Homeostasis

Question 1

Define homeostasis

Answer 1

The maintenance of a stable eqm in the conditions in a body

Question 2

Define negative feedback

Answer 2

When a change is detected by sensory receptors and effectors are stimulated to reverse the change and restore conditions to their base level e.g. blood sugar level

Question 3

Define positive feedback

Answer 3

When a change is detected by sensory receptors and effectors are stimulated to reinforce that change and increase the response e.g. blood clotting

Question 4

Explain homeostasis

Answer 4

1) INPUT: there is a change in conditions away from set point (optimum conditions)
2) RECEPTORS: detect the change e.g. in skin or thermoregulatory centre in hypothalamus
3) CONTROLLER: put information together and initiates appropriate response e.g. thermoregulatory centre
4) EFFECTORS: carry out response to move conditions back to set point
5) OUTPUT: condition returns to set point (feedback)

Question 5

Describe the main ways ectotherms maintain a constant core body temp (with examples)

Answer 5

1) REORIENTATION: moving body with respect to solar radiation varying SA exposed to heat
2) THERMAL GAPING: used by larger ectotherms - open mouth allow heat loss by evaporation from the moist mucous surface - tortoises do same by spreading saliva over neck and front legs
3) COLOUR CHANGES: alter the ability of the body to absorb radiated het energy e.g. Marine Iguana has black pigment
4) BODY RAISING: minimise heat gains by conduction from hot surfaces like rocks and sand by lifting legs or arms off floor
5) BURROWING: enables ectotherms to avoid greater temp fluctuations on the surface of their habitat
6) EXOTHERMIC METABOLIC REACTIONS: butterflies and moth’s vibrate wings before flight to warm muscles as it increases cellular metabolism

Question 6

Whats an ectotherm? (include examples)

Answer 6

An animal that uses its surroundings to warm their bodies

E.G. amphibians, reptiles, all invertebrate animals and fish

Question 7

Whats an endotherm? (include examples)

Answer 7

Animals that rely on their metabolic processes to warm up and usually maintain a very stable core temp
E.G. mammals and birds

Question 8

Describe the main ways endotherms maintain a constant core temp when too hot

Answer 8

1) CHANGES IN BEHAVIOUR: undressing, moving in to shade, rest cycle
2) VASODILATION: sphincters/dilation of superficial arterioles allow blood close to surface so heat loss by radiation and body is cooled
3) SWEAT: when hot sweat glands extract larger volume of fluid from blood and sweat changed into vapour taking latent heat of evaporation from body to do this
4) HAIR: pilo-erector muscles relaxed, hair shafts flatten and allow free circulation of air over hairs - moving air = good conductor of heat

Question 9

Describe the main ways endotherms maintain a constant core temp when too cold

Answer 9

1) CHANGES IN BEHAVIOUR: dressing, moving out of shade, activity cycle
2) VASOCONSTRICTION: superficial arterioles are constricted so blood is shunted away from surface and heat is conserved
3) SWEAT: sweat glands extract smaller volume of fluid from blood and skin surface comparatively dry - no evaporation = no cooling
4) HAIR: pilo-erector muscles contract, hair shafts perpendicular to skin surface and trapped air is poor conductor of heat so warm skin insulated

Question 10

How do endotherms detect a temperature change?

Answer 10

The peripheral temperature receptors in the skin detect changes at surface temperature
Temperature receptors in the hypothalamus detect the temperature of blood deep in the body
The combination of these two receptors gives the body great sensitivity and allows it to pre-empt possible problems that might result from changes in external environment

Question 11

Define the term excretion

Answer 11

The removal of waste products of metabolism from the body

E.G. CO2, Bile pigments and nitrogenous waste products (urea)

Question 12

Whats the importance of removing waste products of metabolism

Answer 12

Metabolic waste is toxic if it remains in cells or tissue fluid
CO2 lowers pH
Urea alters pH (more alkaline) and denatures proteins
Bile pigment contains some of the breakdown products of RBC which are toxic to nervous system

Question 13

Describe protein metabolism in the liver

Answer 13

1) TRANSAMINATION: changing amino acids to different types of amino acid - only 12 types of amino acid can be made by transamination and others come from diet
2) DEAMINATION: breaking down of amino acids
amino acid + oxygen –> keto acid + ammonia
fish stop here as easy to dilute NH3 by surrounding water
3) ORNITHINE CYCLE:
2NH3 + CO2 –> CO(NH2)2 + H2O

Question 14

Describe carbohydrate metabolism (when glucose levels are high (>120 mg100cm-3)

Answer 14

1) GLYCOGENESIS: Insulin receptors on surface of liver to detect insulin, glucokinase is activated and phosphorylates glucose so too big to leave cell
Phosphofructokinase and glycogen synthase activated and glucose polymerase to glycogen

Question 15

Describe carbohydrate metabolism (when glucose levels are low (<80mg 100cm-3)

Answer 15

1) GLYCOGENOLYSIS: glucagon receptors on liver cells detect glucagon and glucagon hydrolysed to glucose and released into the blood
2) GLUCONEOGENESIS: glucose made from amino acids and lipids

Question 16

What are sinusoids and how are they adapted?

