up straight
* More air is trapped between the hair and skin = more insulation
* Shivering
* Increases metabolism by respiration = increase of release of heat energy

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* Increased metabolism – metabolic rate increases
* Adrenaline is release by the adrenal gland
* Long term exposure to low temperatures causes the release of thyroxine by the thyroid gland which causes sustained increase in metabolism
* Behavioural changes: seeking warmer places e.g. radiators

regulating body temp: cooling down

* Vasodilation
* Allows more blood to the surface of skin
* Appear more flushed
* Nerve impulses from the hypothalamus to the smooth muscles in the arterioles cause them to dilate
* Blood is diverted to the capillaries near the skin so more heat is lost by radiation
* Increased sweating
* Panting
* Pilorelaxation
* Nerve impulses cause erector pili muscles to relax, causing hairs on the body surface to be lie down
* Less air is trapped between the hair and skin = less insulation

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* Decreased metabolism
* Adrenaline is release by the adrenal gland
* Long term exposure to high temperatures causes the release of thyroxine by the thyroid gland which causes sustained increase in metabolism.

negative feedback system

Causes the corrective measures to be turned off to return the system to a normal level giving a greater degree of control

e.g. rise in blood temperature, blood glucose

positive feedback

fluctuation triggers changes that result in an ever greater deviation from the normal level

adv of having multiple negative feedback systems

means that you can actively increase or decrease a level so it returns to normal = more control

endocrine system

thyroid gland

produces thyroxine

endocrine system

pituitary gland

master gland situated at the base of the brain

endocrine system

pancreas

produces insulin

endocrine system

adrenal gland

produce adrenaline

endocrine system

testes

produces testosterone

endocrine system

ovaries

produce oestrogen

why does blood glucose conc need to be regulated

1. Main respiratory substrate
2. Affects blood water potential

high blood glucose =

symptoms

hyperglycaemia

causes muscle to break down, weight loss, tiredness

low blood glucose =

symptoms

hypoglycaemia

causes sweating, hunger, irritability, double vision

glycogenesis

* Glycogenesis is the synthesis of glycogen from glucose molecules: glucose → glycogen
* Insulin triggers this process after it detects an increased blood glucose concentration
* The synthesis of glycogen removes glucose molecules from the bloodstream and decreases the blood glucose concentration to within a normal range
* Glycogen acts as a compact and efficient carbohydrate storage molecule

glycogenolysis

* Glycogenolysis is the breakdown of glycogen to produce glucose molecules: glycogen → glucose
* Glucagon triggers this process after it detects a decreased blood glucose concentration
* It activates enzymes within the liver that breakdown glycogen molecules into glucose
* The breakdown of glycogen releases more glucose molecules to the bloodstream and increases the blood glucose concentration to within the normal range

Gluconeogenesis

* Gluconeogenesis is the synthesis of glucose molecules from non-carbohydrate molecules: glycerol/amino acids → glucose
* Glucagon also triggers this by activating enzymes within the liver
* These enzymes convert other molecules, such as fatty acids and amino acids, into glucose molecules
* Glucose molecules are released into the bloodstream which increases the blood glucose concentration to within the normal range

function of pancreas

1. Releases enzymes such as amylase, protease, and lipase
2. Produces hormones involved in controlling blood glucose levels

what do β cells secrete

insulin

what do α cells secrete

glucagon

These α and β cells act as the ___________ and initiate the response for controlling blood glucose concentration.

The liver, muscle and fat cells act as the ___________ in response to insulin.

receptors

effectors

role of insulin

Insulin also helps to increase the uptake of glucose in the liver by activating glycogenesis: glucose → glycogen

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* Once glucose has entered a liver cell an enzyme rapidly converts it to glucose phosphate
* Different enzymes then convert glucose phosphate into glycogen
* This helps to lower glucose concentration within the liver cell
* A steep diffusion gradient is maintained between the blood in the capillaries and the liver cells
* increases permeability of cells to glucose

how does insulin increase permeability of cells to glucose

* increases number of glucose carrier proteins
* triggers conformational change which opens glucose carrier proteins

how does inulin increase the glucose concentration

* activates enzymes for glycogenesis in liver and muscles
* stimulates fat synthesise in adipose tissue

role of glucagon

Glucagon raises blood glucose concentration when it’s too low.


