Micturition Reflex, Glomerular Filtration, and Renal Regulation Concepts

Class Announcements

  • No Class on Friday

    • Micturition Reflex Video Lecture will be uploaded (approx. 15 minutes long).

    • Quiz with about 5 questions will be assigned after the lecture.

  • Cat and Cat Draft Submission

    • Submit rough drafts by the end of the day for review.

    • Feedback will be provided before Friday morning or while traveling to Indy.

  • Lab Tomorrow

    • Dissect kidneys:

    • Wear closed-toe shoes.

    • Bring safety glasses or goggles if available.

  • Assignments Reminder:

    • Cat and Cat due on Sunday.

    • Discussion due on Tuesday.

Information Review and Concepts

  • Kahoot Question Review

    • Discussion on glomerular filtration pressures.

Glomerular Filtration Pressures

  • Hydrostatic Pressure

    • Hydrostatic pressure in glomerular capillaries acts as a pushing force, measured at 55 mmHg.

    • Hydrostatic pressure in the capsular space measured at 15 mmHg, counters glomerular pressure.

    • Blood colloid osmotic pressure inside capillaries is 30 mmHg pulling fluids back.

    • Colloid osmotic pressure in capsular space is essentially 0 mmHg since proteins are not filtered.

  • Net Outward Pressure

    • Calculation of net outward pressure: 55 mmHg (glomerular) - 15 mmHg (capsular) - 30 mmHg (colloid) = 10 mmHg.

    • Sufficient pressure to sustain glomerular filtration.

Glomerular Filtration Rate (GFR)

  • Definition: Amount of filtrate produced by both kidneys per minute.

  • Average GFR: 120-125 mL/min resulting in 180 liters of filtrate per day.

    • Filtrate is not equivalent to urine due to reabsorption processes.

  • Factors Influencing GFR:

    • Large Surface Area of glomerulus aids filtration - mainly constant.

    • High Permeability of filtration membrane allows passage of substances smaller than proteins.

    • Continuous Filtration Pressure is maintained throughout glomerular capillaries preventing reabsorption at venule ends, promoting sustained filtration.

Importance of GFR

  • Health Impact:

    • Essential for waste elimination and balance of water and electrolytes.

    • Fluctuations in GFR can lead to kidney failure; unchecked decline can affect homeostasis and result in death.

  • Key Pressure for GFR: Hydrostatic pressure in glomerular capillaries can be adjusted to maintain GFR.

Blood Pressure Considerations

  • Systolic Pressure Range: Normally ranges between 80 - 120 mmHg.

  • Mean Arterial Pressure (MAP): Must stay above 60 mmHg to maintain regular GFR.

  • Consequences of Low MAP:

    • Below 60 mmHg leads to shock and decreased renal function, risking kidney cell damage and compromised filtration.

Renal Blood Flow (RBF)

  • Normal RBF: Approximately 1,050 mL/min representing 21% of total cardiac output.

  • Plasma Flow: Of the RBF, 578 mL is plasma; equivalent to roughly 20% of total plasma filtered into renal tubule.

  • Daily Urine Output: Average production of 1,296 mL (approx. 1.3 liters per day).

Factors Proportional to GFR

  • Directly Proportional Factors:

    1. Net Filtration Pressure (primarily hydrostatic pressure).

    2. Ability to adjust total surface area available for filtration (minor adjustments possible).

    3. Permeability of the filtration membrane (fixed in normal conditions).

Juxtaglomerular Complex

  • Structure Components:

    • Macula Densa: Chemoreceptors in distal convoluted tubule detecting sodium chloride concentration.

    • Granular Cells (Juxtaglomerular Cells): Mechanoreceptors in afferent arteriole detecting stretch and secreting renin.

    • Extraglomerular Mesangial Cells: Relay signals between macula densa and granular cells.

  • Importance: Regulates GFR based on systemic blood pressure changes.

Regulation of GFR

  • Intrinsic Controls (Autoregulation):

    • Myogenic Mechanism:

    • Muscle cells in blood vessels react to stretch; contraction occurs with systemic blood pressure rise, reducing afferent arteriole size to lower GFR.

    • Relaxation occurs when blood pressure drops, increasing afferent arteriole size to maintain GFR.

    • Tubuloglomerular Feedback Mechanism:

    • Macula densa detects sodium chloride concentration; high levels trigger release of ATP/adenosine causing vasoconstriction of afferent arteriole, thus lowering GFR.

  • Extrinsic Controls: Regulated by the nervous system and hormones; used when systemic blood pressure drastically affects renal function, beyond the kidneys' ability to autoregulate.

Future Topics to Discuss

  • Continuing discussion on extrinsic controls affecting GFR regulation.

  • Focus on reabsorption processes within the kidney, what is typically reabsorbed and how.