Answer 16

The vessels the hepatic portal vein is divided into
Compared to normal capillaries:
- walls more porous
- composed of one layer of very thin cells
- many pores between cells
- no basement membrane
Blood flowing through sinusoids is in close contact with hepatocytes

Question 17

Describe the process of detoxification in the liver

Answer 17

1) BREAKDOWN OF HYDROGEN PEROXIDE
- hepatocytes contain enzyme catalase which splits hydrogen peroxide into water and oxygen
2) BREAKDOWN OF ETHANOL
- hepatocytes contain enzyme alcohol dehydrogenase that dehydrogenates ethanol to ethanal (alongside the reduction of NAD to NADH)
- ethanal is dehydrogenated to ethanoate by aldehyde dehydrogenase (alongside reduction of NAD to NADH
- ethanoate then enters the Krebs Cycle

Question 18

What dangers does excessive alcohol consumption hold?

Answer 18

• Can lead to fatty liver

- More dangerously can lead to cirrhosis which is irreversible

Question 19

Describe the structure of the kidney

Answer 19

Has three main areas

1) CORTEX: dark outer layer where filtering of blood takes place and has dense capillary network carrying blood from renal artery to nephrons
2) MEDULLA: lighter in colour - it contains the tubules of nephrons that form pyramids in the kidney and the collecting ducts
3) PELVIS: the central chamber where urine collects before passing out the ureter

Question 20

Describe the structure of nephrons

Answer 20

1) BOWMANS CAPSULE: cup-shaped structure that contains the glomerulus - site of ultrafiltration
2) PCT: the first coiled region of the tubule after the BC - site of reabsorption
3) LOOP OF HENLÉ: a long loop of tubule that creates a region with a v high solute conc in tissue fluid
4) DCT: a second twisted tubule where the fine-tuning of water balance takes place - the permeability of the walls to water varies in response to ADH (anti-diuretic hormone)
5) COLLECTING DUCT: the urine passes down the collecting duct through the medulla to the pelvis. more fine-tuning takes place

Question 21

Describe the structure of the wall of the glomerulus and the wall of the bowman’s capsule

Answer 21

1) GLOMERULUS: walls of capillaries made of single layer of squamous endothelium cells which are porous to aid movement of particles through basement layer
Basement layer made up of network of collagen and other proteins
2) BOWMAN’S CAPSULE: cells that make up bowman’s capsule are podocytes which have extensions called pedicels that wrap around capillaries forming slits preventing platelets and large proteins getting into tubule

Question 22

Explain the process of ultrafiltration

Answer 22

• Blood supply from afferent arteriole (larger lumen than efferent where blood leaves)
• creates hydrostatic pressure which forces molecules out of capillary through pores in endothelium
• large molecules with RMM larger than 69000 remain in blood
• Basement membrane acts as second sieve before podocytes complete ultrafiltration with pedicels creating slits to prevent entry of larger molecules
• Due to loss of water in blood some water returns by osmosis
• However overall Effective Filtration Pressure forces almost all small molecules out of blood until returned by selective reabsorption.

Question 23

Describe the structure of the wall of the PCT

Answer 23

• Blood in capillaries runs close to PCT
• The epithelium of the wall of the PCT has microvilli (brush border) for increased SA for reabsorption
• Cells lining the PCT have many mitochondria to provide ATP for active transport
• Cells have sodium channel and sodium ion protein pump

Question 24

Explain process of selective reabsorption

Answer 24

• Returns all glucose, amino acids, vitamins and hormones back to the blood by active transport
• Since this requires ATP, cells surrounding PCT have many mitochondria
• Na+ are pumped by active transport out of the filtrate and back into the blood creating an electrochemical gradient causing ions in filtrate to move towards the epithelial cells and through protein channels by facilitated diffusion
• Whilst doing so another molecule e.g. glucose is co-transported and can pass through the pump into the blood
• Water returns to the blood by osmosis directly across epithelial cells which is aided by increased SA from microvilli