1. alpha cells in Islets of Langerhans in the pancreas detect decrease and secrete glucagon into the bloodstream
2. glucagon binds to surface receptors in liver cells and activates enzymes for glycogenolysis: glycogen → glucose and gluconeogenesis: glycerol/amino acids→glucose
3. glucose diffuses from liver to bloodstream

Negative feedback mechanism

High levels of glucose

1. The blood supplied to the islets on Langerhans though the hepatic portal vein is high in glucose, and is detected by the β cells in the pancreas so the beta cells secrete insulin and the alpha cells stop secreting glucagon.
2. Insulin then binds to receptors on liver and muscle cells (the effectors).
3. The liver and muscle cells respond to decrease the blood glucose concentration

* glycogenesis is activated
* increase cellular glucose uptake
* stimulate adipose tissue to synthesise fat


4. Blood glucose concentration then returns to normal.

Negative feedback mechanism

Low levels of glucose

1. The blood supplied to the islets on Langerhans though the hepatic portal vein is low in glucose, and is detected by the α alpha cells in Islets of Langerhans in the pancreas detect decrease and secrete glucagon into the bloodstream
2. glucagon binds to surface receptors in liver cells and activates enzymes for glycogenolysis: glycogen → glucose and gluconeogenesis: glycerol/amino acids→glucose
3. glucose diffuses from liver to bloodstream
4. Blood glucose concentration then returns to normal.

define glucose transporters

channel proteins which allow glucose to be transported across a cell membrane

Skeletal and cardiac muscle cells contain a glucose transporter called….

GLUT4

* When insulin levels are ___, GLUT4 is stored in vesicles in the cytoplasm of cells
* When insulin levels are ___, insulin binds to receptors on the cell-surface membrane, and triggers the movement of GLUT4 to the membrane.

Glucose can then be transported into the cell through the GLUT4 protein by _____________

low

high

facilitated diffusion

when is adrenaline released

when there’s a low concentration of glucose in your blood, when you’re stressed and when you’re exercising

what effect does adrenaline have on blood glucose concentration

1. It activates glycogenolysis (glycogen → glucose)
2. It inhibits glycogenesis (glucose → glycogen ).
3. glucose diffuses from liver into bloodstream

secondary messenger model diagram

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describe the secondary messenger model (SHORT)

1. ^^**Glucagon/adrenaline** (the first messenger)^^ binds to receptors in the cell surface membranes of **liver cells,** as they have specific tertiary structures making them complementary

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2. conformational change activates a **G protein**

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3. This activated G protein **activates the enzyme adenylyl cyclase** which **converts ATP into** %%**cyclic AMP (cAMP)** (which is the second messenger )%%

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4. **cAMP activates protein kinase A** which **activates a cascade of reactions** that breaks down glycogen into glucose (**glycogenolysis**) = amplifies the original signal

how does adrenaline and glucagon activate glycogenolysis

by the secondary messenger model

why does the enzyme only occurs in liver cells

there are no glucagon receptors on muscle cells

gluconeogenesis is the conversion of:

glycerol/amino acids → glucose

activated with glucagon

what is the glycogenolysis the conversion

glycogen → glucose

activated with glucagon

glycogenesis is the conversion of

glucose → glycogen

activated by insulin

what does adrenaline inhibit

glycogenesis

what is diabetes

condition in which the homeostatic control of blood glucose has failed or deteriorated

their insulin function is disrupted which allows the glucose concentration in the blood to rise

* The kidneys are unable to filter out this excess glucose in the blood and so it often appears in the urine
* The increased glucose concentration also causes the kidneys to produce large quantities of urine, making the individual feel thirsty due to dehydration

describe type 1 diabetes

* **Type 1 diabetes** is a condition in which the inability of pancreas to produce insulin
* It normally begins in childhood due to an **autoimmune response** whereby the body’s immune system (T cells) **attacks the β cells** of the islets of Langerhans in the pancreas.
* The β cells detect high blood sugar and **synthesise insulin**
* The lack of insulin also affects **glycogen** stores which results in an individual feeling **fatigued**
* After eating, the blood glucose level rises and stays high — this is called hyperglycaemia and can result in death if left untreated.

