Understanding Kidney Functions: Filtration, Reabsorption, and Secretion, Study Guides, Projects, Research of Anatomy

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Chapter 25

The Urinary

System

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Kidney Functions

• Removal of toxins, metabolic wastes, and excess ions from the blood
• Regulation of blood volume, chemical composition, and pH
• Gluconeogenesis during prolonged fasting
• Endocrine functions
  • Renin: regulation of blood pressure and kidney function
  • Erythropoietin: regulation of RBC production
• Activation of vitamin D 10/30/2013 MDufilho 2

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Figure 25.9 A schematic, uncoiled nephron showing the three major renal processes that adjust plasma composition.

Cortical radiate artery

Afferent arteriole Glomerular capillaries Efferent arteriole

Glomerular capsule Renal tubule and collecting duct containing filtrate Peritubular capillary

To cortical radiate vein Three major renal processes: (^) Urine Glomerular filtration Tubular reabsorption Tubular secretion

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Figure 25.5 Location and structure of nephrons. (3 of 7)

Basement membrane

Podocyte

Fenestrated endothelium of the glomerulus

Glomerular capsule: visceral layer

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Figure 25.10c The filtration membrane.

Capillary

Fenestration (pore)

Filtration membrane

- Capillary endothelium - Basement membrane - Foot processes of podocyte of glomerular capsule

Filtration slit Slit diaphragm

Three layers of the filtration membrane

Foot processes of podocyte

Filtrate in capsular space

Plasma

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Terminology

• Renal Fraction = % of blood flow passing through kidneys = 20 – 25%
• Filtration Fraction = % plasma (in renal fraction) that passes through the filtration membrane to become filtrate = 20%

Figure 25.11 Forces determining net filtration pressure (NFP).

Efferent arteriole

Glomerular capsule

Afferent arteriole

NFP = Net filtration pressure = outward pressures – inward pressures = (HPgc) – (HPcs + OPgc) = (55) – (15 + 30) = 10 mm Hg

HPgc = 55 mm Hg

OPgc = 30 mm Hg

HPcs = 15 mm Hg

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Glomerular Filtration Rate (GFR)

• Volume of filtrate formed per minute by the kidneys (120–125 ml/min)
• Governed by (and directly proportional to)
  • Total surface area available for filtration
  • Filtration membrane permeability
  • NFP

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Clinical Applications – What if…..

• Hemorrhage –
  • Blood Pressure?
  • NFP?
  • GFR?
  • Result
• Fight or flight situation
  • Sympathetic stimulation?
  • Blood Pressure?
  • Result?
• Kidney Infection
  • Result

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Regulation of Glomerular Filtration

• GFR is tightly controlled by two types of mechanisms
• Intrinsic controls (renal autoregulation)
  • Act locally within the kidney
• Extrinsic controls
  • Nervous and endocrine mechanisms that maintain blood pressure, but affect kidney function

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Figure 25.8 Juxtaglomerular complex (JGC) of a nephron.

Glomerular capsule Efferent arteriole

Afferent arteriole

Glomerulus

arteriole^ Efferent Juxtaglomerular complex Macula densa cells of the ascending limb of nephron loop

• Extraglomerular
• Granular cells arterioleAfferent

Parietal layer of glomerular capsule Capsular space

Foot processes of podocytes Podocyte cell body (visceral layer) Red blood cell Proximal tubule cell

Lumens of glomerular capillaries Endothelial cell of glomerular capillary Glomerular mesangial cells Juxtaglomerular complex Renal corpuscle

mesangial cells

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Intrinsic Controls: Tubuloglomerular Feedback Mechanism

Macula densa cells monitor Na+ and Cl- in

filtrate

• Macula densa cells of the JGA respond to NaCl by releasing a vasoconstricting chemical that acts on the afferent arteriole   GFR
• The opposite occurs if GFR decreases.

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Extrinsic Controls: Sympathetic Nervous System

• Under extreme stress
  • Norepinephrine is released by the sympathetic nervous system
  • Epinephrine is released by the adrenal medulla
  • Both cause constriction of afferent arterioles, inhibiting filtration and triggering the release of renin

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Extrinsic Controls: Renin-Angiotensin Mechanism

• Triggered when the granular cells of the JGA release renin angiotensinogen (a plasma globulin) renin  angiotensin I angiotensin converting enzyme (ACE)  angiotensin II

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