PHYSIOLOGY - Renal Function (Filtration)

Question 1

What are the three main functions of the nephron?

Answer 1

Filtration, reabsorption and secretion

Question 2

How much of the glomerular filtrate is made up of plasma?

Answer 2

20% of the glomerular filtrate is made up of plasma

Question 3

What are the two types of nephron?

Answer 3

Cortical nephron
Juxtamedullary nephron

Question 4

Which type of nephron is most important in determining the concentration of urine?

Answer 4

Juxtameduallry nephron is the most important for determining urine concentration

Question 5

Where does filtration occur within the nephron?

Answer 5

Filtration occurs within the glomerulus within Bowman’s capsule (renal corpuscle)

Question 6

During glomerular filtration, which substances are filtered into the filtrate?

Answer 6

Water
Glucose
Electrolytes
Low molecular weight proteins

Question 7

During glomerular filtration, which substances are retained in the circulation?

Answer 7

Water
Cellular components
Medium to high molecular weight proteins

Question 8

What drives filtration to occur?

Answer 8

Renal filtration is driven by high levels of hydrostatic pressure within the capillaries of the glomerulus

Question 9

Which factors determine the glomerular filtration rate?

Answer 9

Blood flow, Hydrostatic pressure and Osmotic forces (i.e. Starling’s forces)

Question 10

What are the four Starling’s forces which influence glomerular filtration rate (GFR)

Answer 10

Glomerular capillary hydrostatic pressure
Osmotic pressure of Bowman’s space
Hydrostatic pressure of Bowman’s space
Oncotic pressure

Question 11

Why is it important to regulate the glomerular filtration rate (GFR)?

Answer 11

To maintain the extra- and intracellular fluid composition as well as to maintain endogenous waste excretion

Question 12

How does vasoconstriction of the afferent arteriole affect the glomerular filtration rate (GFR)?

Answer 12

Vasoconstriction of the afferent arteriole reduces the blood flow into the glomerulus and thus reduces the hydrostatic pressure leading to a reduced glomerular filtration rate (GFR)

Question 13

How does vasoconstriction of the efferent arteriole affect the glomerular filtration rate (GFR)?

Answer 13

Vasoconstriction of the efferent arteriole decreases the blood flow out of the glomerulus thus increasing the hydrostatic pressure. These opposing effects lead to a relatively unchanged glomerular filtration rate (GFR)

Question 14

What is the purpose of autoregulation in the kidneys?

Answer 14

Autoregulation maintains the arteriole resistance of the afferent arterioles to stabilise the glomerular filtration rate (GFR) in response to acute changes in blood pressure

Question 15

What are the two components of autoregulation in the kidneys?

Answer 15

Myogenic reflex
Tubuloglomerular feedback

Question 16

How does the myogenic reflex maintain the homeostatic glomerular filtration rate in response to an acute increase in blood pressure?

Answer 16

An acute increase in blood pressure is detected by stretch receptors in the smooth muscle of the afferent arterioles which stimulates a reflex contraction of the afferent arterioles, reducing both the blood flow and the hydrostatic pressure to stabilise the glomerular filtration fate

Question 17

How does the myogenic reflex maintain the homeostatic glomerular filtration rate in response to an acute decrease in blood pressure?

Answer 17

An acute decrease in blood pressure is detected by stretch receptors in the smooth muscle of the afferent arterioles which stimulates a reflex dilation of the afferent arterioles, increasing both the blood flow rate and the hydrostatic pressure to stabilise the glomerular filtration rate

Question 18

Describe how tubuloglomerular feedback maintains the homeostatic glomerular filtration rate (GFR) in response to an acute decrease in blood pressure (remember it is the opposite for increased blood pressure)

Answer 18

1. Decreased blood pressure leads to a decreased glomerular filtration rate (GFR) and thus a decreased volume of filtrate passing through the tubules
2. A decreased volume of filtrate and thus a decrease in NaCl concentration in the tubule stimulates the macula densa cells to secrete paracrine factors
3. These paracrine factors cause dilation of the afferent arteriole to increase blood flow into the glomerulus and thus increase hydrostatic pressure, stabilising the glomerular filtration rate (GFR)

Question 19

Which mechanism maintains a homeostatic glomerular filtration rate (GFR) in response to a chronic systemic change in blood pressure?

Answer 19

Renin-angiotensin-aldosterone mechanism

Question 20

Why is autoregulation not sufficient enough to maintain a chronic systemic change in blood pressure?

Answer 20

Because autoregulation only regulates the afferent arteriole whereas a chronic systemic change in blood pressure effects both the afferent and efferent arteriole

Question 21

Describe how the renin-angiotensin-aldosterone mechanism maintains the homeostatic glomerular filtration rate (GFR) in response to an acute decrease in blood pressure (remember it is the opposite for increased blood pressure)

Answer 21

1. Decreased blood pressure leads to a decreased glomerular filtration rate (GFR) and thus a decreased volume of filtrate passing through the tubule
2. A decreased volume of filtrate and thus a decrease in NaCl concentration in the tubule stimulates renin secretion from the juxtameduallry cells
3. Renin coverts angiotensin produced by the liver into angiotensin I
4. Angiotensin I travels to the lungs where angiotensin converting enzyme (ACE) coverts it into angiotensin II
5. Angiotensin II acts on the kidneys to stimulate vasoconstriction of the efferent arteriole which will increase the hydrostatic pressure in the glomerulus to counteract the low blood flow due to the decreased blood pressure

Question 22

Why is it important to estimate the glomerular filtration rate (GFR)?

Answer 22

The glomerular filtration rate (GFR) is an important clinical indicator of the extent and progression of renal disease

Question 23

What is the calculation used to estimate the glomerular filtration rate?

Answer 23

GFR = urinary excretion of a substance per minute / plasma concentration of a substance

Question 24

Why is it essential to use a urinary excreted substance which is only filtered and neither reabsorbed or secreted to estimate the glomerular filtration rate (GFR)?

Answer 24

Because using a substance that is reabsorbed and/or secreted will lead to an under- or overestimation of the glomerular filtration rate (GFR)

Question 25

Which urinary excreted substance is ideal for estimating the glomerular filtration rate (GFR)?

Answer 25

Inulin

Question 26

Creatinine is another urinary excreted substance which is used to estimate the glomerular filtration rate (GFR). What is the disadvantage of using this substance?

Answer 26

Creatinine is secreted into the urine leading to an overestimation of the glomerular filtration rate (GFR)

Question 27

What is clearance in regards to renal function?

Answer 27

Renal clearance of a substance refers to how quickly a particular substance is removed from the plasma by the kidneys and excreted in urine

Question 28

What is the equation that is used to estimate the rate of renal clearance?

Answer 28

Clearance of X = (urinary concentration of X) x (volume of urine per min) / plasma concentration of X

X = chosen substance