Renal System Lecture Review Flashcards

The Renal System

Introduction to the Kidney

• Presented by Professor Lam Sau Kuen, Dept of Pre-Clinical Sciences, MK FMHS.
• Date: August 29, 2025 (Friday).
• Course: Health Sciences, June 2025.

Anatomy of the Kidneys

Surface Markings

• Location: Between the vertebral levels of $ext{T10}$ and $ext{L4/L5}$.
• Right Kidney: Generally slightly lower than the left due to the liver.
• Left Kidney: Associated with the spleen.
• Reference Points:
  • $ext{T10}$, $ext{T11}$, $ext{T12}$, $ext{L4}$, $ext{L5}$.
  • Scapular line.
  • 12th rib (typically crosses the superior aspect of the kidney).
  • Crest of ilium.
  • Dimple indicating posterior superior iliac spine.

Location in Abdomen

• Superior to Kidneys: Adrenal glands (suprarenal glands).
• Right Kidney Relationships: Inferior to the liver.
• Left Kidney Relationships: Associated with the spleen, inferior to the diaphragm.
• Major Vessels:
  • Abdominal aorta: Gives rise to renal arteries.
  • Inferior vena cava: Receives renal veins.
  • Common iliac artery and vein (inferior to kidneys).
• Drainage: Ureters transport urine from kidneys to the urinary bladder.

Sectioned Kidney Structure

• Two Main Areas:
  • Cortex: The outer region.
  • Medulla: The inner region, composed of renal pyramids.
• Nephron: The functional unit of the kidney.
  • Humans have approximately $ext{1.2} imes ext{10}^6$ nephrons per kidney, totaling about $ext{2.4} imes ext{10}^6$ nephrons per person.
• Internal Architecture:
  • Renal Capsule: Outermost protective layer.
  • Renal Cortex: Extends between renal pyramids forming renal columns.
  • Renal Medulla: Contains renal pyramids with their bases facing the cortex and apices (renal papillae) pointing towards the renal pelvis.
  • Renal Papilla: The apex of a renal pyramid, where collecting ducts release urine.
  • Minor Calyx: Collects urine from a single renal papilla.
  • Major Calyx: Formed by the convergence of several minor calyces.
  • Renal Pelvis: A large funnel-shaped structure formed by the union of major calyces, continuous with the ureter.
  • Ureter: Tube carrying urine from the renal pelvis to the urinary bladder.

Main Functions of the Kidney

1. As an Excretory Organ

• Formation of Urine: The primary excretory product.
• Excretion of Urea: A major metabolic waste product from protein catabolism.
• Excretion of Other Metabolic Wastes: Includes creatinine, uric acid, bilirubin metabolites.
• Excretion of Exogenous Substances: Drugs, toxins, food additives, etc.

2. As a Regulatory Organ

• Control of Body Fluids:
  • Maintains osmotic pressure of body fluids.
  • Regulates volume of body fluids.
• Control of Electrolytes: Manages concentrations of various electrolytes (e.g., $ext{Na}^+, ext{K}^+, ext{Ca}^{2+}, ext{Cl}^-$) in body fluids.
• Long-Term Control of Blood Volume and Pressure: Through regulation of water and sodium balance.
• Control of Acid-Base Balance: Adjusts $ext{H}^+$ and $ext{HCO}_3^-$ excretion/reabsorption to maintain physiological pH.

3. As an Endocrine Organ

• Erythropoietin: Hormone stimulating red blood cell production in bone marrow.
• Calcitriol (1,25-dihydroxycholecalciferol or 1,25 Vitamin D3): The active form of Vitamin D, crucial for calcium and phosphate homeostasis.

4. As a Secretory Organ

• Renin: An enzyme that initiates the Renin-Angiotensin-Aldosterone System (RAAS), important for blood pressure regulation.
• Prostaglandins: Local hormones with various effects, including modulating renal blood flow and sodium excretion.
• Bradykinin and Other Kinins: Vasoactive peptides involved in vasodilation and increased vascular permeability.

