Homeostasis Flashcards
Define homeostasis
Homeostasis=maintenance of constant internal environment
Keep set point stable within narrow limits in body
Irrespective of changes in external environment
*Factors such as temperature, water potential, glucose concentration must be maintained in the tissues fluid because it affects cell function
*So that internal environment can:
➡️Be stable
➡️Function optimally
*Control systems often work by using a negative feedback mechanism
What is negative feedback and what are the components needed for negative feedback?
Components needed:
*Stimulus
*Receptor
*Control center
*Effector
*Response
➡️Restoration of norm or set point
*Continuous monitoring of the factor affecting the internal environment
*Resulting in many “corrective actions”
*Factors thus fluctuates around the norm/set point stable
What is a stimulus?
Stimulus refers to internal or external change in factor away from norm/set-point
What is a receptor?
Cells/tissue/organ which detect the stimulus
What is a coordinating centre?
Consists of tissue which receives and processes messages, in the form of hormones or nerve impulses from the receptors and determines the appropriate response.
Only if the stimulus reaches a certain threshold/is strong enough, it sends messages to an effector.
What is an effector?
Tissues/organs which receive messages from coordinating centre and carry out a corrective reaction.
What is a response?
The reaction carried out by the effectors.
In negative feedback response counteract the stimulus. Return to set point/ norm
Compare positive vs negative feedback response
Positive feedback:
*Response reinforce the original stimulus
*Very uncommon
*Response worsen/intensifies the initial change
E.g. Labour pains, ripening of fruit, inhalation of CO2
Negative feedback:
*Response counteracts the original stimulus
*Common in the body
*To maintain homeostasis/stable internal environment
E.g. maintaining blood glucose levels, temperature,oxygen levels, water content in blood etc
What is excretion
Removal of unwanted products of metabolism
→Toxic,poisonous, will cause damage to tissues
Main excretory products:
a)Carbon dioxide
→From aerobic respiration
→Excreted via bloodstream and lungs
b)Urea (nitrogenous waste)
→Produced in liver
→From excess amino acids
→Excreted via kidneys
c)Creatinine (Nitrogenous waste)
→Small amounts produced in liver
→From certain amino acids
→Most used as energy storage in muscles
→Excreted via kidneys
d)Uric acid (Nitrogenous waste)
→Produced in liver
→From excess purines of nucleotides
→Excreted via kidneys
Describe Urea
*Main nitrogenous excretory product
*Formed from excess amino acids
*In liver cells
- Deamination
→Remove amine group and a H atom from amino acid
→Produce ammonia,NH3
→Toxic if allowed to accumulate - Urea cycle(aka ornithine cycle)
NH3 + CO2 →Urea→Excreted (kidneys)
*Keto acid remains→Respired or converted to glucose/glycogen/fat
What are the blood vessels anmd excretory tubes related to the kidney?
Blood vessels:
Renal artery(in)
Renal vein (out)
Excretory tubes:
*Ureter -Urine from kidney into urinary bladder
*Urethra-Urine out from urinary bladder
Describe te structure of the kidney
*Capsule
→Tough, protective layer
*3 main regions:
a)Cortex
b)Medulla
c)Pelvis
*Nephrons-tiny tubes in the kidney in coretx and medulla
Describe the structure of a nephron
1) Bowman’s capsule @ cortex aka renal capsule
2) Proximal convulated tubule @ cortex
3) Loop of Henle @ Medulla
subdivided to:
a)Descending limb
b)Ascending limb
4)Distal convulated tubule @ cortex
5)Collecting duct @ medulla
→Ureter @ pelvis
*Branch of renal artery
→Afferent arteriole
→Glomerulus (tangle of capillaries in the “cup” of the bowman’s capsule)
→Efferent arteriole
→Network of blood capillaries
Surrounding the rest of the nephron
→Branch of renal vein
What are the two stages involved in the mechanism of extraction in the kidneys?
2 stages:
1) Ultrafiltration
*Filtering of small molecules, incl. urea out of the blood @ bowman’s capsule
2)Selective reabsorption
*Absorbing any useful molecules from fluid in nephron
@proximal convulated tubules,loop of Henle, distal convulated tubule and collecting duct
What is ultrafiltration?
