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A2 Biology 1 > Homeostasis > Flashcards

Homeostasis Flashcards

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1
Q

Define homeostasis

A

the maintenance of a constant internal environment.

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2
Q

why is homeostasis important?

A
  1. All cellular activity is controlled by enzymes, and if the pH or temperature of an enzyme deviates from the optimum value then the reaction rates drop and cells don’t function as efficiently. Consequently, organisms have a reduced chance of survival.
  2. Changing conditions can also cause osmotic problems in cells and body fluids.
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3
Q

give some factors that are controlled by homeostasis

A

pH, temperature, blood glucose levels, blood pressure, oxygen levels, and osmotic balance.

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4
Q

what fluids should be kept constant in homeostasis?

A

As all cells are bathed in an extracellular tissue fluid, this (and the blood due to the permeable nature of capillaries) are the area that must be kept constant irrespective of external conditions.

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5
Q

Describe how less ‘complex’ animals maintain a constant internal environment.

A

Less ‘complex’ animals will have less ability to maintain a constant internal environment, eg. Insect body temperature normally varies with external environment. These animals avoid large swings in body conditions by living in areas where the external environment is relatively constant, for example the sea.

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6
Q

What are the three features of a homeostatic response?

A
  1. Control system with receptors
  2. Corrective mechanism that is set about to return the factor to its normal level if receptors detect a departure from the set point.
  3. corrective mechanism involves a negative feedback system
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7
Q

Where can you find the control system and receptors that are involved in homeostasis? How do the control centre and receptors communicate?

A

The receptors can be in the brain or localised throughout the body, but the control centre (monitor) is usually in the brain

Communication between the monitor and receptors can be under nervous or hormonal control. Eg. Temperature is under nervous control, while blood-glucose levels are under hormonal control.

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8
Q

Give an example of a corrective mechanism in homeostasis

A

If mammals overheat, corrective mechanisms include sweating and vasodilation of skin blood capillaries

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9
Q

Define negative feedback

A

a system where a change from the set level (eg. 37ºC for body temperature) triggers a response which opposes the change and restores the set level. The return to the set level is detected and the corrective response is switched off to prevent over correction.

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10
Q

Explain negative feedback in control of body temperature

A

Stimulation of sweat glands and vasodilation of blood capillaries reduces in mammals as the body temperature returns to normal.

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11
Q

Describe the stages of the negative feedback response

A
  1. Conditions cause the level of a factor to change from the norm
  2. Change is detected by receptors
  3. Receptors communicate this information by means of a hormone or nerve impulse to effectors (or target organ in case of hormones)
  4. Corrective mechanism operates to restore the factor back towards the norm
  5. Receptors detect when the norm has been established and the corrective mechanism is turned off
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12
Q

List the organs in the urinary system

A 
Kidneys
Renal artery and renal vein
Ureters
Bladder
Urethra
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13
Q

Describe the function of the kidneys

A

filters blood for toxic substances

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14
Q

Describe the function of the renal artery and renal vein

A
  • renal artery carries mineral rich, oxygenated blood from heart to kidneys for nutrition and cellular respiration
  • renal veins carry deoxygenated blood after waste products have been removed via glomerular filtration from kidneys to heart
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15
Q

describe the function of the ureters

A

carries urine from the kidney to the urinary bladder

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16
Q

describe the function of the bladder

A

stores urine before it is released from the body

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17
Q

describe the function of the urethra

A

transports urine from the bladder, allowing the bladder to empty when urinating

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18
Q

Name the two functions of the urinary system

A
  1. Excretion

2. Osmoregulation

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19
Q

What is excretion?

A

removal of toxic waste of metabolism

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20
Q

What products are excreted from the urinary system?

A

mainly the nitrogenous waste urea, which is a product of breakdown of excess amino acids and nucleic acids in the liver. Other waste products excreted by the kidney include creatinine, which is produced from the breakdown of creatine phosphate (a molecule important in ATP synthesis) in the muscles.

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21
Q

Define osmoregulation

A

the control of water potential of body fluids

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22
Q

How does osmoregulation occur in the kidneys?