The Nephron

Structure of a Nephron

• Glomerulus: A tuft of capillaries where filtration occurs.
• Bowman's Capsule (Glomerular Capsule): A cup-shaped structure surrounding the glomerulus, collecting the filtrate.
• Proximal Convoluted Tubule (PCT): Highly coiled segment immediately distal to Bowman's capsule, involved in extensive reabsorption and secretion.
• Loop of Henle: A U-shaped tubule extending into the medulla, crucial for establishing the medullary osmotic gradient.
  • Descending Limb (Thin DL): Permeable to water, impermeable to solutes.
  • Ascending Limb (Thin AL, Thick AL): Impermeable to water, actively transports solutes out.
• Distal Convoluted Tubule (DCT): Coiled segment after the Loop of Henle, involved in fine-tuning reabsorption and secretion, often under hormonal control.
• Collecting Duct (Not strictly part of nephron, but functionally associated): Receives filtrate from multiple DCTs, extends through the medulla, and plays a role in final urine concentration.

Types of Nephrons

There are two main types of nephrons:

i. Cortical Nephrons (approximately 85% of nephrons):
* Capsule Location: Located in the outer cortex.
* Loop of Henle: Short, extends only a short distance into the outer medulla.
* Peritubular Capillaries: Surround the tubules.
* Countercurrent Mechanism: Generally not involved in the countercurrent mechanism for concentrating urine.

ii. Juxtamedullary Nephrons (approximately 15% of nephrons):
* Capsule Location: Close to the junction between the cortex and medulla.
* Loop of Henle: Long, extends deep into the medulla, sometimes reaching the tip of the renal pyramid.
* Peritubular Capillaries + Vasa Recta: Possess specialized peritubular capillaries called vasa recta that run parallel to the long loops of Henle.
* Countercurrent Mechanism: Actively involved in the countercurrent mechanism for concentrating urine.
* Note: All glomeruli and capsules, regardless of nephron type, are located in the renal cortex.

Blood Supply to the Kidney

Overall Pathway

• Blood enters the kidney via the renal artery (a branch of the abdominal aorta).
• Within the kidney, blood is filtered in the glomeruli.
• Filtered blood then flows through peritubular capillaries and vasa recta, where reabsorption and secretion occur.
• Deoxygenated blood exits the kidney via the renal vein (draining into the inferior vena cava).

Detailed Intrarenal Blood Flow

1. Renal artery: Supplies blood to the kidney.
2. Segmental arteries: Branches of the renal artery within the renal sinus.
3. Interlobar arteries: Pass between renal pyramids (in renal columns).
4. Arcuate arteries: Arch over the bases of the renal pyramids, at the corticomedullary junction.
5. Interlobular arteries (Cortical Radiate Arteries): Branch off arcuate arteries and extend into the cortex.
6. Afferent arteriole: Branches from interlobular arteries, leading to the glomerulus.
7. Glomerulus (Capillary bed): Site of filtration.
8. Efferent arteriole: Carries blood away from the glomerulus.
9. Peritubular capillaries: Surround the proximal and distal tubules in the cortex.
10. Vasa recta: Specialized capillaries extending from efferent arterioles of juxtamedullary nephrons, running parallel to the Loops of Henle deep in the medulla.
11. Interlobular veins (Cortical Radiate Veins): Collect blood from peritubular capillaries and vasa recta.
12. Arcuate veins: Receive blood from interlobular veins.
13. Interlobar veins: Receive blood from arcuate veins.
14. Renal vein: Formed by the convergence of interlobar veins, drains into the inferior vena cava.

