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A-Level OCR Biology > Chapter 15 - Homeostasis > Flashcards

Chapter 15 - Homeostasis Flashcards

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1
Q

What is the definition of homeostasis?

A

The maintainence of a dynamic equilibrium inside of the body

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2
Q

Why do our bodies maintain a dynamic equilibrium as opposed to a static equilibrium?

A

Because it would be impossible to maintain an entirely stable state, as minute changes happen constantly

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3
Q

2 examples of positive feedback in the body

A

Platelet activation in blood clotting

Oxytocin levels during labour

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4
Q

3 examples of negative feedback in the body

A

Blood glucose control
Osmoregulation
Internal temperature control

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5
Q

What are the 5 stages of the feedback loop?

A 
Input
Receptor
Coordination
Effector
Output
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6
Q

What are the two main processes the kidneys are involved in?

A

Excretion and osmoregulation

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7
Q

What are the kidneys made up of?

A

Millions of filtering units called nephrons

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8
Q

What are the 3 main areas of the kidneys?

A

The cortex
The medulla
The pelvis

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9
Q

What is the appearance of the cortex?

A

The dark outer layer of the kidneys

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10
Q

What is the appearance of the medulla?

A

The lighter in colour, inner section of the kidneys

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11
Q

What occurs in the cortex?

A

The filtering of the blood

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12
Q

What is the glomerulus?

A

A tangle of capillaries, in which ultrafiltration takes place

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13
Q

Where is the glomerulus found?

A

Within the Bowman’s capsule

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14
Q

What is the proximal convoluted tubule?

A

The first coiled region of the tubule after the Bowman’s capsule

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15
Q

What area of the kidneys is the proximal convoluted tubule found?

A

The cortex

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16
Q

What is the Bowman’s capsule?

A

The cup-shaped structure that contains the glomerulus, into which blood enters during ultrafiltration

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17
Q

What primarily happens in the proximal convoluted tubule?

A

Many of the substances needed by the body are reabsorbed into the blood

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18
Q

What is the name of the region between the proximal and distal convoluted tubule?

A

The loop of Henle

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19
Q

What is the distal convoluted tubule?

A

The second coiled region of the tubule

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20
Q

What primarily occurs in the distal convoluted tubule?

A

The fine-tuning of the water balance

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21
Q

What does the permeability of the walls of the distal convoluted tubule depend on?

A

The levels of antidiuretic hormone (ADH)

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22
Q

What is the collecting duct?

A

Where urine passes down, through the medulla to the pelvis

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23
Q

What is the difference in the levels of urea as blood enters and leaves the kidneys?

A

Blood leaving the kidneys has greatly reduced levels of urea

24
Q

What arteriole does blood enter the glomerulus through?

A

The afferent arteriole

25
Q

What arteriole does blood leave the glomerulus through?

A

The efferent arteriole

26
Q

What is the difference between the afferent and efferent arteriole, and what is the effect of this?

A

The efferent arteriole is much narrower, which means there is considerable pressure in the capillaries of the glomerulus

27
Q

What is the effect of the different lumen sizes between the afferent and efferent arterioles?

A

It creates a filtration pressure in the glomerulus, which forces blood out of the capillaries into the Bowman’s capsule

28
Q

What makes up the first sieve/filter of ultrafiltration?

A

Gaps in the capillary endothelium and the basement membrane

29
Q

How do gaps in the capillary endothelium and the basement membrane act as a sieve?

A

Substances in the blood that are too large to fit through will remain in the glomerulus and leave out of the efferent arteriole

30
Q

What makes up the second sieve/filter of ultrafiltration?

A

Podocytes

31
Q

What are podocytes?

A

Specialised cells found on the wall of the Bowman’s capsule that act as an additional filter in ultrafiltration

32
Q

How do podocytes act as an additional filter?

A

They have extensions called pedicels that wrap around the capillaries forming small slits, ensuring that anything that has made it past the first sieve that shouldn’t have, does not get through to the Bowman’ capsule

33
Q

What elements of the blood can’t make it through ultrafiltration?

