72 Glomerular filtration rate (GFR)

Learning Objectives

After reading this section you should be able to-

  • Define glomerular filtration rate (GFR) and explain the role of blood pressure, capsule fluid pressure, and colloid osmotic (oncotic) pressure in determining GFR.
  • Describe factors that can change blood pressure, capsule fluid pressure, and colloid osmotic (oncotic) pressure and thereby change glomerular filtration rate (GFR).
  • Explain the role of the juxtaglomerular apparatus (JGA) in tubuloglomerular feedback.
  • Trace the changes in filtrate osmolarity as it passes through the segments of the nephron.
  • Explain the role of the nephron loop (of Henle), its permeability to water, and the high osmolarity of the interstitial fluid in the renal medulla in the formation of dilute urine

Filtrate is produced by the glomerulus when the hydrostatic pressure produced by the heart pushes water and solutes through the filtration membrane. Glomerular filtration is a passive process as cellular energy is not used at the filtration membrane to produce filtrate. Recall that the filtration membrane lies between the blood in the glomerulus and the filtrate in the Bowman’s (glomerular) capsule and this filtration membrane is highly fenestrated allowing the passage of small molecules such as water, sodium, glucose, etc. The volume of filtrate formed by both kidneys per minute is termed glomerular filtration rate (GFR). Approximately 20% of your cardiac output is filtered by your kidneys per minute under resting conditions. The work of the kidneys produces about 125 mL/min filtrate in men (range of 90 to 140 mL/min) and 105 mL/min filtrate in women (range of 80 to 125 mL/min). This corresponds to approximately 180 L/day in men and 150 L/day in women. However, extensive reabsorption processes ensure that only 1–2 liters of urine are excreted daily.

Factors Determining GFR

Glomerular filtration rate (GFR) is influenced by three main pressures: blood pressure (glomerular hydrostatic pressure), capsule fluid pressure (capsular hydrostatic pressure), and colloid osmotic (oncotic) pressure.

Blood pressure (glomerular hydrostatic pressure) is the force exerted by the blood against the walls of the glomerular capillaries. It promotes the filtration of blood by pushing water and solutes through the filtration membrane into the Bowman’s capsule.

Capsular fluid pressure (capsular hydrostatic pressure) is the pressure exerted by the filtrate already in the Bowman’s capsule. This pressure opposes the glomerular hydrostatic pressure, reducing the net filtration rate.

Colloid osmotic (oncotic) pressure is the pressure exerted by proteins (mainly albumin) in the blood plasma. It opposes the glomerular hydrostatic pressure by drawing water back into the glomerular capillaries from the filtrate.

Glomerular filtration occurs when glomerular (blood) hydrostatic pressure exceeds the hydrostatic pressure of the glomerular capsule and the blood colloid osmotic pressure. The sum of all of the influences, both osmotic and hydrostatic, results in a net filtration pressure (NFP). Glomerular hydrostatic pressure is typically about 55 mmHg pushing fluid into the glomerular capsule. This outward pressure is countered by a typical capsular hydrostatic pressure of about 15 mmHg and a blood colloid osmotic pressure of 30 mmHg. To calculate the value of NFP:

NFP = Glomerular blood hydrostatic pressure (GBHP) – [capsular hydrostatic pressure (CHP) + blood colloid osmotic pressure (BCOP)] = 10 mm Hg

That is: NFP = GBHP – [CHP + BCOP] = 10 mm Hg

Or: NFP = 55 – [15 + 30] = 10 mm Hg (Figure 72.1).

This figure shows the different pressures acting across the glomerulus.
Figure 72.1 – Net Filtration Pressure: The NFP is the sum of osmotic and hydrostatic pressures.

Factors Influencing Blood Pressure, Capsule Fluid Pressure, and Colloid Osmotic Pressure

Factors that can change these pressures include systemic blood pressure, protein levels in the blood, and obstructions in the urinary system. High blood pressure increases glomerular hydrostatic pressure, raising GFR, while low blood pressure has the opposite effect. Decreased protein levels reduce colloid osmotic pressure, increasing GFR. Obstructions in the urinary system increase capsular hydrostatic pressure, reducing GFR.

Role of the Juxtaglomerular Apparatus (JGA) in Tubuloglomerular Feedback

The juxtaglomerular apparatus (JGA) is a specialized structure formed by the distal convoluted tubule and the afferent arteriole. It helps regulate blood pressure and GFR through tubuloglomerular feedback. Macula densa cells in the distal convoluted tubule monitor sodium chloride concentration in the filtrate. When sodium chloride levels are high, these cells signal juxtaglomerular cells to release renin, which leads to the production of angiotensin II, causing vasoconstriction of the afferent arteriole and reducing GFR. This feedback mechanism maintains GFR within optimal ranges.

Changes in Filtrate Osmolarity in the Nephron

As the filtrate passes through the nephron, its osmolarity changes significantly. In the proximal convoluted tubule, water and solutes are reabsorbed, keeping the osmolarity similar to blood plasma. In the descending limb of the nephron loop (Loop of Henle), water is reabsorbed, increasing filtrate osmolarity. The ascending limb is impermeable to water but actively reabsorbs sodium and chloride, decreasing filtrate osmolarity. In the distal convoluted tubule and collecting duct, further reabsorption and secretion fine-tune the osmolarity of the filtrate.

Formation of Dilute Urine: The Nephron Loop’s Osmotic Role

The nephron loop plays a crucial role in the formation of dilute urine. The descending limb is permeable to water but not to solutes, leading to water reabsorption and increased osmolarity of the filtrate. The ascending limb is impermeable to water but actively transports sodium and chloride out of the filtrate, creating a dilute filtrate. The high osmolarity of the interstitial fluid in the renal medulla, maintained by the nephron loop, allows the kidneys to concentrate urine when needed.

In circumstances where the body needs to conserve water, such as in a hydrated state, the permeability of the collecting duct to water is regulated by antidiuretic hormone (ADH). If ADH is low or absent, the collecting duct remains impermeable to water, and dilute urine is formed, allowing for the excretion of excess water while retaining essential solutes.

In essence, the nephron loop, with its selective permeability characteristics, actively contributes to the kidneys’ ability to modulate urine concentration based on the body’s hydration status. This finely tuned osmotic regulation exemplifies the kidneys’ adaptive response to maintain fluid and electrolyte balance in diverse physiological conditions.

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 25
definition

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

Basic Human Physiology Copyright © by Jim Davis is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book