Module 11: Renal system (Part 2)

Question 1

What does the loop of Henle consist of?

Answer 1

• The descending section that extends deep into the medulla of the kidney
• An ascending section that loops back into the cortex of the kidney

Question 2

Describe the changes in the concentration of the interstitial fluid from the cortex to the medulla

Answer 2

• near the cortex, the concentration of the interstitial fluid is 300mOsm/kg water
• as the nephron descends into the medulla, the concentration increases to 1200mOsm/kg water

Question 3

describe the reabsorption of ions and water in the descending loop of Henle, and explain the filtrate concentration change

Answer 3

the descending loop of Henle is permeable to water and not permeable to other ions
- with this and the large concentration gradient water will move out via osmosis to the interstitial space
- very few ions move out of the lumen down the concentration gradient
- this results in loss of water from the filtrate increasing the filtrate concentration to 1200mOsm/kg water by the time it reaches the ascending limb

Question 4

describe the reabsorption of ions in the ascending loop of Henle, and explain the filtrate concentration change

Answer 4

the ascending loop of Henle is not permeable to water, so no water is reabsorbed here. its permeable to Na+, K+, and Cl- ions.
- 25% of all Na+, K+, and Cl- is reabsorbed here
these ions are transported out of the filtrate by the Na+/K+ pumps in the tubule cells
- the pumps create the concentration gradient for Na+ from the filtrate to the tubule cells
- the gradient drives a special co transporter that carries K+, Cl -, and Na+ into the tubule simultaneously
- due to the activity of the pump and co transporter, K+ concentration in the tubule cells increases dramatically. some will be secreted back into the filtrate by diffusion through leaky channels
- Na+ is also reabsorbed by the Na+/H+ exchanger, reabsorbing Na+ while secreting H+
- Filtrate concentration changes from 1200mOsm/kg water to 100mOsm/kg water

Question 5

describe the reabsorption of water and Na+ in the distal convoluted tubule

Answer 5

Na+ reabsorption is regulated by aldosterone, aldosterone…
- 12% of Na+ is reabsorbed here
1. increases activity of the Na+/K+ pump on the basal side, decreases Na+ concentration in the cell
2. increases more Na+ channels on the luminal side, more Na+ diffuses into the cells down concentration gradient
reabsorption of water here is controlled by the antidiuretic hormone (ADH/vasopressin)
- depends on the level of hydration of the individual
amount absorbed ranges from 0-15%

Question 6

describe the secretion of K+ in the distal convoluted tubule

Answer 6

K+ is secreted into the lumen in response to aldosterone, aldosterone…
- increases the activity of the Na+/K+ pump, increases K+ concentration in the cell
- increases the number of Na+ channels, also increases the number of K+ channels on the luminal side
- K+ secretes back into the lumen

Question 7

describe the reabsorption of Na+ and water in the collecting duct

Answer 7

• only 10% of Na+ and water is reabsorbed here, its always under control of hormones
• Na+ reabsorption is controlled by aldosterone
• Water reabsorption depends on the presence of antidiuretic hormone (ADH)
• increased aldosterone or ADH will increase Na+ or water reabsorption respectively

Question 8

describe the secretion of K+ in the collecting duct

Answer 8

K+ secretion also takes place here due to the presence of aldosterone
- same happens as distal convoluted tubule
- increased Na+/K+ pump activity on the basal side
- more K+ channels on the luminal side
- K+ will leak out down its concentration gradient into the lumen of the collecting duct

Question 9

List all reabsorption & secretion along nephron

Answer 9

proximal tubule:
- reabsorption of glucose, amino acids, and ions including: Na+, K+, Cl-, and water
loop of henle:
- descending: permeable to water, water moves into interstitial fluid via osmosis
- water reabsorption and some ions
- ascending: permeable to ions, reabsorption of K+, Cl-, Na+
- secretion of K+ and H+
distal tubule:
- water reabsorbed via ADH
- Na+ reabsorbed due to aldosterone
- K+ secretion by aldosterone
collecting duct:
- Na+ reabsorption via aldosterone
- Water reabsorption via ADH
- K+ secretion via aldosterone

Question 10

What organ’s main function is water balance regulation, what controls it and where is it produced/released?

Answer 10

the kidney’s main function is to regulate the amount of water in the body
- water loss must be balanced by the water gained
- regulated by the anti diuretic hormone (ADH/vasopressin)
- ADH is produced in the hypothalamus and released from the posterior pituitary
- relies on negative feedback system

Question 11

what are osmoreceptors and where are they located?