A

controlling the volume and concentration of urine produced

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23
Q

Name the tissue layers of the kidney

A
  • Tough outer fibrous capsule
  • Cortex
  • Medulla
  • Pelvis
  • Ureter
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24
Q

What is the nephron?

A

the functional unit of the kidneys - over a million present in each kidney

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25
Q

Name the functional parts of the nephron

A
  • Bowman’s capsule (glomerulus)
  • Proximal convoluted tubule
  • Descending and ascending limbs of the loop of Henlé
  • Distal convoluted tubule
  • Collecting duct
  • Vasa recta
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26
Q

What is the vasa recta?

A

blood capillaries branching off efferent arteriole

slow moving blood for exchange of substances from within the nephron

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27
Q

What are the two main steps of kidney excretion?

A
  1. Ultrafiltration

2. Selective reabsorption

28
Q

what is ultrafiltration?

A

filtering the contents of the blood with the assistance of high pressure

29
Q

what is selective reabsorption?

A

reabsorbing useful molecules that shouldn’t be lost in the urine

30
Q

where does ultrafiltration take place?

A

the boundary between the glomerular capillaries and Bowman’s capsule

31
Q

why is the blood in the glomerular capillaries under very high hydrostatic pressure?

A
  1. Differing diameters of the afferent and efferent arteriole: afferent arteriole has a wider diameter compared to the efferent. This creates a bottleneck effect causing an increase in blood pressure within the glomerulus.
  2. Short distance between the kidneys and the heart ensures there is little decrease in pressure
  3. Coiled nature of glomerular capillaries restricts blood flowing easily, resulting in increased pressure
32
Q

What will the basement membrane let through into Bowman’s capsule?

A

any molecules with an RMM less than 68000

This includes urea, glucose, sodium ions and other salts, and amino acids

33
Q

define glomerular filtrate

A

liquid that is forced out of the glomerulus and into Bowman’s capsule

34
Q

what remains in the blood plasma after ultrafiltration?

A

Blood cells and large plasma proteins (eg clotting factors and albumin)

35
Q

What are the ultrafiltration layers?

A
  1. Capillary endothelium
  2. Basement membrane
  3. Podocytes
36
Q

What is the capillary endothelium?

A

endothelium of the glomerulus is composed of squamous endothelium with pores between them - makes them more permeable than normal capillaries

37
Q

what is the basement membrane?

A

the effective filter - an extracellular gel-like matrix that acts as a molecular sieve

38
Q

what are podocytes?

A

specialised cells that are the inner wall of Bowman’s capsule. They have a structure that allow any substances that have passed through the basement membrane to flow freely into the capsular space

39
Q

what type of epithelium is present in the nephron?

A

cuboidal epithelium (except in Bowman’s capsule, which is lined with podocytes)

40
Q

What is the hydrostatic pressure in the glomerulus?

A

The pressure potential involved in ultrafiltration

caused by the differing diameter of the afferent and efferent arterioles

41
Q

Name the two pressures that work against the hydrostatic pressure

A

colloidal osmotic pressure

glomerular filtrate pressure

42
Q

what is the colloidal osmotic pressure?

A

the pressure created by the remaining plasma proteins in the blood, that pulls fluids back into the blood by osmosis (solute potential is bigger in filtrate than in blood)

43
Q

what is the glomerular filtrate pressure?

A

pressure that builds up in the capsular space - this forces blood into the glomerulus.

44
Q

Describe the relative water potentials in the capillaries and Bowman’s capsule that are required for filtration to occur

A

For filtration to occur, the water potential within the glomerular capillaries (blood plasma) must exceed the water potential in Bowman’s capsule (glomerular filtrate) - ie. glomerular filtrate must have a more negative water potential.

45
Q

Describe the adaptions of the PCT for its job of absorption

A
  • lined with microvilli on lumen side - increased SA for absorption
  • numerous mitochondria to provide energy for respiration
46
Q

How does selective reabsorption occur in the PCT?

A
  1. Small molecules (eg glucose, amino acids, ions) diffuse into cuboidal epithelium cells of the tubule.
  2. They are then transported by active transport into extracellular spaces and basal channels
  3. Substances in extracellular spaces then diffuse into the blood capillaries and are then circulated.
47
Q

Why is water reabsorbed in the PCT?