*Filtering of small molecules, incl. urea
*Out of the blood in glomerulus
→Into filtrate in bowman’s capsule space/lumen
*Glomerular filtrate is produced
*Flows along the entire nephron
→Into ureter
Describe the structure of glomerular wall and bowman’s capsule wall
Blood in glomerulus is seperated from lumen of bowmans capsule by 3 layers:
1) Endothelium of blood capillaries of the glomerulus
*With many more gaps/fenestrations
2)Basement membrane
*Mesh of collagen and glycoprotein fibres
*Acts as main selective barrier/filter
3)Epithelial cells of bowman’s capsule
*Inner lining of bowman’s capsule (podocytes)
*Wrap around capillaries of the glomerulus
*Podocytes have many finger-like projections that forms gaps/filtration slits
How is the structure of the kidney adapted for ultrafiltration?
1) Many large gaps in capillary endothelium + filtration slits between foot processes of podocytes
*Allow movement of substances from blood plasma easily into bowman’s capsule lumen
2) Diameter of the lumen of afferent arteriole is wider than efferent arterioles
*leads to high blood pressure/hydrostatic pressure in the glomerulus than the bowman’s capsule
*Fluid forced out of glomerulus into bowman’s capsule
3)Basement membrane acts as a filter
→Prevents RBCs,WBCs and large plasma proteins (RMM>68000Da) from passing through
Resulting glomerular filtrate contains:
*No cells and large proteins
*Soluble molecules: water,amino acids, glucose, urea, inorganic ions (Na+,K+,Cl-),uric acid,creatinine,vitamins
→Glomerular filtrate passes through gaps betweens podocytes and into renal capsule
What is selective reabsorption?
*Necessary
*To reabsorb essential substances from filtrate
*Back into blood
*Selective reabsorption so only certain substances are reabsorbed
*E.g. Glucose,amino acids,vitamins,Cl-,Na+,H2O
@proximal convulated tubule, loop of henle. distal convulated tubule and collecting duct
Describe selective reabsorption @ proximal convulated tubule (PCT)
*Main site for glucose/amino acid/vitamin/Cl- reabsorbtion
*Walls made of single layer of cuboidal epithelial cells
1)Active transport of Na+ ions
*From PCT cells into blood in capillary
*VIA Na+/K+ pumps
*Concentration of Na+ ions in the PCT cell decreases
2)Na+ ions in PCT lumen diffuse down its gradient into cells lining the PCT
*By faciliated diffusion
*Via co-transported carrier proteins
*Na+ co-transported with glucose/amino acids/vitamins/Cl- ions into cell
3)Glucose/amino acid/vitamins/Cl- ions diffuse into blood via transport protein
*By faciliatated diffusion
What is the result of selective reabsorption @ proximal convulated tubule (PCT)?
*Glucose is
→ALL actively reabsorbed into blood
→No glucose in urine
*Amino acids,vitamins and Cl- ions
→Actively reabsorbed
*Water, urea
→Some passively reabsorbed
*Uric acid and creatinine → NOT reabsorbed
→Creatinine→Actively secreted/transported into lumen of PCT
What are the adaptations of Proximal convulated tubules cells for selective reabsorption?
a)Numerous microvilli (facing lumen)
-Large surface arwa for absorption
b)Presense of different transport proteins in membranes (facing lumen)
i.e cotransporters,pumps, aquaporins
c)High density of mitochondria
-Provides energy in the form of ATP for active transport
d) High infolding of basal membranes (facing blood capillaries)
e)Tight junctions holding adjacent cells together
-Seperate proteins of front and basal membrane
-So fluid cannot pass between cells,substance must pass through cells
Describe the structure of the loop of Henle
Loop of Henle is located at the medulla
Mainly for water reabsorption
2 parts:
1)Descending limb
*Permeable to both water and Na+ and Cl- ions
2)Ascending limb
*Impermeable to water
*Permeable to Na+ and Cl- ions
How does selective reabsorption occur at the loop of Henle?
@Ascending limb
1. Na+ and Cl- ions move out of the tube
→By active transport
→Into tissue fluid of medulla space
2. High concentrations of Na+ and Cl- ions in the medulla space
→Renal fluid becomes more dilute and enters distal convulated tubule
→Longer loop results in higher concentration of solute built up in the medulla space, more water reabsorption, more conc urine formed!
@Descending limb
*Permeable to both water and Na+ and Cl- ions
Due to high concentrations of solute in the medulla
3. Water moves out into medulla tissue fluid
→By osmosis
→Water is reabsorbed
4. Urea, Na+ and Cl- ions in medulla space diffuse into descending limb
→Fluid in the descending limb becomes very concentrated as it moves down the loop.
