L2 Renal Circulation (P2 Hard)

Renal Module Information

• Course code: IMP-07- 20318
• Phase: I
• Year/semester: 2nd year / Semester 3
• Academic year: 2022-2023

Lecture 2: Renal Circulation and Autoregulation of Renal Blood Flow

• Lecturer: Prof. Dr. Aziza Khalil
• Topic: Renal circulation and its significance, regulation of renal blood flow, plasma clearance and its measurement

Intended Learning Outcomes (ILOs)

• Students will be able to describe renal circulation and its significance.
• Students will be able to discuss the regulation of renal blood flow.
• Students will be able to identify plasma clearance and its measurement.

Renal Circulation

• Renal artery → interlobar arteries → arcuate arteries → interlobular arteries
• Afferent arteriole → glomerulus → efferent arteriole (arterial blood)
• Peritubular capillaries and vasa recta
• Capillaries drain into interlobular vein → arcuate vein → interlobar vein → renal vein

Nephron and Tubules

• Diagram of nephron with labels for proximal tubule, juxtaglomerular apparatus, efferent arteriole, afferent arteriole, artery, vein, peritubular capillaries, loop of Henle, distal tubule, collecting duct, renal corpuscle (glomerulus, Bowman's capsule), cortex, and medulla.

Renal Circulation (additional points)

• It is a portal circulation (2 sets of capillaries).
• Glomerulus specialized for filtration.
• Reabsorption occurs in peritubular capillaries

Renal Blood Flow

• 20-25% of cardiac output
• 1140 ml/min
• 90% supplies the cortex

Renal Fraction

• 21% proportion of cardiac output (COP) that goes to the kidney

Renal Plasma Flow

• 625 ml/min (55% of renal blood flow; RBF)
• Calculated by para-aminohippuric (PAH) acid clearance

Plasma Clearance

• Volume of plasma cleared of a substance by the kidney per minute
• Formula: C_x = (U_x × V)/ P_x
  • U_x = Urine concentration of substance x
  • V = Rate of urine flow
  • P_x = Plasma concentration of substance x

PAH Acid Clearance

• Para-aminohippuric acid (PAHA) clearance: used to measure renal plasma flow, reflecting renal function.
• PAHA is completely removed from blood passing through kidneys (via glomerular filtration and tubular secretion)
• Rate of PAHA clearance from blood reflects total renal plasma flow

Renal Plasma Flow Calculation (ERPF)

• ERPF (effective renal plasma flow) = (U × V) / P

Urine Formation

• 3 basic processes:

  • Glomerular filtration (GF)
  • Tubular reabsorption (TR)
  • Tubular secretion (TS)

• 20% of plasma entering glomerulus is filtered (glomerular filtrate = 125 ml/min)

• 80% of plasma entering glomerulus is not filtered and leaves via efferent arteriole

Regulation of Renal Blood Flow (RBF)

• Autoregulation: aims to maintain constant RBF & GFR despite changes in mean arterial blood pressure (75–160 mmHg). Mechanisms include myogenic and tubuloglomerular feedback.
• Extrinsic control (sympathetic): Intentionally adjusts GFR to regulate arterial blood pressure (ABP).

Autoregulation Mechanisms

• Myogenic: a direct response to stretch induced by changes in blood pressure
• Tubular glomerular feedback: The macula densa cells in the distal tubule sense changes in NaCl concentration. This triggers feedback mechanisms (vasoconstriction/vasodilation of afferent arterioles) to maintain constant GFR.

Autoregulation within ABP Ranges (75-160 mmHg)

• GFR and RBF remain relatively constant despite fluctuations in ABP within this range.

Extrinsic control

• Adjusts GFR as needed to control Blood pressure by targeting afferent arteriole to either constrict or dilate.
• Overrides autoregulation within the normal BP range.
• Mediated by the sympathetic nervous system

Additional Points to Remember

• Renal blood flow (RBF) is approximately 1/4 of cardiac output.
• RBF is regulated by extrinsic and intrinsic factors, including autoregulation.
• Clearance of a substance represents the volume of plasma cleared from that substance per minute.
• Renal plasma flow is measured using para-aminohippuric acid (PAH) clearance.