Renal System

Question 1

Kidney Overview:

Answer 1

• Two kidneys on the posterior wall of the abdomen, outside the peritoneal cavity.
• Two ureters (about 25 cm) that transport urine from the kidneys to the bladder by peristalsis.
• The bladder stores urine (up to 700–800 mL) until micturition, which involves voluntary contraction of the bladder through the urethra

Question 2

Kidney Functions:

Answer 2

• Water and Electrolyte Balance: Regulates Na⁺, K⁺, Cl⁻, Ca²⁺, phosphate, and magnesium ions.
• Acid-Base Balance: Adjusts pH by excreting hydrogen ions and reabsorbing bicarbonate.
• Waste Excretion: Eliminates metabolic waste products (e.g., nitrogen) and foreign substances (e.g., drugs, pesticides).
• Hormone Secretion:
  1. Renin: Controls angiotensin formation.
  2. Erythropoietin: Stimulates RBC production.
  3. 1,25-dihydroxy Vitamin D3: Involved in calcium metabolism​

Question 3

Kidney Structure:

Answer 3

• Divided into cortex, medulla, and pelvis.
• Cortex: Outer layer containing glomeruli and convoluted tubules for filtration.
• Medulla: Inner region with the loop of Henle and collecting ducts.
• Pelvis: Funnel-shaped top part of the ureter where kidney tubules drain​

Question 4

Nephron Structure:

Answer 4

The nephron consists of vascular components (afferent and efferent arterioles, glomerulus, peritubular capillaries) and tubular components (proximal tubule, loop of Henle, distal tubule, collecting duct).

Filtration occurs in the glomerulus; tubular filtrate flows through renal tubules, where reabsorption and secretion occur​

Question 5

Nephron Filtration and Urine Formation:

Answer 5

Plasma volume (~3L) filters at a rate of 125 mL/min (GFR), totaling 180 L/day. Filtration, reabsorption, secretion, and excretion processes maintain homeostasis. Without tubular reabsorption, plasma and essential solutes would be lost quickly​.

Question 6

Reabsorption and Secretion in the Tubules:

Answer 6

Proximal Tubule: Reabsorbs 67% of Na⁺ and water, along with nutrients like glucose and amino acids.

Loop of Henle: Descending limb reabsorbs water; ascending limb is impermeable to water but reabsorbs Na⁺ and Cl⁻.

Distal Tubule & Collecting Duct: Fine-tunes Na⁺ and water reabsorption under hormonal control (aldosterone and ADH)​

Question 7

Hormones Regulating Tubular Reabsorption:

Answer 7

Antidiuretic Hormone (ADH): Increases water permeability in the collecting duct, reducing urine volume.

Aldosterone: Enhances Na⁺ reabsorption in the distal tubule, increasing blood volume and pressure​

Question 8

Tubular Secretion

Answer 8

Tubular secretion is reabsorption in reverse which is the movement of solutes from the peritubular capillaries into the tubules

Occurs almost completely in PCT (Proximal convoluted tubule)

Selected substances are moved from peritubular capillaries through tubule cells out into filtrate
– K+, H+, NH4+, creatinine, organic acids and bases
– Substances synthesized in tubule cells also are secreted (example:
HCO3–)

Question 9

Tubular secretion is important for:

Answer 9

– Disposing of substances, such as drugs or
metabolites, that are bound to plasma proteins
– Eliminating undesirable substances that were passively reabsorbed (example: urea and uric acid)
– Ridding body of excess K+ (aldosterone effect)
– Controlling blood pH by altering amounts of H+ or HCO3– in urine

Question 10

Glomerular Filtration Barrier:

Answer 10

Composed of:
- Fenestrated Endothelial Cells: Allow small molecules through but block larger ones.
- Basement Membrane: Negatively charged, repels proteins.
- Podocytes with Slit Diaphragms: Form the final filtration layer, allowing free passage of small solutes and blocking large molecules like proteins​

Question 11

Glomerular Filtration Rate

Answer 11

Normal GFR is ~125 mL/min.

GFR is determined by the net filtration pressure, membrane permeability, and surface area.

Creatinine clearance is a common clinical measure of GFR

Question 12

Forces Governing GFR:

Answer 12

Hydrostatic Pressure in Glomerular Capillaries: Favors filtration (approx. 55 mm Hg).

Plasma Oncotic Pressure: Opposes filtration.

Bowman’s Capsule Hydrostatic Pressure: Also opposes filtration.

Starlings Law: GFR depends on net filtration pressure and the permeability of the glomerular membrane​

Question 13

Factors Affecting GFR:

Answer 13

Severe Burns: Decreased plasma oncotic pressure increases GFR.

Urinary Tract Obstruction: Increases Bowman’s capsule pressure, reducing GFR.

