Urinary System and Kidneys

Question 1

Outline the primary mechanism for sodium reabsorption in the proximal tubule.

Answer 1

Co-transport, active transport alongside H+.

Question 2

Explain the relationship between sodium reabsorption and water reabsorption in the nephron.

Answer 2

Sodium reabsorption in the nephron creates an osmotic gradient across the tubular epithelium. Water follows sodium by osmosis.

Question 3

Describe the stimulus for erythropoietin (EPO) production by the kidneys and explain the hormone’s target and effect.

Answer 3

Stimulus: Low oxygen levels in the renal cortex trigger.
Target/Effect: Bone marrow to produce more red blood cells, improving oxygen delivery to tissues.

Question 4

Why is it important to use a substance that is neither reabsorbed nor secreted when measuring GFR?

Answer 4

Rate of filtration must equal rate of excretion.

Question 5

Describe two key adaptations of the proximal tubule that enhance its resorptive capacity.

Answer 5

• Microvilli brush border: significantly increases the surface area available for reabsorption.
• High [mitochondria] to provide the ATP needed for active transport processes.

Question 6

Describe the main actions of atrial natriuretic peptide (ANP) on the kidneys and adrenal cortex.

Answer 6

Kidneys: Increases GFR and decreases renin production.
Adrenal Cortex: Inhibits secretion of aldosterone, leading to increased sodium and water excretion.

Question 7

Explain the effect of increasing afferent arteriolar resistance on glomerular filtration rate.

Answer 7

Reduces glomerular blood flow and glomerular capillary hydrostatic pressure, leading to a decrease in glomerular filtration rate.

Question 8

How does molecular weight influence the rate at which a substance is filtered?

Answer 8

Rate of filtration is inversely proportional to molecular weight.

Large molecules filtered at a slower rate.

Question 9

Define glomerular filtration rate (GFR) and state its normal range in healthy humans.

Answer 9

Volume of fluid filtered from the glomerular capillaries into Bowman’s capsule per unit of time. In healthy humans, GFR typically ranges from 90-125 ml/min (across both kidneys).

Question 10

How are glucose and amino acids reabsorbed in the proximal tubule, and what happens when their transport maximum (Tm) is reached?

Answer 10

Co-transported with sodium ions across apical membrane, facilitated diffusion across basolateral membrane.
When transport maximum is reached, excess glucose and amino acids are excreted in urine.

Question 11

What are the main nitrogenous waste products excreted by the kidneys in urine?

Answer 11

• Urea (from protein metabolism)
• Creatinine (from muscle metabolism).

Question 12

What are the three components of the glomerular filtration barrier?

Answer 12

• Fenestrations in capillary epithelium.
• Basement membrane.
• Podocytes.

Question 13

Outline the steps involved in the conversion of angiotensinogen to angiotensin II. Include the enzymes involved.

Answer 13

• Renin is secreted by the juxtaglomerular cells.
• Renin acts as an enzyme to convert angiotensinogen (produced by the liver) into angiotensin I.
• Angiotensin I is then converted into angiotensin II by angiotensin-converting enzyme (ACE).
• ACE is found predominantly in the lungs.

Question 14

In what part of the nephron does aldosterone exert its primary effects, and what are the consequences of aldosterone action on potassium levels?

Answer 14

• Primarily acts on the distal convoluted tubule and the cortical portion of the collecting duct.
• Stimulates the secretion of potassium ions into the tubular lumen and the reabsorption of sodium ions, therefore regulating plasma potassium levels.

Question 15

Explain the difference between tubular reabsorption and tubular secretion.

Answer 15

Tubular Reabsorption: substances move from the tubular fluid (filtrate) back into the blood plasma.
Tubular Secretion: substances move from the blood plasma into the tubules, downstream from the glomerulus.

Question 16

Explain how the absence of ADH affects urine volume and osmolality.

Answer 16

Collecting ducts become largely impermeable to water (very few aquaporins) → significant decrease in water reabsorption → production of a large volume of dilute urine.

Question 17

Describe the ECG changes that may be observed in a patient with moderate hyperkalaemia.

Answer 17

• Loss of the P wave (it may be hidden within the QRS complex).
• Prolongation of the QRS complex.

Question 18

How does tubular reabsorption affect the renal clearance of a substance? Provide an example.

Answer 18

Tubular reabsorption decreases the renal clearance of a substance.
Example: Substance that is freely filtered at the glomerus but completely reabsorbed back into blood will have a clearance of zero (none in urine).

Question 19

What percentage of the glomerular filtrate is reabsorbed in the proximal tubule, and is this process under hormonal control?

Answer 19

60-70% of glomerular filtrate, process is not under hormonal control.

Question 20

Describe the path of blood flow through the nephron, beginning with the afferent arteriole and ending with the renal venules.