how to treat type 1 diabetes

Regular blood tests

Insulin injections

Appropriate diet

Insulin pumps delivering insulin continuously

The insulin used by diabetics can be fast-acting or slow-acting

describe type 2 diabetes

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* Type II diabetes is more common than type I
* It usually develops in those aged 40 and over however more and more young people are developing the condition, also more likely in people with a family history.
* In type II diabetes the pancreas still produces insulin, but the glycoprotein receptors have reduced in number or no longer respond to it or insufficient insulin produced by the pancreas
* This reduced sensitivity to insulin occurs in the liver and fat storage tissues
* The lack of response to insulin means there is a reduced glucose uptake which leads to an uncontrolled high blood glucose concentration
* This can cause the β cells to produce larger amounts of insulin which ultimately damages them

what is a risk of type 2 diabetes

obesity

consumes a diet high in carbohydrates, and the over-production of insulin triggers the development of insulin resistance

what is the correlation between blood pressure and diabetes

* Individuals with poorly controlled diabetes often suffer from high blood pressure
* The high blood glucose concentration lowers the water potential of the blood which causes more water to move from the tissues into the blood vessels by osmosis
* As a result, there is a larger volume of blood within the circulatory system which causes blood pressure to increase

Explain why the protein insulin must be administered intravenously rather than orally.

Insulin is a protein, if it was taken orally it would be digested by the enzyme protease found in the gut before entering the bloodstream.

Osmoregulation

the control of the water potential of the blood via homeostatic mechanisms

what are the 2 functions of the kidneys responsible

1. Osmoregulatory organ – they regulate the water content of the blood
2. Excretory organ – excrete toxic water products of metabolism (urea) and substances in excess

what are the 3 main areas of the kidney

1. cortex
2. medulla
3. renal pelvis

cortex

lighter region of the kidney made up of Bowman’s capsules, convoluted tubules, glomerulus and blood vessels

medulla

a darker inner region of the kidney made up of loops of Henle, collecting ducts and blood vessels

explain the role of the collecting duct

reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins

explain why it is important to maintains an Na+ gradient

counter current multiplies - filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential

maintains water potential gradient for maximum reabsorption of water

renal pelvis

cavity that collects urine into the ureter

fibrous capsule

protects kidney

renal vein

returns the deoxygenated and filtered blood to the heart via the inferior vena cava

renal artery

supplies blood carrying the waste to the kidneys from the heart via the aorta

ureter

tube carrying urine to the bladder

what is the nephron

which is a functional unit of the kidney, responsible for the formation of urine

Afferent arteriole

* tiny vessel originating from the renal artery
* supplying the nephron with blood
* it enters the Bowman’s capsule where it forms the glomerulus

efferent arteriole

* E for exit.
* A vessel that leaves the renal capsule.
* It is thinner than the afferent arteriole and causes an increase in pressure in the glomerulus because the exit is narrower.
* They carry blood away from the glomerulus and from the blood capillaries

glomerulus

* branched knot of capillaries from which fluid is forced out from the blood.
* They recombine to form the efferent arteriole.

blood capillaries

* a concentrated network that surround both convoluted tubules and loop of Henle.
* They reabsorb water, mineral salts, glucose and salts.
* They merge to form the renal vein.

Proximal convoluted tubules (PCT)

* twisted network near the Bowman’s capsule.
* Series of loops that are surrounded by an extensive network of blood capillaries.
* They are made of up the epithelial cells, which each have microvilli

Distal convoluted tubules (DCT)

* series of loops that are surrounded by blood capillaries.
* The walls are made of epithelial cells but it has fewer capillaries than the PCT.

