Basic Kidney Physiology

Question 1

What are the basic homeostatic principles of water and electrolyte homeostasis?

Answer 1

• Intake/loss must be in balance (over any significant period)
• Failures result in consequences such as : haemorhagge, severe dehydration

Question 2

What are the typical water intakes and outputs for an adult male doing normal daily activities at 23 C ?

Answer 2

Intake = water around 1200ml, food around 1000ml, metabolic around 300ml
Total- around 2.5 L/day

Output = urine around 1500ml, sweat around 100ml, faeces around 200ml, insensible loss around 700ml
Total - around 2.5L/day

Question 3

What does water intake and output depend on?

Answer 3

• Intake depends on thirst, appetite, availability of food, water
• Much of output depends on temp. , humidity, activity

Question 4

What will happen to water output during heavy exercise in hot/humid conditions?

Answer 4

Sweat loss can be more than 2L/hour and more than 10L/day.

Urine output may be reduced may be reduced in these conditions.

Question 5

What is the typical intakes/output of salt?

Answer 5

• This is hard to measure in western diets

- UK recommendations is adults should eat no more than 6g of salt/day although many exceed this.

Question 6

Is renal blood flow big or small and why?

Answer 6

Renal blood flow is huge as it metabolises kidney as well as being filtered.

Question 7

What is the blood pressure in the glomerular capillaries like?

Answer 7

Is high pressure : 50-60mmHg

Question 8

What is the sequence of blood vessels for kidney function?

Answer 8

Afferent arteriole –> glomerular caillaries –> efferent arterioles –> tubular capillaries –> venule.

Question 9

What are the 2 types of nephron?

Answer 9

• Superficial

- Juxta-medullary (water reabsorption more effective in this as they are longer).

Question 10

What are the 4 sections of the nephron?

Answer 10

• PCT
• Loop of Henle
• DCT
• Collecting duct

Question 11

Deescribe ultrafiltration in the glomerulus of kidney nephron?

Answer 11

• Driven by high blood pressure in glomerular capillaries.
• Filtration of small molecules and water through slits between podocytes.
• Filtration barrier are podocytes which give off processes which interdigitate with each other. Limit is space between podocyte processes.
• Normal glomerular filtration rate : 90-140ml/min

Question 12

What is the PCT lined with to help function?

Answer 12

• Lined with simple cuboidal epithelial cells with brush border to increase SA on apical side.
• Helps with reabsorption of Na and K ions etc.

Question 13

Describe active reabsorption in PCT?

Answer 13

-Occurs by active pumping through co-transporters, aqueos channels, membrane pumps.
-There is also substantial water reabsorption.
-By end of PCT, there has been complete reabsorption of glucose, amino acids.
Substantial reabsorption of Na and water.
-Volume of filtrate reduced by 2/3rds.

Question 14

What are the thickness of walls like in the loop of henle?

Answer 14

• Thinner wall during descent into the medulla

- Thicker wall during ascent from medulla..

Question 15

Describe conter current concentration process in loop of henle?

Answer 15

• Thicker walls in ascending limb pump solutes out of flitrate and they can diffuse back to start of loop. They are actively pumped out of the tubule.
• Solute diffuses into start, counter current mechanism recycles solutes.
• Ion pumping develops high osmotic pressure at the tip of loop.
• No net reabsorption here.

Question 16

What is the structure and function of the DCT like?

Answer 16

Similar structure and function to PCT, no brush border here.

Question 17

Describe more solute reabsorption and secretion in DCT?

Answer 17

• No need for glucose transporters as they were reabsorbed in PCT.
• Less intense electrolytes and water reabsorption.
• DCT ion pumping can be controlled by hormones like aldosterone to fine tune NA and K exchange.

Question 18

Describe the concentration of urine that occurs in collecting ducts?

Answer 18

• CD’s pass close to tips of loop of henle.
• If CD’s are permeable to water, then moves out of duct to concentrate filtrate.
• Duct permeability set by hormone ADH/AVP.

Question 19

What does ADH/AVP do to increase water permeability?

Answer 19

Inserts aquaporins into the luminal membrane to allow water movement.
Rpaid insertion/removal allows quick responses.

Question 20

How is salt balance controlled by hormones?

Answer 20

-Aldosterone increases when electrolyte conc. falls
-Aldosterone increases reabsorption of NA and CL ions from loop, distal tubule and duct cells.
Also increases K secretion.
-When electrolyte reabsorption increases then so does water reabsorption.

Question 21

How is flitration pressure - blood pressure controlled by hormones?

Answer 21

Hypofiltration initiates secretion of renin by the juxtaglomerular apparatus.
Renin splits angiotensin to make 1 which is converted to angiotensin II which is a powerful vasoconstrictor.
This system reguates renal blood flow and glomerular filtration rate
-Sympathetic nerves enhance this action

Question 22

Where is aldosterone secreted?

Answer 22

BY glomerulosa cells of the adrenal cortex.

Question 23

What is normal plasma osmolarity?

Answer 23

Around 300mOsm

Question 24

How is control of blood osmolarity-blood volume controlled by hormones?

Answer 24

• When water intake is restricted, plasma osmolarity increases.
• More ADH/AVP is secreted by hypothalamus
• This increases water permeability of CD’s.
• More water reabsorbed
• Conc. urine produced

Vice versa for higher water intake.

Question 25

What is max conc. of urine and max and min urine output?

Answer 25

Max conc. of urine = around 1200mOsm
Min urine output = around 1ml/min
Max urine output = around 20ml/min

Question 26

What si the half life of ADH?

Answer 26

Around 15 mins