6.4 Homeostasis

Question 1

What does homeostasis involve?

Answer 1

Physiological control systems that maintain the internal environment within limits

Question 2

What is negative feedback?

Answer 2

A deviation from the norm initiates a corrective mechanism to restore the norm

Question 3

What is positive feedback? Examples?

Answer 3

A deviation from the norm causing further deviation from the norm

Blood clotting
Puberty
Contractions in childbirth
Hyper/hypothermia

Question 4

What happens when there’s an increase above the norm in temperature? Eg hot environment or exercise

Answer 4

Detected by thermoreceptors in the thermoregulatory centre of the brain

Responses:
Vasodilation 
Sweat
Take off a layer 
Find shade

Question 5

What happens when there’s a decrease below the norm in temperature? Eg cold environment

Answer 5

Detected by thermoreceptors in the thermoregulatory centre of the brain

Responses:
Shiver - muscle contraction 
Vasoconstriction 
No Sweat
Hairs stand on end
Curl up in a ball - decreases SA:V ratio

Question 6

Why is hypothermia and hyperthermia examples of positive feedback?

Answer 6

Hypothermia - as you cool down reactions slow, so less heat is released from aerobic respiration, so you cool down even further

Hyperthermia - as you heat up reactions increase, so more heat is released from aerobic respiration, so you heat up even further

Question 7

Why must blood glucose levels be maintained at relatively constant levels?

Answer 7

Glucose is needed for aerobic respiration

Concentration of blood glucose affects water potential into and out of red blood cells
Low blood glucose = lysis
High blood glucose = crenation

Question 8

What happens when there’s a increase above the norm in blood glucose concentration? Eg eating carbohydrates, too much glycolysis

Answer 8

Detected by receptors in the pancreas - islets of langerhans
Beta cells release insulin

Increase uptake of glucose into cells
Convert glucose to glycogen
Increased rate of respiration
Convert into lipids

Takes place in the liver

Question 9

What happens when there’s a decrease below the norm in blood glucose concentration? Eg Lack of carbohydrates, too much exercise

Answer 9

Detected by receptors in the pancreas - islets of langerhans
Alpha cells release glucagon

Break down the glycogen into glucose
Break down lipids and proteins (alternative respiratory substrates)

Take place in the liver and muscles

Question 10

How can we increase the uptake of glucose into cells?

Answer 10

Insulin stimulates vesicles near the membrane to fuse
These vesicles contain glucose transport proteins
The cell membrane now has more glucose carrier proteins to transport more glucose

Insulin binds to its complementary receptor - changing the shape of the glucose carrier proteins
The proteins open and glucose enters via facilitated diffusion

Question 11

How is glucose converted into glycogen?

Answer 11

Insulin binds to its complementary receptor which activates enzymes present in the liver and muscle cells to convert glucose into glycogen

Question 12

How do we break down glycogen into glucose?

Answer 12

Glucagon binds to a receptor in the liver
Which activates enzymes to break down glycogen into glucose

It’s a second messenger model so doesn’t enter the liver

Question 13

What does adrenaline do?

Answer 13

Adrenaline binds to an intrinsic protein causing a change in shape of Adenylyl Cyclase
This breaks down ATP into cyclic AMP (adenosine monophosphate)
cAMP activates protein kinase A which catalyses glycogen into glucose

Adrenaline effectively activates kinase A
It is produced in the adrenal gland

Question 14

What is the 2nd messenger system?

Answer 14

A substrate doesn’t do the job itself but it sends or activates something else to do the job

Question 15

Information about Type 1 diabetes?

Answer 15

Cause - genetic mutation of Beta cells in islets of langerhans means they don’t produce enough insulin

Treated with insulin injections

Question 16

Information about Type 2 diabetes?

Answer 16

Cause - linked to obesity as high blood pressure can damage insulin receptors so liver and muscle cells don’t respond to insulin

Treated by diet and exercise (food with a low glycemic index)

Question 17

What is used in osmoregulation?

Answer 17

The kidney’s

The nephron

Question 18

Describe the cross section of a kidney?

Answer 18

Out to in

Renal cortex
Renal Medulla 
Renal pyramid’s on the
Renal pelvis
Ureter 

Renal vein 
Renal artery (coming out from the side)

Question 19

What are the sections of the nephron?

Answer 19

Bowman’s Capsule 
(Glomerulus)
Proximal convoluted tubial
Loop of Henle
Distal convoluted tubial
Collecting duct

Question 20

What is a nephron?

Answer 20

Microscopic filtering units

Question 21

What is the nephron’s first stage?

Answer 21

In the Bowman’s capsule - Blood with enters the afferent arteriole ( which has a wider lumen)
Into the Glomerulus (bundle of capillaries) where ultrafiltration takes place - it has three processes to create the filtrate
The capillary endothelial layer, connective tissue fibre and Podocytes
The filtrate produced contains: water, urea, mineral ions and some glucose/amino acids (small enough to remain) things like red/white blood cells are too big

Question 22

Describe the layers of ultrafiltration?

Answer 22

The blood goes through
The pore in the capillary endothelial layer
Then straight through the mesh of connective tissue
And through the shortest diffusion pathway of the podocyte

Question 23

What is the nephron’s second stage?

Answer 23

The filtrate leaves out the efferent arteriole (with a smaller lumen) = high hydrostatic pressure
It enters the proximal convoluted tubial
Selective reabsorption of glucose and amino acids (potentially a bit of water/ions)
They are reabsorbed by co-transport
It has a large surface area (microvilli), lots of mitochondria and ribosomes to carry the molecules across the membrane

Question 24

What is the nephron’s third stage?

Answer 24

In the loop of Henle ions and water are reabsorbed
In the thin part of the ascending limb ions move out via facilitated diffusion (this is also impermeable to water
In the thick part of the ascending limb the ions move out via active transport due to the lower concentration gradient created
Due to ions moving into the surrounding tissue fluid a low water potential is created (lots of solute) therefore water move out of the thin descending limb (impermeable to ions)

Question 25

During the loop of Henle how is this constant concentration gradient maintained

Answer 25

A counter current mechanism is used as these process happen continually in a cycle not one after the other

They all affect each other and the countercurrent allows the constant turnover

Question 26

What is the nephron’s fourth stage?

Answer 26

In the distal convoluted tubial as much water as needed is reabsorbed
Here is where the effect of the diuretic hormone will take place leading to increased uptake of water via osmosis

Question 27

What is the nephron’s fifth stage?

Answer 27

The collecting duct
This uses a counter current multiplier
The molecules left are: water, urea and ions forming urine
Which is temporarily stored in the bladder and leaves via the urethra

Question 28

What is an antidiuretic hormone used for?

Answer 28

To make you have less urine (wee less)

More water is absorbed back into the blood

Question 29

Describe the process of implementing the antidiuretic hormone?

Answer 29

Blood water potential falls
Osmoreceptors in the hypothalamus detect this change and send an impulse to the posterior pituitary gland which then releases ADH into the blood
They bind to complementary kidney receptors so more water is absorbed as aqua porins in vesicles fuse with the membrane and form water channels
Now blood water potential increases - pituitary gland releases less ADH and less water is reabsorbed from the kidneys