Kidney Function: Filtration, Absorption and Secretion

Question 1

What are the two main functions of the kidney?

Answer 1

Excretion of metabolites:
-urea from proteins catabolism
uric acid from nucleic acid breakdown
- creatinine from muscle creatine
- end products of hormone breakdown
Control of body fluid composition:
- volume regulation
- osmoregulation
- pH regulation

Question 2

What hormones act on the kidney?

Answer 2

ADH, aldosterone, natriuretic peptides, parathyroid hormon and fibroglast growth hormone 23.

Question 3

What hormones does the kidney produce?

Answer 3

Renin, vitamin D3 (calciferol), erythropeotin, prostaglandins, alpha-klotho

Question 4

Outline the gross structure of the kidney.

Answer 4

Kidney sits between T12 and L3.

Two distinct tissue regions: medulla and cortex.

Question 5

What is a nephron?

Answer 5

Micro-structure of the kidney.

Single unit component of the kidney filtration system.

Question 6

Name the structures that make up the nephron.

Answer 6

Renal corpuscle 
Proximal convoluted tubule
Proximal straight tubule
Descending thin limb of Henle
Ascending thin limb of Henle
Thick ascending limb of Henle
Distal convoluted tubule
Cortical collecting duct
Medullary collecting duct

Question 7

Describe the structure and function of the renal corpuscle.

Answer 7

Made up of the glomerus (compact network of arteries) and the Bowman’s capsule.
Blood enters the glomerulus via the afferent arteriole.
Blood exits the glomerulus via the efferent arteriole.
Ultrafiltration occurs at the renal corpuscle from blood into the Bowman’s capsule.

Question 8

How does ultrafiltration occur at the renal corpuscle?

Answer 8

Afferent arteriole has a wider lumen than the efferent arteriole so pressure increases as it passes through the glomerulus - forced fluid into the Bowman’s capsule.
Endothelium, basal lamina and tubular epithelium act as filters for fluid.
Podocytes have long processes which act as filtration slits.

Question 9

Name the 2 different types of nephron and where they are found and their function.

Answer 9

Cortical (85%):
 - outer 2/3 of cortex. 
 -Short loop of Henle.
Juxtamedullary (15%):
 - inner 1/3 of cortex.
 - long loop of Henle
 - Produces concentrated urine

Question 10

Name 3 main cells in the juxtaglomerular apparatus and their function.

Answer 10

Extraglomerular mesangial cells - type of smooth muscle cell. Autoregulation of blood flow to the kidney.
Juxtaglomerular cells - synthesise, store and secrete renin.
Macula densa - specialised epithelial cells of the distal convoluted tubule which sodium concentration and fluid in the tubule.

Question 11

Where are the peri-tubular capilaries?

Answer 11

Around the Loop of Henle - called the vasa recta

Question 12

What percentage of plasma is filtered into the Bowman’s capsule during glomerular filration?

Answer 12

20%

Question 13

What is tubular secretion?

Answer 13

Secretion of solutes from the peritubular capillaries into the tubules.

Question 14

What is tubular reabsorption?

Answer 14

Movement of material from the filtrate in tubules into the peritubular capillaries.

Question 15

What substances are filtered/secreted/reabsorbed by the kidney?

Answer 15

Glucose - filtered into the nephron but completely reabsorbed. No glucose in the urine.
Urea is excreted.

Question 16

How is the glomerular filtration rate used?

Answer 16

Good indicator of kidney function.

Estimates how much blood passes through the gomeruli each minute.

Question 17

How is the glomerular filtration rate regulated by the body?

Answer 17

Neuronal and hormonal input.
Constriction of the afferent arteriole reduces hydrostatic pressure and reduces GFR.
Glomerular mesangial cells can alter surface area for filtration by constricting to reduce surface area - under control of sympathetic nervous system.

Question 18

What 3 components does the glomerular filtration rate depend on?

Answer 18

1. Starling forces - hydrostatic pressure and colloid osmotic/oncotic pressure.
2. Surface area of filtration interface.
3. Hydraulic permeability of capillaries.

Question 19

What is a normal Glomerular Filtration Rate (GFR)?

Answer 19

125ml/min of filtrate formed

Question 20

Where does reabsorption mainly occur?

Answer 20

Proximal tubule

convoluted and straight

Question 21

What adaptations does the proximal tubule have to allow reabsorption to occur?

Answer 21

Walls are single layer of collumnar cells - numerous microvilli to increase surface area.
Many mitochondria in epithelial cells to generate ATP for primary and decondary active transport.

Question 22

What substances are reabsorbed in the proximal tubule?

Answer 22

Glucose
Amino acids
Phosphates/sulphates
Urea
Cl-, K+, Ca2+

Question 23

How does reabsorption occur in the proximal tubule?

Answer 23

• Na+-coupler transporters for glucose, phosphates, sulphates, amino acids,
• Passive reabsorption of urea, Cl-, K+, Ca2+

Question 24

How does organic anion secretion occur in the proximal tubule?

Answer 24

1. Organic anion (OA-) enters epithelial cell in exchange for dicarboxylate (DC-)
2. DC- accumulate in cells by metabolism and Na+ coupled co-transporter.
3. OA- enters tubule lumen via ATP-dependent transporters.

Question 25

How does organic cation secretion occur in the proximal tubule?

Answer 25

1. Organic cation enters cell via facilitated organic cation transporters (OCT1 and OCT2)
2. OC+ enters tubule lumen via multi drug and toxin extrusion proteins (MATEs) antiporter in exchange for H+.