Question 25

Describe the role of the Loop of Henlé

Answer 25

• The cells in the walls of the ascending limb pump Na + Cl ions out of the filtrate into the interstitial tissue creating a low water potential between the limbs
• Water moves by osmosis from filtrate in descending limb into interstitial tissue where its reabsorbed into the blood (water can leave ascending limb as wall is too thick)
• At the hairpin of the loop the max conc of fluid is achieved. Salts leave the loop and build up in the tissue of the medulla creating v low water potential

Question 26

Describe the role of the DCT

Answer 26

• Ions reabsorbed into the blood.
• Cells here same as PCT
• Ions are reabsorbed under influence of hormones e.g. aldosterone (from adrenal gland)

Question 27

Describe osmoregulation

Answer 27

• In hypothalamus the osmoregulatory centre monitors conc of water
• If blood has low water potential cells called osmoreceptors lose water and shrivel
• They are neurosecretory cells - release hormone ADH to pituitary gland and the target cells for ADH are those in the collecting duct
• The pituitary gland releases ADH into the blood
• ADH binds to target cells of wall of the collecting duct
• Second messengers inside the cells cause phosphorylase enzymes to be activated
• Vesicles containing water-specific channels called aquaporins then move to and fuse with the cell surface membrane
• The aquaporins increase the permeability of the wall of the collecting duct and more water leaves the filtrate and is reabsorbed into the blood
• Reduced vol of urine (concentrated)

Question 28

List the causes of kidney failure

Answer 28

• Infection
• Hypertension/ high blood pressure
• Injury
• Drugs
• Genetic disorder e.g. polycystic kidney disease

Question 29

List the symptoms of kidney failure

Answer 29

• Blood in urine
• Large proteins in urine
• High blood pressure
• Tissue fluid build up - oedema
• Build up of urea or ions
• Loss of bone density
• Joint pain
• Anaemia

Question 30

What is creatinine?

Answer 30

• Breakdown product of muscles but must be removed as TOXIC

- During kidney failure its not removed from blood due to low filtration rate

Question 31

Whats GFR? (Glomerular Filtrate Rate)

Answer 31

• The rate at which blood is filtered from the glomerulus into the Bowman’s capsule
• Used to monitor kidney function
• More accurate then serum creatinine test as considers age, gender and health

Question 32

Describe the method to collect eGFR

Answer 32

1) collect urine over 24 hours
2) collect blood sample after 24 hours
- If normal levels of creatinine found then individual has healthy kidney function
- If low levels of creatinine found then its a sign kidney’s are failing

Question 33

Whats Haemodialysis?

Answer 33

• The process of removing toxic waste and excess fluid from the blood using an artificial kidney machine that filters and ‘cleans’ the blood
• Blood is collected from a fistula (loop of an artery and a vein)
• The dialysis solution flows in opposite direction on the opposite side of the tube and any substances in excess will diffuse across
• Treatment is required 3 times a week, each lasting 5 hours

Question 34

Whats peritoneal dialysis?

Answer 34

• Method of removing impurities using the patients abdomenal peritoneum
• A catheter into the abdomen is used to introduce dialysis fluid between the skin and the peritoneum
• The peritoneum acts as the partially permeable membrane allowing dialysis to occur across the natural membrane
• Can be carried out at home
• Performed several times a day

Question 35

What are the pros and cons of kidney dialysis?

Answer 35

PROS:
- it doesn’t require major surgery
CONS:
- Treatment 3 to 4 times a week
- Treatment had the strictest diet and fluid limits of all
- Dialysis takes a long time (about 4 hours)
- More expensive then transplant
- Makes the patient very tired following dialysis

Question 36

What are the pros and cons of kidney transplants?

Answer 36

PROS:
- cheaper than dialysis (30,000 instead of 500,000)
- only one operation needed and no more work needed for 8-10 years
CONS:
- good tissue match needed
- major surgery
- risk of rejection of the donor kidney so immunosuppressant drugs must be taken daily
- Some religions don’t allow transplants
- Donor kidneys only last 10-15 years

Question 37

How do pregnancy dipsticks work?

Answer 37

• The stick is dipped into the urine to the line
• HCG-specfic antibodies bound to gold are carried up. If there is any HCG present it binds to the anti-bodies and is carried up
• If the stick is working, a pink line appears in the control region - HCG specific antibody bound to gold is carried upwards and captured by antibody specific to it, which are immobilised here.
• If the urine contains HCG it binds to the HCG-specific antibody and gold at the end of the dipstick and its carried upwards. When this meets immobilised HCG-specific antibody. It is bound and a pink line appears. This shows the person is pregnant