Quantitative Aspects of Renal Blood Flow

• Cardiac Output (CO): Kidneys receive approximately ext{20-25%} of cardiac output (which is about $ext{5 L/min}$ in an adult).
• Renal Blood Flow (RBF): Approximately $ext{1.2 L/min}$.
• Renal Plasma Flow (RPF): Approximately $ext{660 ml/min}$. (Calculated considering a hematocrit of ext{ extasciitilde}45 ext{%} . So, $ext{RPF} = ext{RBF} imes (1 - ext{Hematocrit}) = 1200 ext{ ml/min} imes (1 - 0.45) = 1200 imes 0.55 = 660 ext{ ml/min}$).
• Glomerular Filtration Rate (GFR): Approximately $ext{125 ml/min}$.
• Filtration Fraction (FF): The fraction of total renal plasma flow that is filtered by the glomeruli.
  • ext{FF} = ext{GFR} / ext{RPF} = ext{125 ml/min} / ext{660 ml/min} ext{ extasciitilde} 0.19 ext{ or } 19 ext{%} .
  • Clinically, FF is often approximated as ext{ extasciitilde}20 ext{%} .
• The remaining plasma ($ext{535 ml/min}$) flows into the peritubular capillaries for reabsorption and secretion processes.

Formation of Urine

Three Main Processes

Urine formation involves a combination of three fundamental processes:

1. Glomerular Filtration: Bulk flow of plasma from glomerular capillaries into Bowman's capsule.
2. Tubular Reabsorption: Selective movement of substances from the tubular lumen back into the peritubular capillaries (blood).
3. Tubular Secretion: Selective movement of substances from the peritubular capillaries (blood) into the tubular lumen.

General Functions of Each Nephron Segment

• Glomerulus & Bowman's Capsule: Site of filtration. Plasma is filtered; water and dissolved substances pass, while proteins and RBCs are generally retained in blood.
• Proximal Convoluted Tubule (PCT): Extensive tubular transport (reabsorption and secretion). Both active and passive processes occur. Generally, not under hormonal control.
• Loop of Henle (Thin DL, Thin AL, Thick AL): Functions as part of the counter-current system to establish the medullary osmotic gradient, which is essential for concentrating urine.
• Distal Convoluted Tubule (DCT), Cortical Collecting Duct (CCD), Medullary Collecting Duct (MCD):
  • These segments determine the final urine volume and concentration.
  • Regulate $ext{Na}^+$ and $ext{H}_2 ext{O}$ reabsorption.
  • Regulate $ext{K}^+$ and $ext{H}^+$ secretion.
  • These functions are largely under hormonal control, primarily by aldosterone (for $ext{Na}^+, ext{K}^+$) and Antidiuretic Hormone (ADH) (for $ext{H}_2 ext{O}$).

Glomerular Filtration

Overview

• As blood passes through the glomerular capillaries, plasma is filtered into Bowman's capsule.
• Water and dissolved small substances (e.g., ions, glucose, amino acids, urea) are filtered.
• Large molecules like proteins and cellular components like red blood cells (RBCs) are typically not filtered.

Glomerular Filtration Rate (GFR)

• Definition: The rate of filtration of plasma from all nephrons in both kidneys per unit time.
• Normal Values in Humans:
  • Males (M): $ext{125 ml/min}$.
  • Females (F): $ext{110 ml/min}$.
  • More precisely: $ext{125 ml/min/1.73 m}^2$ (adjusted for body surface area).
  • Clinically, a value around $ext{100 ml/min}$ is often cited.

Factors Determining Filtration

Glomerular filtration is determined by the interplay of several factors:

1. Effective Filtration Pressure: The net pressure driving fluid across the glomerular capillary membrane.
   • This is the result of Starling forces.
   • Hydrostatic Pressure (P): The physical pressure of fluid, primarily from systemic mean arterial pressure (MAP).
   • Oncotic (Colloid Osmotic) Pressure ($ext{ extPi}$): Pressure exerted by plasma proteins, tending to draw water back into the capillaries.
2. Surface Area of Glomerular Membrane: A larger surface area allows for more filtration.
3. Permeability of Glomerular Membrane: The ease with which substances can pass through the membrane (determined by fenestrations, basal lamina, and podocytes).