Describe how selective reabsorption @ distal convulated tubule occurs.
Distal convulated tubule is located in the cortex
1st part of DCT=Similar to ascending limb of loop of Henle
*Na+ and Cl- ions again actively transported into blood
2nd part of DCT=Similar to collecting duct
*Water is reabsrobed into blood
*Plus secretion of K+,H+ ions and urea into lumen from blood
Describe how selective reabsorption occurs @ collecting duct
*Located at medulla
*Tissue fluid of medullah has high concentrations of solutes
*So water moves out of collecting duct
*High reabsorption of water back into blood
→Formation of urine
*Rate of water reabsorption is controlled by ADH (antidiuretic hormone)
Describe osmoregulation
Osmoregulation=control of the water potential of body fluids
Uses negative feedback mechanism
Stimuli:Water potential of blood is low
Receptor:Osmoreceptors at the hypothalamus detect water potential of blood
Effector:Neuroscretory cells of the hypothalamus send nerve impulse to posterior pituitary glands
*ADH (antidiuretic hormone) released from posterior pituitary
→Enter blood stream
→Target organ: Distal convulated tubule/collecting duct of kidneys
Describe the response to ADH in osmoregulation
- ADH in blood bind to receptors on plasma membrane of disital convulated tubule/collecting duct
- Activate a series of enzyme-controlled reactions/enzyme cascade in cells
→Production of active phosphorylase enzyme
3.Vescicles containing aquaporins fuse with plasma membrane of lumen side
Describe the result of ADH in osmoregulation
Result:
*ADH increases membrane permeablity of collecting duct
*Increases water reabsorption
*More water flows out of distal convulated tubule/collecting duct into blood down water potential convulated gradient
*so smaller volume of more conc urine produced
*Water potential of blood increases
*Returns to norm/set point
What happens if there is an increase in the water potential of blood?
*Osmorecpetors are no longer stimulated
*Neurons stop secreting ADH
*Aquaporins move out of cell surface membrane of collecting duct, back into vesicles in the cytoplasm
*Collecting duct is less permeable to water
*Dilute urine and larger volume of urine produced
*Water potential of blood decreases
*Returns to set point
What are endocrine glands?
Secretory cells
Releases secretions directly into blood capillaries in the glands
Secretions:Hormones
E.g. pituitary glands,thyroid,adrenal,ovary, testes, pancreas
What are Exocrine glands?
Secretory cells
Releases secretion into ducts/tubes (not capillaries)
Secretions: not hormones
E.g. stomach,salivary glands,pancreas
What are hormones
*Secreted by endocrine glands
*Hormones can be globular proteins OR steroids
E.g. Insulin-protein hormone
Testosterone/steroid hormone
Characteristics:
1.Small molecules, chemical messengers
2. Needed in small quantities
3.Secreted quickly upon receiving stimulus
4.Short life span,quickly broken down by enzymes/excreted via urine
5. Transported in the blood stream to target cells
6. Specific- bind to receptors on target cells
Receptors can be on cell surface membrane OR inside cell
What are the types of hormone receptors?
1)Receptors for protein hormones-on plasma membrane
➡️Water soluble, cannot pass through plasma membrane
2)Receptors for steroid hormones-in cytoplasm
➡️Lipid soluble, can pass easily through plasma membrane
Describe the pancreas
The pancreas:
*Acts as BOTH endocrine and exocrine gland
*Exocrine-secretes pancreatic juice
➡️VIA pancreatic duct to duodenum
*Endocrine-secretes insulin and glucagon hormones into blood
➡️Islets of langerhans (group of secretory cells) composed of:
*Alpha cells that secrete glucagon
*Beta cells that secrete insulin
Describe Insulin vs Glucagon
Glucagon and insulin are antagonist hormones
*Glucogan secreted by alpha cells → used to increase blood sugar
*Insulin secreted by beta cells→used to decrease blood sugar
What are the stimuli, receptors and effectors for insulin?
Stimuli: Blood glucose level increases
Receptors: Detected by α and β cells in islet of langerhans of the pancreas
Effectors:β cells secrete more insulin into blood and α cells stop secreting glucagon
Insulin acts on liver cells, muscle cells and adipose (fat) cells
How does insulin work?
1.Insulin bind to receptors on cell surface membrane of liver cells/muscle cell/adipose cell
2.Increase permeability of membrane to glucose in liver and muscle cells.