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reabsoroption:

* water via osmosis
* ions via active transport

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loop of henle

long hairpin loop extending from the cortex into the medulla and back again

Bowman’s capsule

* closed at the end at the start of the nephron. It surrounds a mass of blood capillaries that are known as glomerulus.
* It has specialised cells called podocytes.
* Ultrafiltration of blood plasma into the Bowman’s capsule, blood cells and proteins remain in the glomerulus capillary.

descending of henle

water reabsorbed by osmosis according to the concentration of sodium ions

ascending limb of Henle

sodium ions reabsorbed by facilitated diffusion and then active transport

Role of nephron in osmoregulation

1. Formation of the glomerular filtrate by ultrafiltration
2. Selective reabsorption of glucose and water by the PCT
3. Maintenance of gradient of sodium ion sin the medulla by loop of Henle
4. Reabsorption of water by the DCT and collecting ducts

The process of urine formation in the kidneys occurs in 2 stages

1. ultrafiltration
2. selective reabsorption

quick summary of ultrafiltration

* In the Bowman’s capsule
* Amino acids, water, glucose, urea, ions are filtered out of the blood capillaries of the glomerulus and into the bowman’s capsule to make glomerular filtrate

how are cells of the bowman’s capsule adapted for ultrafilteration

fenestrations between epithelial cells of capillaries

fluid can pass between and under folded membrane of podocytes

quick summary selective reabsorption

* In the proximal convoluted tubule
* Useful molecules are taken back by active transport from the filtrate and returned to the blood as the filtrate flows along the nephron.

describe the process of ultrafiltration

The process of ultrafiltration occurs in the nephrons of the kidneys. The process can be described as follows:


1. Blood enters the kidney through the renal artery which branches to give around 1 million tiny arterioles. Each of these enter the Bowman’s capsule of a nephron through the afferent arteriole.
2. This then divides to give a complex network of capillaries called the glomerulus
3. The walls of the glomerular capillaries are made up of epithelial cells that have pores between them. Blood cells and proteins are not able to pass through as these molecules are too large.
4. This later merge to give the efferent arteriole. The efferent has a smaller diameter than the afferent arteriole. This creates a build-up of hydrostatic pressure inside the glomerulus.
5. Water, glucose, and mineral ions are then squeezed out of the capillary forming the glomerular filtrate.

if there is a presence of proteins in urine what does that mean

blood cells and proteins are not able to pass through these as they are too large

indicates your nephron is damaged

describe the structure of the nephron

bowman’s capsule

PCT

loop of Henle

DCT

collecting duct

what is the glomerular filtration rate

* Rate at which the kidneys are filtering out the blood
* This is met with a significant amount of resistance.
* Overall rate controlled by the outward and inward pressure that the glomerulus is experiencing

Factors which resist the movement

1. Connective tissues and endothelial cells of the blood capillary
2. Epithelial cells of the renal Bowman’s capsule
3. The hydrostatic pressure of the fluid in the real capsule space
4. The low water potential of the blood in the glomerulus

adaptions of the glomerulus

1. Inner layer of the capsule is made of specialised podocytes, these are spaces between them that allows filtrate to pass beneath them and through gaps between their branches. Filtrate passes around them rather than through them
2. The endothelium of the glomerular capillaries has spaces of up to 100nm between them. Fluid passed between rather than through.
3. As a result, the hydrostatic pressure in the blood is enough to overcome the resistance so filtrate passed into the renal capsule.

ultrafiltration occurs by pressure

* As the afferent arteriole is wider than the efferent arteriole the blood pressure is relatively high in the glomerular capillaries, this increases the water potential of the blood plasma in the glomerular capillaries above the water potential of the filtrate in the Bowman’s capsule.

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**Result:** water moves down the water potential gradient from the blood plasma in the glomerular capillaries to the bowman’s capsule.

ultrafiltration occurs by solute concentration

* Whilst the basement membrane allows most solutes within the blood plasma to filter out into the Bowman’s capsule, the plasma protein molecules are too big to get out and stay in the blood.
* The solute concentration in the blood plasma in the glomerular capillaries is higher than the filtrate in the Bowman’s capsule
* This makes water potential of the blood plasma lower than the filtrate in the Bowman’s

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**Result:** water moves down the water potential gradient from the Bowman’s capsule into the blood plasma in the glomerular capillaries.