A

Due to the high concentrations of the ions contained in the cells surrounding the lumen of the PCT an osmotic gradient is produced. This causes water to be reabsorbed from the lumen of the PCT by osmosis.

48
Q

Describe the reabsorption of urea in the PCT

A

Although urea isn’t a useful product and therefore isn’t selectively reabsorbed, some still passes from the nephron and back into the blood by diffusion. However its concentration will relatively increase along the PCT due to water reabsorption.

49
Q

Describe what happens to proteins in the PCT

A

Very small protein molecules (molecular mass <68000) in the glomerular filtrate are also reabsorbed in the PCT by pinocytosis at the base of the microvilli. These proteins are then broken down enzymatically into amino acids.

50
Q

describe the water potential of the glomerular filtrate at the end of the PCT

A

isotonic with the blood in the surrounding capillaries

51
Q

Describe the structure of the loop of Henlé

A
  • Thin descending limb that is permeable to water

- Thicker ascending limb that is impermeable to water

52
Q

Describe what happens to the filtrate in the descending limb of the loop of Henlé

A

-Due to very negative solute potential surrounding the descending limb and its high permeability to water, water moves from the filtrate to the surrounding slow moving vessels of the vasa recta (this means there is little effect on the water potential of the interstitial fluid).

53
Q

Describe the water potential of the filtrate at the tip of the loop of Henlé

A

hypertonic - due to the osmotic removal of water and the diffusion of sodium and chloride ions into the descending limb.

54
Q

Describe the action of the ascending loop of Henlé

A
  • sodium and chloride ions are actively pumped out into the surrounding interstitial space of the medulla
  • medulla tissue gains a very negative solute potential, and the remaining filtrate becomes more dilute
55
Q

How is the ascending limb of the Loop of Henlé adapted for its function?

A

cuboidal epithelium that lines the limb has a lot of mitochondria - provides energy for active transport of ions

56
Q

Describe the water potential of the filtrate at the end of the loop of Henlé

A

hypotonic to blood due to the action of the ascending limb

57
Q

What other part of the nephron functions due to the action of the Loop of Henlé?

A

Collecting duct - LoH facilitates osmotic removal of water from collecting duct as it passes through the medulla, which has gained a very negative water potential

58
Q

How does the Loop of Henlé act as a counter current multiplier?

A

the longer the length of the loop, the more concentrated the urine can become as it can make the medulla even more concentrated and more water can be reabsorbed.

59
Q

Describe how the DCT further regulates the content of the urine

A
  • Varies pH and ionic concentration of filtrate

- Absorbs other excretory waste products such as creatinine from the blood.

60
Q

what is the water potential of pure water?

A

0kPa

61
Q

where is most water absorbed in the nephron?

A

mostly in the PCT

some in descending limb of LoH and collecting duct

62
Q

what hormone is responsible for osmoregulation in the nephron?

A

anti-diuretic hormone (ADH)

63
Q

where is ADH made and stored?

A

made in the hypothalamus

stored in the posterior lobe of the pituitary gland

64
Q

what is a diuretic?

A

a substance that makes you urinate more frequently eg caffeine and alcohol

65
Q

where are the osmoreceptors situated?

A

in the hypothalamus

66
Q

Describe the action of osmoregulation in the nephron in the case of dehydration

A
  1. If osmoreceptors in the hypothalamus detect that the blood is too concentrated/hypertonic/has very negative solute potential then more ADH will be released into the bloodstream.
  2. ADH travels through the bloodstream to the kidneys. In the nephron, the cells lining the collecting ducts and the DCT are sensitive to ADH and in response become more permeable to water. This is due to the insertion of more aquaporins into the cell membranes.
  3. As the cells are now more permeable to water, more water is reabsorbed into the blood, and less continues down the collecting ducts and into the urine.
  4. As a result, a smaller volume of more concentrated urine is formed. More water is retained in the blood and the solute potential starts to increase back towards the set point.
  5. This is detected by the osmoreceptors and the amount of ADH is reduced as a result of negative feedback.

A2 Biology 1 (5 decks)

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