Severe Dehydration: Increases plasma oncotic pressure, reducing GFR​

Question 14

Autoregulation of GFR:

Answer 14

Maintains GFR by controlling glomerular capillary pressure, despite fluctuations in blood pressure.

Myogenic Mechanism: Afferent arteriole constricts in response to increased blood pressure, preventing excessive GFR.

Tubuloglomerular Feedback: Macula densa cells sense NaCl levels; high NaCl leads to afferent arteriole constriction, reducing GFR, while low NaCl dilates the arteriole, increasing GFR​

Question 15

Renal Clearance:

Answer 15

Clearance rate (C) is calculated by:
​C=UV/P

where:
U = concentration in urine
V = urine flow rate
P = plasma concentration of the substance.

Inulin: Used as a precise measure of GFR; freely filtered, neither reabsorbed nor secreted.
Creatinine: Commonly used clinically to estimate GFR, though slightly less accurate than inulin​

Question 16

Loop of Henle and Countercurrent Mechanism:

Answer 16

Countercurrent Multiplier:
- Ascending Limb: Actively transports Na⁺ and Cl⁻ into the medulla, creating a hyperosmotic environment.
- Descending Limb: Permeable to water, which diffuses out to balance medullary osmolarity.

Result: The osmotic gradient in the medulla allows for the production of concentrated urine when necessary​

Question 17

Hormonal Control of Salt and Water Balance:

Answer 17

Vasopressin (ADH):
- Synthesized in the hypothalamus, stored in the posterior pituitary, released when plasma osmolarity is high.
- Inserts aquaporins into the collecting duct, increasing water reabsorption and concentrating urine.

Aldosterone:
- Secreted from the adrenal cortex in response to angiotensin II or high plasma K⁺ levels.
- Enhances Na⁺ reabsorption and K⁺ excretion in the distal tubule, contributing to increased blood volume and blood pressure

Question 18

Renin-Angiotensin-Aldosterone System (RAAS):

Answer 18

Activation:
Renin is released from juxtaglomerular cells in response to low blood pressure or low NaCl sensed by macula densa cells.
Renin converts angiotensinogen to angiotensin I, which is converted to angiotensin II in the lungs by ACE.

Effects of Angiotensin II:
Vasoconstriction: Increases systemic blood pressure.
Aldosterone Release: Promotes Na⁺ and water reabsorption.
ADH Secretion: Enhances water retention

Question 19

Factors Regulating Renin Release:

Answer 19

Renal Baroreceptors: Juxtaglomerular cells in afferent arterioles detect blood pressure; low pressure stimulates renin release.

Sympathetic Stimulation: β₁-adrenergic receptors increase renin secretion during sympathetic activation.

Macula Densa Feedback: Low NaCl concentration in distal tubules prompts renin release, triggering RAAS to increase Na⁺ reabsorption and blood pressure

Question 20

Tubuloglomerular Feedback Mechanism:

Answer 20

Macula Densa Cells: Specialized cells in the distal tubule that monitor NaCl concentration.

Feedback Process:
- High NaCl in tubular fluid signals the afferent arteriole to constrict, reducing GFR.
- Low NaCl promotes afferent arteriole dilation, increasing GFR to maintain sodium balance​

Question 21

Describe the vascular components of the nephron:

Answer 21

Afferent Arteriole: Supplies blood to the glomerulus.
Glomerulus: A network of capillaries where blood filtration occurs.
Efferent Arteriole: Drains blood from the glomerulus to the peritubular capillaries or vasa recta, which facilitate exchange with the tubular fluid

Question 22

Describe the tubular components of the nephron:

Answer 22

Proximal Tubule: Major site for reabsorption of water, ions, and nutrients.
Loop of Henle: Establishes an osmotic gradient in the medulla, allowing concentration of urine.
Distal Tubule: Involved in the selective reabsorption and secretion of ions, influenced by hormones.
Collecting Duct: Final site for water and electrolyte balance, regulated by ADH and aldosterone

Question 23

Explain the difference between tubular reabsorption and secretion:

Answer 23

Reabsorption: Movement of substances from the tubular lumen back into the blood (e.g., Na⁺, water).
Secretion: Transfer of waste and excess ions from the blood into the tubular fluid for excretion (e.g., H⁺, K⁺)

Question 24

Diagram to Show Sodium and Water Reabsorption in the Nephron:

Answer 24

In Image

Question 25

Explanation of Sodium and Water Reabsorption (Roles of Vasopressin and Aldosterone):

Answer 25

Proximal Tubule: Reabsorbs 67% of Na⁺ and water, along with nutrients like glucose and amino acids.

Loop of Henle: Descending limb reabsorbs water; ascending limb is impermeable to water but reabsorbs Na⁺ and Cl⁻.

