Renal system; physiology and function

Question 1

State the functions of the kidney.

Answer 1

• Homeostatic regulation of water and ion content of blood; regulation of osmolarity - maintenance of ion balance, regulation of extracellular fluid volume - blood pressure, regulation of pH
• Excretion of metabolic waste products e.g. urea, creatinine
• Production of hormones; renin - sodium balance, activation of vitamin D - Ca2+ balance and bone, prostaglandins and kinins - renal blood flow

Question 2

Describe the structure of the kidney.

Answer 2

• Interior contains two layers; an outer cortex and an inner medulla
• Layers formed by the organised arrangement of tubules called nephrons

Question 3

What is the nephron?

Answer 3

It’s the functional unit of the kidney.

Question 4

Describe the structure and function of the nephron.

Answer 4

• Bowman’s capsule contains a network of capillaries called the glomerulus; where plasma fluid is ultra-filtrated
• Fluid then flows into the proximal tubule then into the loop of Henle
• Then into the medulla (descending limb) and then back up (ascending limb)
• The fluid then passes into the distal tubule
• Distal tubules drain into the collecting duct which pass from the cortex into the medulla
• Collection ducts drain into the renal pelvis
• Urine flows in the ureter on its way to excretion

Question 5

Describe the vascular supplies to the nephron.

Answer 5

• Blood flows from the afferent arteriole into the glomerulus and leaves into an efferent arteriole and a second set of capillaries known as peritubular capillaries that surround the tubule
• Renal capillaries join to form venules, conducting blood out of the kidney

Question 6

What is the function of the renal portal system?

Answer 6

• To filter fluid out of the blood and into the lumen of the nephron at the glomerulus
• Fluids are then reabsorbed from the tubule back into the blood at the peritubular capillaries

Question 7

State the 4 processes of the nephron.

Answer 7

1. Filtration
2. Reabsorption
3. Secretion
4. Excretion

Question 8

Describe filtration.

Answer 8

The movement of fluid from blood into the lumen of the nephron - takes place in the Bowman’s capsule

Question 9

Describe reabsorption.

Answer 9

The movement of the filtrate from the lumen of the tubule back into the blood, through the peritubular capillaries.

Question 10

Describe secretion.

Answer 10

Removes selected molecules from the blood and adds them to the filtrate in the tubule lumen

Question 11

Describe excretion.

Answer 11

Anything filtered from the glomerulus is destined for excretion, unless it is reabsorbed.

Question 12

What does amount of excretion equal?

Answer 12

amount filtered - amount reabsorbed + amount secreted

Question 13

How to calculate filtration fraction?

Answer 13

Glomerular filtration rate/plasma flow rate

Question 14

What is meant by clearance as a marker of kidney functionality?

Answer 14

A volume of plasma from which a substance has been removed and excreted into urine per unit time.

Question 15

How can we calculate clearance?

Answer 15

Urine concentration x Urine flow rate / plasma concentration

Question 16

What are the three filtration barriers involved in glomerular filtration?

Answer 16

1. Capillary endothelium
2. Basal lamina
3. Podocytes

Question 17

Describe the capillary endothelium.

Answer 17

• Has pores
• Pores are small enough to prevent blood cells from leaving the capillary
• On the surface are negatively charged proteins which will repulse negatively charged plasma proteins

Question 18

Describe the basal lamina.

Answer 18

• A layer of extracellular matrix that separate the capillary endothelium from the epithelium of Bowman’s capsule
• Layer is negatively charged - excluding most plasma proteins from filtration

Question 19

Describe podocytes.

Answer 19

• Present in Bowman’s capsule with long cytoplasmic extension called foot processes that interdigitate the basal membrane leaving narrow filtration slits.
• Proteins will be excluded from filtration according to charge and molecular weight

Question 20

State the four structures involved in reabsorption.

Answer 20

1. Proximal tubule
2. Loop of Henle
3. Distal tubule
4. Collecting tubule

Question 21

Describe the mechanism of glucose and amino acid reabsorption at proximal tubule.

Answer 21

• Recovery of glucose/ AA dependant on Na+/K+ pump
• Basolateral membrane
• Maintains inwardly directed electrochemical gradient for Na+
• Na+ coupled transport systems in luminal membrane enable recovery of glucose and amino acids
• ~65% of filtered Na+ is recovered from tubular fluid
• Creates osmotic gradient and allows recovery of ~65% of water

Question 22

Summarise the substances reabsorbed and secreted in the proximal convoluted tubule.

Answer 22

Reabsorption: ~65% Na+, H2O, K+, Cl-
~100% Glucose, amino acids
~50% urea, urate
~80% HCO3

Secretion of organic anions and cations (H+) (K+)

Question 23

Describe important features of the DESCENDING LIMB of the loop of Henle and why this is important in reabsorption.

Answer 23

• The epithelium is water permeable but impermeable to NaCl
• No active transport
  The means osmolarity will increase as the descending limb progress, reaching a maximum at the loop

Question 24

Describe the ascending limb of the loop of Henle.

Answer 24

• Water impermeable
• Thin ascending limb reabsorbs NaCl passively
• Thick ascending limb moves Na+, K+ and Cl- out of tubule fluid
• Active transport powered by Na+/K+ pump
• Osmolarity decrease as the ascending limb progress reaching a minimum at the top

Question 25

Describe how the loop of Henle reduces osmolarity in the tubule (diluting urine)

Answer 25

1. Isosmotic fluid leaving the proximal tubule becomes more concentrated in the DESCENDING limb
2. Removal of solute in the thick ascending limb create hypo osmotic fluid
3. Permeability to water and solute in the distal tube and collecting duct is regulated by hormones
4. Final urine osmolarity depends on reabsorption in the collecting duct

Question 26

Summarise the substances reabsorbed and secreted in the loop of henle.

Answer 26

Reabsorption: ~25% Na+, K+, Cl-
~15% H2O, urate
HCO3-
Secretion of (H+)

Question 27

Describe the function of the distal tubule in reabsorption.

Answer 27

• NaCl is reabsorbed due to action of Na+/Cl- co-transporter (and Na+/K+ pump)
• Water impermeable
• Osmolarity falls further

Question 28

Describe the effects of diuretics on distal tubule.

Answer 28

Inhibit transport system in distal tubule

Question 29

Describe the function of the collecting duct.

Answer 29

Recovers more Na+ and water via ion channel: epithelial Na+ channel (ENaC) and water channels (aquaporins)

Question 30

Describe the function of aldosterone in controlling sodium balance.

Answer 30

• If Na+ is reduced aldosterone is secreted by the adrenal cortex and increases Na+ recovery via ENaC
• More Na+ is reabsorbed due to insertion of ENaC on the membrane
• Makes urine more concentrated

Question 31

Describe the effect of antidiuretic hormone in controlling the amount of water in the body.

Answer 31

Increases the water reabsorption due to the generation of the water channel; aquaporin
- Makes urine more concentrated

Question 32

Why are cells in the last segment of distal tubule and collecting duct important?

Answer 32

• Cells reabsorb K+ and secret H+ through K+/H+ anti porter.
• Important to regulate pH

Question 33

Summarise the reabsorption and secretion of substances in the distal tubule and collecting duct.

Answer 33

Reabsorption of ~8% NaCl, K+, Cl-
~8-17% H2O, HCO3-

Secretion of H+, K+