The Urinary System: Filtration and Regulation (Chapter 25, Part III)

The Urinary System

Filtrate vs. Urine

The process of urine formation begins with the production of filtrate in the kidneys. Understanding the differences between filtrate and urine is essential for grasping renal physiology.

• Filtrate Formation: About 20% of systemic blood passes through the high-resistance glomerulus each minute, forcing out filtrate. This is a passive, nonselective process driven by glomerular blood pressure.

• Filtrate Composition: Glomerular filtrate is mostly protein-free and contains blood plasma components such as nutrients, essential ions, water, and some wastes.

• Urine Composition: Urine contains metabolic wastes (e.g., creatinine, urea, uric acid, excess ions) but no proteins. Proteins like albumin are too large to pass through the filtration membrane.

• Filtrate vs. Urine: Only about 1% of the filtrate is excreted as urine daily; the rest is reabsorbed.

Example: Glucose and amino acids are present in filtrate but are normally reabsorbed and not found in urine.

Filtrate Formation & Processing

Urine formation involves three major processes that occur in sequence within the nephron:

1. Glomerular Filtration: Water and filtered solutes of less than 3 nm (e.g., amino acids, glucose, nitrogenous wastes) move across the filtration membrane into Bowman’s capsule.

2. Tubular Reabsorption: About 99% of water and useful solutes are reabsorbed from the renal tubules into peritubular capillaries. Waste material continues to become part of urine.

3. Tubular Secretion: Unfiltered wastes, drugs, excess ions, and urea are secreted from peritubular capillaries into renal tubules, becoming part of the urine.

Additional info: Substances that cannot pass through the filtration membrane (e.g., large proteins, blood cells) remain in the bloodstream.

Filtration Membrane

The filtration membrane is a three-part structure that lies between the glomerular capillaries and podocytes (visceral portion of the glomerular capsule):

• Glomerular Capillaries Endothelium: Contains fenestrations (pores) that allow passage of most plasma components but not blood cells.

• Fused Basement Membranes: Composed of endothelial and epithelial layers, acting as a physical and charge barrier to proteins.

• Podocyte Epithelium: Podocytes have filtration slits that further restrict passage of large molecules.

Example: The filtration membrane allows passage of water, glucose, and ions, but not albumin or red blood cells.

Filtration Pressures: Promoting and Opposing

Filtrate formation is governed by several pressures that either promote or oppose filtration:

Pressure	Value	Effect
Glomerular hydrostatic pressure (HPgc)	~55 mm Hg	Promotes filtration (systemic BP provides gradient)
Blood colloid osmotic pressure (OPgc)	~30 mm Hg	Opposes filtration (due to plasma proteins)
Capsular hydrostatic pressure (HPcs)	~15 mm Hg	Opposes filtration (due to fluid in capsule)

• Net Filtration Pressure (NFP): The difference between pressures promoting and opposing filtration.

$NFP = HP_{gc} - (OP_{gc} + HP_{cs})$

Typical NFP is about 10 mm Hg.

Glomerular Filtration Rate (GFR)

GFR is the volume of plasma filtrate (in mL) delivered into Bowman’s capsule per minute. It is a key indicator of kidney function.

• Normal GFR: 120–125 mL/min

• Clinical Importance: Measuring GFR can detect early kidney damage, diagnose chronic kidney disease, and monitor medication effectiveness.

• Factors Affecting GFR: Systemic blood pressure, renal artery pressure, glomerular hydrostatic pressure, net filtration pressure, and filtrate processing time.

Example: Both acute (hypotension) and chronic (hypertension) changes in BP can threaten kidney function by altering GFR.

Glomerular Filtration Regulation

Precisely maintaining GFR is essential for optimal filtrate processing, including substance recovery and waste removal.

• Substance Recovery: Occurs via tubular reabsorption.

• Waste Removal: Occurs via tubular secretion.

• Effects of Altered GFR: Too high or too low GFR can impair reabsorption or waste excretion.

Altered GFR	Flow Rate	Time for Reabsorption	Effect
High	Increased	Decreased	Amino acids, glucose not reabsorbed (lost)
Low	Decreased	Increased	Uric acid, creatinine are reabsorbed (wastes retained)

GFR Control Mechanisms

Both intrinsic and extrinsic mechanisms regulate GFR to maintain homeostasis and respond to physiological needs.

• Intrinsic (Renal Autoregulation): The kidney adjusts its own blood flow to maintain a nearly constant GFR despite changes in systemic BP. Mechanisms include myogenic response and tubuloglomerular feedback.

• Extrinsic (ANS-Mediated): The central nervous system (CNS) and hormones adjust GFR in response to systemic BP changes, affecting both NFP and GFR.

$F = \frac{\Delta P}{R}$

Additional info: F = flow, ΔP = pressure difference, R = resistance.

GFR: Intrinsic Controls

Intrinsic controls maintain GFR through two main mechanisms:

• Myogenic Mechanism: Changes in systemic BP cause afferent arteriole smooth muscle to contract or relax, maintaining needed NFP. Stretch receptors in arteriole walls adjust absorption and secretion rates.

• Tubuloglomerular Feedback: Changes in filtrate flow rate are detected by juxtaglomerular apparatus (JGA) cells in response to changes in osmolality (e.g., Na+ concentration). This feedback adjusts arteriole diameter to stabilize GFR.

GFR: Extrinsic Controls

Extrinsic controls involve the nervous system and hormonal regulation:

Nervous System (ANS)

• At Rest: Renal blood vessels are maximally dilated (parasympathetic release), and kidney autoregulation prevails. NFP is maintained at 10 mm Hg.

• Under Stress: Renal blood vessels constrict due to norepinephrine release (sympathetic stimulation), but NFP is maintained so filtrate formation continues.

Hormonal Regulation

• Renin-Angiotensin Mechanism: JGA granular cells release renin in response to decreased systemic BP. Renin triggers the formation of angiotensin II, which stimulates the hypothalamus to release ADH and the adrenal cortex to release aldosterone.

• ADH (Antidiuretic Hormone): Increases water reabsorption in the kidneys, raising blood volume and BP.

• Aldosterone: Increases Na+ reabsorption, which also increases water reabsorption.

Key Principle: "Wherever sodium goes, so does water." Increased Na+ reabsorption leads to increased water retention, raising systemic BP.