Distal Tubule & Collecting Duct: Fine-tunes Na⁺ and water reabsorption under hormonal control (aldosterone and ADH)​

Vasopressin (ADH): Increases water reabsorption in the collecting duct by inserting aquaporins.

Aldosterone: Enhances Na⁺ reabsorption in the distal tubule, promoting water retention​

Question 26

Describe the Three Zones of the Kidney:

Answer 26

Cortex: Outer layer with glomeruli and convoluted tubules.

Medulla: Contains loops of Henle and collecting ducts.

Pelvis: Urine collection area that drains into the ureter​

Question 27

Diagram of Blood Flow into and out of the Kidney:

Answer 27

Blood flow: Renal artery → Segmental artery → Interlobar artery → Arcuate artery → Cortical radiate artery → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries/vasa recta → Renal vein

Question 28

Structure and Function of the Nephron:

Answer 28

Structure: Composed of the glomerulus (filtration) and renal tubules (reabsorption, secretion, and excretion).

Function: Filters blood, reabsorbs essential nutrients, secretes waste products, and concentrates urine​

Question 29

Labelled Diagram of the Tubular Components of the Nephron:

Answer 29

Proximal Tubule: Bulk reabsorption.
Loop of Henle: Osmotic gradient.
Distal Tubule: Hormone-regulated adjustments.
Collecting Duct: Final concentration of urine​

Question 30

What is the Glomerular Filtration Rate (GFR)? How is it Measured?

Answer 30

GFR: Volume of plasma filtered per minute (~125 mL/min).
Measurement: Estimated using substances like creatinine or inulin that are freely filtered and not reabsorbed or secreted.
GFR = UV/P
​(U = urine concentration, V = urine flow rate, P = plasma concentration)​

Question 31

Structure of the Glomerular Filtration Barrier:

Answer 31

Composed of endothelial cells (fenestrated), basement membrane (negatively charged), and podocytes (with slit diaphragms).
Role: Selective filter allowing water, ions, and small solutes but blocking large molecules like proteins​

Question 32

Forces Driving and Opposing Glomerular Filtration:

Answer 32

Driving Force: Glomerular capillary hydrostatic pressure (~55 mm Hg).
Opposing Forces: Plasma oncotic pressure and Bowman’s capsule hydrostatic pressure.
Net Filtration Pressure: Controls the rate of filtration

Question 33

Factors Controlling Glomerular Capillary Pressure:

Answer 33

Arterial Pressure: Directly affects blood flow into the glomerulus.
Afferent & Efferent Arteriole Resistance: Constriction or dilation of these arterioles modulates glomerular pressure​

Question 34

Changes in GFR in Severe Burns, Urinary Tract Obstruction, and Dehydrating Diarrhea:

Answer 34

Severe Burns: Decreased plasma oncotic pressure, increasing GFR.
Urinary Tract Obstruction: Raises Bowman’s capsule hydrostatic pressure, decreasing GFR.
Dehydrating Diarrhea: Increases plasma oncotic pressure, reducing GFR​

Question 35

Myogenic Mechanism (with Diagram):

Answer 35

Mechanism: Afferent arteriole constricts in response to increased blood pressure, stabilizing GFR by reducing glomerular capillary pressure.

Question 36

Tubuloglomerular Feedback Mechanism (with Diagram):

Answer 36

Mechanism: Macula densa cells detect increased NaCl, triggering afferent arteriole constriction to lower GFR; low NaCl has the opposite effect.

Question 37

Renin-Angiotensin-Aldosterone System (RAAS):

Answer 37

Activation: Triggered by low blood pressure or NaCl. Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II.
Effects of Angiotensin II: Increases blood pressure via vasoconstriction, stimulates aldosterone (enhancing Na⁺ reabsorption), and ADH release (increasing water reabsorption)​.

Question 38

Factors Stimulating Renin Release:

Answer 38

Renal Baroreceptors: Detect low afferent arteriole pressure.
Sympathetic Stimulation: Increases renin via β₁-receptors on juxtaglomerular cells.
Macula Densa: Low NaCl levels increase renin release to maintain blood pressure​.

Question 39

Juxtaglomerular Apparatus (with Diagram):

Answer 39

Components:
- Macula Densa: Senses NaCl.
- Juxtaglomerular Cells: Release renin.
- Extraglomerular Mesangial Cells: Facilitate signaling within the apparatus.

Question 40

Define Intrarenal Baroreceptors:

Answer 40

Definition: Juxtaglomerular cells in the afferent arteriole, acting as baroreceptors that detect blood pressure changes, modulating renin release​

Question 41

Countercurrent Multiplier System (with Diagram):

Answer 41

Mechanism: NaCl is actively transported out of the ascending limb of the loop of Henle, creating a hyperosmotic medullary interstitium that draws water from the descending limb. This mechanism concentrates urine and conserves water.