Starling Forces and Net Filtration Pressure

Assuming no oncotic pressure in Bowman's capsule ($ext{ extPi}_{BC} = 0 ext{ mmHg}$) due to minimal protein filtration:

• Pressures Favoring Filtration:
  • Glomerular Capillary Hydrostatic Pressure ($ext{P}_{GC}$): Approximately $ext{45 mmHg}$. This is the main driving force for filtration.
• Pressures Opposing Filtration:
  • Bowman's Capsule Hydrostatic Pressure ($ext{P}_{BC}$): Approximately $ext{10 mmHg}$. Pressure exerted by fluid already in the capsule.
  • Glomerular Capillary Oncotic Pressure ($ext{ extPi}_{GC}$): Varies along the capillary length due to protein concentration changes.
    • At the afferent end: Approximately $ext{27 mmHg}$.
    • At the efferent end: Approximately $ext{35 mmHg}$ (as fluid is filtered, protein concentration increases).

Calculation of Net Filtration Pressure (Peff):
$ext{P}</em>{eff} = ( ext{P}<em>{GC} + ext{ extPi}</em>{BC}) - ( ext{P}<em>{BC} + ext{ extPi}</em>{GC})$

• At the Afferent End of the Glomerular Capillary:

  • $ext{Net Pressure} = ( ext{45 mmHg} + ext{0 mmHg}) - ( ext{10 mmHg} + ext{27 mmHg})$
  • $ext{Net Pressure} = ext{45 mmHg} - ext{37 mmHg} = ext{8 mmHg}$
  • This positive net pressure drives filtration.

• At the Efferent End of the Glomerular Capillary:

  • $ext{Net Pressure} = ( ext{45 mmHg} + ext{0 mmHg}) - ( ext{10 mmHg} + ext{35 mmHg})$
  • $ext{Net Pressure} = ext{45 mmHg} - ext{45 mmHg} = ext{0 mmHg}$
  • At this point, the net filtration pressure approaches zero, indicating cessation of net filtration along the latter part of the capillary.

The Glomerular Filtration Rate (GFR) can be expressed by the equation:
$ext{GFR} = ext{K}<em>f imes ( ext{Net Ultrafiltration Pressure})$ Where $ext{K}</em>f$ is the filtration coefficient, representing the product of the hydraulic conductivity and the effective surface area of the glomerular capillaries.
Referring to slide 22, the net ultrafiltration pressure (APUF) is given as $ext{P}<em>{GC} - ext{P}</em>{BC} - ext{ extPi}_{GC}$, assuming Bowman's capsule oncotic pressure is negligible.

Factors That Influence GFR

• Blood Flow: Increased renal blood flow generally leads to increased GFR, assuming other factors are constant.
• Capillary Hydrostatic Pressure ($ext{P}_{GC}$): Directly influenced by systemic mean arterial pressure (MAP).
  • Renal Autoregulation: Within a MAP range of $ext{80 - 180 mmHg}$, GFR remains relatively constant due to intrinsic autoregulatory mechanisms (e.g., myogenic mechanism, tubuloglomerular feedback).
  • MAP < 80 mmHg: Below this range, autoregulation becomes less effective, leading to a decrease in GFR.
  • MAP < 40-50 mmHg: GFR can drop to zero, potentially leading to renal failure.
• Hydrostatic Pressure in Capsule ($ext{P}<em>{BC}$): An increase in $ext{P}</em>{BC}$ (e.g., due to ureteral obstruction) opposes filtration and decreases GFR.
• Oncotic Pressure in Capillary ($ext{ extPi}<em>{GC}$): An increase in $ext{ extPi}</em>{GC}$ (e.g., severe dehydration or increased protein concentration) reduces net filtration pressure and decreases GFR.
• Oncotic Pressure in Capsule ($ext{ extPi}<em>{BC}$): Normally negligible (ideally zero). If proteins leak into Bowman's capsule (e.g., in glomerular disease), an increase in $ext{ extPi}</em>{BC}$ would favor filtration, but this is pathological.
• Membrane Permeability: Increased permeability (e.g., inflammation) can increase GFR, while decreased permeability (e.g., thickening of the glomerular basement membrane) can decrease GFR.
• Surface Area of Membrane: A decrease in functional surface area (e.g., loss of nephrons or contraction of mesangial cells) reduces GFR.