*Trigger vesicles with glucose transported proteins (GLUT proteins) to move and fuse with plasma membrane
*More facilitated diffusion of glucose into cells
3. Increase glucose uptake/absorption from blood
*Stimulate activation of enzyme glucokinase→phosphorylates glucose
*Glucose trapped in cells
4. Increase rate of respiration of glucose
5. Increase conversion of glucose→glycogen (glycogenesis)
*By activating two enzymes (phosphofructokinase,glycogen synthetase)
→Store in liver and muscles
6. Increase protein and lipid synthesis
7. Inhibit secretion of glucagon from α cells
→Inhibit glycogen breakdown into glucose (glycogenolysis)
8.Inhibit production of glucose from proteins and fats (gluconeogenesis)
Result:
*Decrease in glucose concentration and return to norm/set point
What is the stimuli, recpetors and effectors of glucagon?
Stimuli: Blood glucose level decreases
Receptors: Both α and β cells in islet of langerhans of the pancreas
Effectors:
α cells secrete glucagon into blood
β cells stop secreting insulin
Glucagon acts on liver cells ONLY
How does glucagon work?
- Glucagon binds to receptor on cell surface membrane of liver cells
*Receptor changes shape - Activates G proteins,
*Which activates adenyl cyclase - Adenyl cyclase produces cyclic AMP (cAMP)
*From ATP
*cAMP acts as the second messenger
*Activates protein kinase
*Triggers an enzyme cascade/ a series of enzyme-controlled reactions
*Signal is amplified
Response:
4. cAMP activates enzyme glycogen phosphorylase
*Increase breakdown of glucogen to glucose (glycogenolysis)
5. Use fatty acids and proteins as respiratory substrate instead of glucose
6. Increase production of glucose from proteins and fats (gluconeogenesis)
*Glucose diffuse through glucose transported proteins-GLUT proteins from liver
*Liver releases glucose into blood
*Increase in blood glucose conc. and return to norm/set point
Describe adrenaline
*Fight or flight hormone
*Produced during excercise and stress
*Secreted by adrenal gland into blood
*To increase glucose levels in blood
→So muscles can undergo aerobic/anaerobic respiration and produce more ATP
Describe diabetes melitus
*High glucose concentration in blood
There are two types
*Symptoms same in both forms
*High glucose concentration in blood and urine
→Due to glucose not taken up by cells
→Less glucose converted to glycogen/fat
→Not all glucose can be reabsorbed in kidneys
*Decrease in water potentiol of blood
→Water and salts move out of cells down concentration gradient
→Dehyrdration, production of dilute urine, loss of saltys and cramps
→Detected by osmoreceptors in the hypothalamus→feeling thirsty
*Fat and proteins used in respiration instead of glucose→weight loss
→Build up of keto acids/ketones in blood →blood pH lowers, can cause coma
How does urine analysis work?
Urine can be collected from patient to test
Presense of glucose and keto acids/ketons in urine
*Not all glucose is reabsorbed at the PCT
*May have diabetes mellitus
Long term presense of proteins in urine:
*Most protein molecules do not pass through the basement membrane at the bowman’s capsule
*Other protein should be reabsorbed at the PCT
*May have kidney infection or disease affecting the glomeruli
*Also associated with high blood pressure
*Short-term presense common during during high fever, vigorous excercise, pregancy
How do dipstick tests work?
*Used to measure glucose concentration in urine (not blood!)
*Specific test for glucose detection
1)Glucose oxidase and peroxidase immobolised onto pad on disptick
2)Dip stick lowered into urine
3)Glucose catalysed by glucose oxidase→gluconolactone + hydrogen peroxide
Hydrogen peroxide + chromogen(colourless chemical) catalysed by peroxidase→darker compound
4)Compare with colour chart
*Darkness of colour is porportional to concentration of glucose
*The more glucose present, the darker the colour!
Describe how biosensors work
*Biosensors can directly measure glucose concentration in blood
*Reusable, more precise
1)Glucose oxidase immobolized on pad
2)Place small smaple of blood on pad and insert into machine
3)Glucose is catalysed by glucose oxidase→gluconolactone + hydrogen peroxide
*Small electric current is generated at the same time
4)Current detected by electrode
*Current amplified and reading is produced
*Gives numerical value of blood glucose concentration
*More glucose present, greater current, greater reading from bio sensor