Answer 20

• Enters through the afferent arteriole leading to glomerular capillaries.
• Exits the glomerular capillaries via the efferent arteriole.
• Flows into peritubular capillaries or the vasa recta, before draining into the renal venules.

Question 21

Describe the two main pathways through which substances can move across the tubular epithelium.

Answer 21

Transcellular Pathway: through cells, substances must cross both apical and basolateral membranes.
Paracellular pathway: between cells, through gap junctions.

Question 22

Why is the regulation of extracellular fluid (ECF) osmolality and volume crucial for normal bodily function?

Answer 22

• Prevents water movement into or out of cells: ensures cellular function is not impaired. - Maintains adequate blood pressure and tissue perfusion.

Question 23

Explain how bicarbonate ions are indirectly reabsorbed in the proximal tubule.

Answer 23

In the tubular fluid, bicarbonate combines with secreted hydrogen ions to form carbonic acid, catalysed by apical carbonic anhydrase. Carbonic acid breaks down into CO₂ and H₂O, which diffuse into the cell. Inside the cell, they reform carbonic acid, which dissociates into bicarbonate and hydrogen ions. The bicarbonate is then reabsorbed into the blood.

Question 24

List the Starling forces that influence glomerular filtration and indicate whether each force promotes or opposes filtration.

Answer 24

• Glomerular capillary hydrostatic pressure (PGC): promotes filtration.
• Bowman’s capsule hydrostatic pressure (PT): opposes filtration
• Glomerular capillary colloid osmotic pressure (πGC): opposes filtration.
• Bowman’s capsule colloid osmotic pressure (πT): promotes filtration.

Question 25

Describe the movement of water in the descending and ascending limbs of the loop of Henle.

Answer 25

**Descending Limb:** Permeable to water. Water moves out of the tubular fluid, causing it to become more hypertonic. **Ascending Limb:** Impermeable to water, but actively transports sodium chloride out, causing the tubular fluid to become more dilute (hypotonic).

Question 26

What is the significance of the counter current exchange mechanism in the vasa recta?

Answer 26

Counter current exchange mechanism in the vasa recta = capillaries running parallel to the loops of Henle. **Significance:** helps to maintain the medullary osmotic gradient → allows for the exchange of solutes (like NaCl) and water, preventing the dissipation of the established gradient.

Question 27

Briefly describe the roles of the ureters, bladder, and urethra in the urinary system.

Answer 27

- Ureters transport urine from the kidneys to the bladder via peristalsis. - Bladder stores urine until ready for elimination. - Urethra carries urine from the bladder to the outside of the body.

Question 28

Describe the mechanism by which ADH influences water reabsorption in the collecting duct of the nephron.

Answer 28

ADH promotes the insertion of aquaporin channels into the apical membrane of the collecting duct cells → more water is reabsorbed back into blood.

Question 29

How does charge influence the rate at which a substance is filtered?

Answer 29

Filtration barrier has a negative charge. Positively charged molecules > Neutral molecules > negatively charged molecules.

Question 30

What are the key differences between cortical and juxtamedullary nephrons in terms of their location and function?

Answer 30

- Cortical nephrons have short loops of Henle that stay mostly in the cortex and primarily handle reabsorption and secretion. - Juxtamedullary nephrons have long loops that extend deep into the medulla and are essential for producing concentrated urine.

Question 31

What are the 3 primary stimuli that lead to the secretion of renin from the juxtaglomerular cells?

Answer 31

- Increased sympathetic activity - A decrease in sodium ions detected by the macula densa - Decreased stretch of the afferent arteriole due to low blood pressure or volume.

Question 32

What is the role of the juxtaglomerular apparatus, and which two types of cells are key components of it?

Answer 32

**Roles:** Regulates blood pressure and glomerular filtration rate. **Key Components:** - Macula densa cells (in the distal tubule): detect sodium levels. - Juxtaglomerular cells (in the afferent arteriole): release renin in response to low blood pressure or low sodium.

Question 33

What triggers the release of atrial natriuretic peptide (ANP) from the heart?

Answer 33

Increased blood volume → increased stretch of the atrial myocardial cells in the heart → release of atrial natriuretic peptide (ANP) into the circulation.

Question 34

Explain why inulin is not typically used in the clinical setting even though it is an ideal substance for GFR measurement.

Answer 34

Inulin is not produced in the body: it has to be injected IV which can be very inconvenient in practice. Inulin is also very expensive.

Question 35

How does aldosterone increase sodium reabsorption in the distal tubule and collecting duct?

Answer 35

It stimulates transcription of apical sodium channels and basolateral Na⁺/K⁺ ATPase, increasing Na⁺ reabsorption and K⁺ secretion.

Question 36

Explain the role of osmoreceptors in the hypothalamus when plasma osmolality increases.

Answer 36

They detect increases in plasma osmolality and stimulate both the **thirst centre**, leading to increased fluid intake, and the **posterior pituitary** to release ADH.