Answer 11

osmoreceptors are sensors to water
- located in the hypothalamus and respond to changes in body fluid concentrations
- recall, dehydration/over hydration of a cell can cause changes in fluid concentrations

Question 12

What happens during dehydration?

Answer 12

will concentrate the body fluids (hypertonic solution) causing osmoreceptors to lose water and shrink
- shrinking osmoreceptors signal the posterior pituitary to release more ADH
- ADH will cause kidneys to reabsorb more water from the distal tubule and collecting ducts

Question 13

What happens during over hydration?

Answer 13

will dilute the body fluids (hypotonic solution) causing the osmoreceptors to swell
- less ADH will be released, less water reabsorbed in the distal tubule and collecting duct
- more water excreted in the urine

Question 14

What do dehydration and over hydration do to blood volume and pressure?

Answer 14

dehydration - results in a lower blood volume and pressure
over hydration - results in an increase in blood volume and pressure

Question 15

What are volume receptors and baroreceptors? Where are they located? what can they control?

Answer 15

• changes in blood volume can be detected by volume receptors in the left atrium
• changes in blood pressure can be detected by baroreceptors in the aortic arch and carotid sinuses
• these receptors can also control the release of ADH, low blood volume causes a release of ADH, high blood volume decreases ADH release

Question 16

ADH effect on cells

Answer 16

ADH causes water reabsorption by the distal convoluted tubule and the collecting ducts
- ADH stimulates these regions to manufacture special water channels called aquaporins which insert into the luminal membrane
- due to the concentration gradient (low solutes outside, high solutes inside) more water will be reabsorbed by osmosis, less excreted in the urine

Question 17

Explain what happens in each scenario for water balance:
1. increased plasma osmolarity
2. decreased blood volume
3. decreased blood pressure

Answer 17

1. increased plasma osmolarity
   - detected by osmoreceptors in hypothalamus
   - release of ADH from posterior pituitary
   - reabsorption of water in kidney
   - osmolarity and blood volume returns to normal
2. decreased blood volume
   - detected by volume receptor in atria
   - rest is the same as + plasma osmolarity
3. decreased blood pressure
   - detected by baroreceptors in carotid sinuses and aortic arch
   - increase in cardiac output and total peripheral resistance
   - blood pressure returns to normal

Question 18

What does alcohol do to ADH?

Answer 18

it inhibits the release of ADH from the pituitary gland
- less water reabsorbed, more water excreted in the urine

Question 19

How are sodium levels regulated?

Answer 19

sodium levels are regulated by the renin-angiotensin system (RAS) and by aldosterone
- they are combined and called the renin-angiotensin-aldosterone system (RAAS)

Question 20

What is aldosterone, what does it do?

Answer 20

a steroid hormone produced by the adrenal glands in response to…
- low blood Na+ or high blood K+ levels
- angiotensin II
- adrenal corticotropic hormone (ACTH)
returns Na+ and K+ concentration to normal by causing reabsorption of Na+ and secretion of K+ in distal convoluted tubule and collecting ducts

Question 21

What does the renin angiotensin system (RAS) do?

Answer 21

the RAS system regulates Na+ balance in the body by increasing the reabsorption of Na+ in the proximal convoluted tubule and ascending loop of Henle
- involves series of chemical reactions converting angiotensinogen to angiotensin II

Question 22

Describe the development of the RAS system

Answer 22

1. inactive protein angiotensinogen (produced in the liver) reacts with renin
   - renin produced by juxtaglomerular cells located in the wall of afferent/efferent arterioles
   - renin released when blood pressure or plasma Na+ levels are low
2. Renin converts angiotensinogen to angiotensin I
3. Angiotensin I is then converted to the active hormone angiotensin II by angiotensin converting enzyme (ACE, produced in the lungs)

Question 23

What does angiotensin II do? What causes its production?

Answer 23

increases reabsorption of Na+ in the proximal convoluted tubule and ascending loop of Henle
- produced when Na+ levels or blood pressure is low
- increases activity of the Na+/H+ exchanger, reabsorbing more Na+ and secreting more H+
- also stimulates secretion of aldosterone

Question 24

Angiotensin II’s effects on the body in response to low Na+ levels or low blood pressure

Answer 24

1. one of the most potent vasoconstrictors in the body, vessels will be constricted causing an increase in total peripheral resistance which will increase blood pressure
2. stimulates sensation of “thirst”, drinking will increase blood volume which increases blood pressure
3. stimulates release of ADH, also causes the absorption of water and sodium from the digestive tract