describe selective reabsorption

1. Sodium ions are actively transported out of the epithelial cells into the blood which carry them away. This lowers the concentration of Na+ in epithelial cells.
2. Sodium ions diffuse down a concentration gradient from the lumen of the PCT into the epithelial cells through carrier proteins via facilitated diffusion. These carrier proteins are co-transporters so carry another molecule (glucose, amino acid, or chloride ion) through co-transport.
3. The molecules that have been co-transported into the epithelial cells of the PCT then diffuse into the blood (blood is constantly moving so there is concentration gradient).
4. As a result, all the glucose and other important molecules, as well as water, are reabsorbed.

outline the transport process involved in selective reabsorption

^^glucose from glomerular filtrate (co-transport with Na+ ions)^^ ^^→^^ @@cells lining PCT (active transport) →@@ intercellular spaces (diffusion) %%→ blood capillary lining tubule%%

what adaptions does the epithelium of the wall of the PCT

* 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)
* Co-transporter proteins
* A high number of mitochondria
* Tightly packed cells

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* Useful solutes, like glucose, are reabsorbed along the PCT by active transport and facilitated diffusion.
* Water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate.

Molecules reabsorbed in the PCT

* All glucose in the glomerular filtrate is reabsorbed into the blood
* No glucose should be in the urine
* Amino acids, vitamins, and inorganic ions are reabsorbed
* The movement of all these solutes from the PCT into the capillaries increases the water potential of the filtrate and decreases the water potential of the blood in the capillaries
* This creates a steep concentration water potential gradient and causes water to move into the body by osmosis
* A significant amount of urea is reabsorbed too
* The concentration of urea in the filtrate is higher that in the capillaries, causing urea to diffuse from the filtrate back into the blood.

Reabsorption of water and salts

* As the filtrate dips through the Loop of Henle, necessary salts are reabsorbed back into the blood by diffusion
* As salts are reabsorbed back into the blood, water follows by osmosis
* Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body need at the time
* After the reabsorption of amino acids, water, glucose and inorganic ions is complete, the filtrate eventually leaves the nephron and is called urine which passes along the ureter to the bladder

loop of Henle summarised

1. active transport of Na+ and Cl- ions out of the ascending limb
2. water potential of interstitial fluid decreases
3. osmosis of water out of descending limb (ascending limb is impermeable to water)
4. water potential of filtrate decreases going down, descending limb: lowest in medullary region highest at top of ascending limb.

loop of henle

1. Sodium ions actively removed from the ascending limb using ATP from mitochondria in the cells that line its walls
2. This creates low water potential (high osmolarity) in the region of the medulla between the two limbs. Normally, water would leave the ascending limb by osmosis because of this. However, the thick walls of it prevent this.
3. The walls of the descending limb are very permeable to water, so it passes out of the filtrate by osmosis into the interstitial space then to the blood capillaries and is carried away.
4. The filtrate loses water as it moves down the descending limb lowering its water potential (increasing its osmolarity). It reaches its lowest water potential at the top of the loop.
5. At this point, sodium ions diffuse out of the filtrate as it moves up the ascending limb, and they are also actively pumped out. The filtrate decreases in the number of solutes dissolved per litre (osmolarity) but increases in its water potential the further up the medulla it goes.
6. In the interstitial space between the ascending limb and collecting duct there is a gradient of water potential with the lowest concentration of ions in the cortex. The further into the medulla you go the increasingly higher concentration of ions you’ll find (lower water potential).
7. The collecting duct is permeable to water and so as the filtrate moves down it, water passes out through osmosis. It goes into the blood vessels that surround it and is carried away.
8. As water passes out of the filtrate, the water potential is lowered (osmolarity increases). However, the water potential is also lowered in the interstitial space as it is being removed by the blood vessels. Water therefore continues to be removed for the entire length of the collecting duct. The counter current multiplier ensures there is always a water potential gradient drawing water out of the tubule.

When the water potential of the blood decreases:

1. change in water potential triggers the osmoreceptors to signal the hypothalamus, which send a signal to the posterior pituitary gland.


1. This causes the posterior pituitary to release antidiuretic hormone (ADH) into the blood.


1. makes cells lining the collecting duct more permeable to water
2. binds to receptor → activates phosphorylase → vesicles with aquaporins on membrane fuse with csm
3. makes lining of collecting duct more permeable to urea
4. water potential in interstitial fluid decreases
5. more water reabsorbed = more concentrated urine