A

Red blood cells
White blood cells
Platelets
Large plasma proteins

34
Q

What substances do make it through ultrafiltration?

A 
Urea
Glucose
Salt ions
Water
Amino acids
35
Q

What does the word ‘ultrafiltrate’ refer to?

A

The blood and substances that enters the Bowman’s capsule through ultrafiltration

36
Q

What is the purpose of reabsorption?

A

To return the necessary substances (such as glucose) which have passed through ultrafiltration back into the bloodstream

37
Q

What substances are completely reabsorbed in the proximal convoluted tubule and how?

A

All of the glucose, amino acids, vitamins and hormones are reabsorbed by active transport

38
Q

What other substances are reabsorbed in the proximal convoluted tubule?

A

85% of Na+ Cl- ions and water

39
Q

How are Na+ removed in the proximal convoluted tubule and what is the effect of this?

A

Na+ ions are removed by active transport, which increase the water potential inside the tubule, causing water to leave by osmosis.
This loss of water increases solute potential causing Cl- ions to also leave by diffusion

40
Q

Where do substances go once they have left the tubule in reabsorption?

A

They go into the tissue fluid, and then diffuse down steep concentration gradients into the extensive capillary network which surrounds the tubules

41
Q

How are steep concentration gradients maintained in the capillary network surrounding the tubules in the nephron?

A

The concentration gradients are maintained by the constant supply of fresh blood through the capillaries

42
Q

What adaptions does the proximal convoluted tubule have and how do they help?

A
  • Covered with microvilli, which increase the SA over which substances can be reabsorbed
  • Many mitochondria which provide ATP needed for active transport of substances out of the tubule
43
Q

What leads on from the proximal convoluted tubule?

A

The descending loop of Henle

44
Q

What is the descending loop of Henle permeable and impermeable to, and what is the effect of this?

A

It is permeable to water, and impermeable to salt ions. This results in water leaving the descending loop, which means the ion concentration increases as you go down the descending loop

45
Q

What is the hypertonic point of the loop of Henle?

A

The bottom of the loop, in which the salt ion concentration is at its highest, and the water potential is at its lowest

46
Q

What happens to the salt ion concentration as you go up the ascending loop of Henle?

A

Salt ion concentration will decrease, as salt ions diffuse out of the ascending loop down the concentration gradient

47
Q

In the ascending loop of Henle, what happens when salt ion concentration is isotonic with salt ion concentration in the tissue fluid?

A

The ions are actively pumped out into the medulla tissue

48
Q

Why does water not also leave the ascending limb as ion concentration decreases (hence water potential increases)?

A

As the ascending limb of the loop of Henle is impermeable to water

49
Q

Simplistically, how does the loop of Henle have a countercurrent multiplier system?

A

It works like a positive feedback system.
The pumping of salt ions out of the ascending loop causes the medulla tissue fluid to have a very low water potential.
This creates a steeper concentration gradient for water to leave the descending loop, meaning more water leaves.
This means the salt ion concentration in the tubule as you go down and around the loop of Henle will be even higher.
This increased salt ion concentration will mean even more salt ions leave the ascending loop, creating an even steeper water concentration gradient between the medulla tissue fluid and the descending limb.
And so on…

50
Q

What is the result of the countercurrent multiplier system?

A

High salt concentration in the medulla tissue fluid, and a low salt concentration in the blood entering the distal convoluted tubule

51
Q

Where does the balancing of water levels happen in the kidney?

A

The distal convoluted tubule and the collecting duct

52
Q

How are salt levels balanced in the distal convoluted tubule?

A

If more salt is needed by the body, Na+ ions will be actively pumped out, and Cl- ions will follow by diffusion down the electrochemical gradient

53
Q

How are water levels balanced in the distal convoluted tubule and collecting duct?

A

ADH will control the amount of aquaporins on the tubule, which will control the amount of water that is reabsorbed into the blood

54
Q

What are aquaporins?

A

Channel proteins that facilitate the transport of water out of the tubule, hence making the tubule more or less permeable to water

55
Q

Where is ADH secreted from?

A

The pituitary gland

A-Level OCR Biology (21 decks)

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