Understanding the Mechanisms of Urine Formation and Glomerular Filtration Rate (GFR), Exercises of Human Physiology

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Objectives

• Describe that the mechanism of urine formation include three basic

processes; glomerular filtration, tubular reabsorption and tubular

secretion

• Define GFR and quote normal value

• Identify and describe the factors controlling GFR in terms of starling

forces, permeability with respect to size, shape and electrical charges

and ultra-filtration coefficient

• Describe Intrinsic and extrinsic mechanism that regulate GFR

• Describe autoregulation of GFR & tubuloglomerular feedback

mechanism

Glomerular filtration rate (GFR): is the volume of plasma filtrate produced by both kidneys per minute.

• The average per minute is : 125 ml/min ( 20 % of renal plasma flow)
• The average per day is: 180 L/day (gallons)
• 99 % will reabsorbed (178.5 L/day) from filtrate and only 1 % will excreted (1.5 – 2 L/day) So, most filtered water must be reabsorbed or death would ensue from water lost through urination.
• These values varies with: kidney size, lean body weight and number of functional nephrons
• The relation between GFR an Net Filtration Pressure:
  • ↑ NFP → ↑GFR
  • ↓ NFP → ↓ GFR
• Normally changes in GFR is a result from change of blood pressure.

If GFR is too high:

Fluid flows through tubules too rapidly to be absorbed (will not absorbed very well). It will lead to: 1 - Urine output rises 2 - Creates threat of dehydration and electrolyte depletion

If GFR is too low:

Fluid flows sluggishly through tubules. It will lead to: 1 - Tubules reabsorb wastes that should be eliminated 2 - Azotemia develops (high levels of nitrogen-containing substances in the blood).

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A. glomerular hydrostatic pressure (= 60 mmHg). It promotes filtration. Increased by: (increase GFR) 1 - increase Arterial blood pressure (slightly affect due to autoregulation) 2 - Afferent arteriole vasodilation 3 - Moderete efferent arteriole vasoconstriction( 1 ). Decreased By: (decrease GFR) 1 - Afferent arteriole vasoconstriction 2 - Efferent arteriole vasodilation A. hydrostatic pressure in Bowman’s capsule (= 18 mmHg). It opposes filtration. Increased by: (decrease GFR) 1 - Urinary obstruction (such as: Kidney stones) 2 - Kidney edema A. colloid osmotic pressure of glomerular plasma proteins (= 32 mmHg). It opposes filtration. Increased by: (decrease GFR) 1 - Dehydration 2 - Decrease renal blood flow 3 - Severe efferent vasoconstriction Decreased by: (increase GFR) 1 - Hypoprotenemia 2 - increase renal blood flow ( 1 ) Severe efferent vasoconstriction which is a pathological condition will decrease GFR due to increase in colloid osmotic pressure of glomerular plasma proteins

Autoregulation of GFR and Renal Blood Flow

Importance of GFR Autoregulation in Preventing Extreme Changes in Renal Excretion (Additional Reading from guyton) In the absence of autoregulation, a relatively small increase in blood pressure (from 100 to 125 mm Hg) would cause a similar 25 percent increase in GFR (from about 180 to 225 L/day). If tubular reabsorption remained constant at 178.5 L/day, this would increase the urine flow to 46.5 L/day (the difference between GFR and tubular reabsorption)—a total increase in urine of more than 30 - fold. Because the total plasma volume is only about 3 liters, such a change would quickly deplete the blood volume. In reality, changes in arterial pressure usually exert much less of an effect on urine volume for two reasons: ( 1 ) renal autoregulation prevents large changes in GFR that would otherwise occur ( 2 ) there are additional adaptive mechanisms in the renal tubules that cause them to increase their reabsorption rate when GFR rises, a phenomenon referred to as glomerulotubular balance

Several hormones and autacoids can influence GFR and renal blood flow 1 - afferent arterioles, appear to be relatively protected from angiotensin II due to release of vasodilators, especially nitric oxide and prostaglandins ,which counteract the vasoconstrictor effects of angiotensin II 2 - efferent arterioles are highly sensitive to angiotensin II. Because angiotensin II constricts efferent arterioles in most physiologic conditions, increased angiotensin II levels raise glomerular hydrostatic pressure while reducing renal blood flow.

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1 - How much volume filtered per day? A- 125 ml/min B- 180 L/day C- 200 L/day D- 125 L/day 2 - When the Glomerular Filtration Rate increase the net filtration pressure is? A-Decrease B-Increase C-Inverse proportional D-Constant 3 - If GFR is low the body will react by : A-Increase fluid flow through tubules B-Rise in urine output C-Fluid flows sluggishly through tubules D-Dehydration 4 - GFR controlled by adjusting glomerular blood pressure through : A-renin and angiotensin B-parasympathetic control C-Estrogen D-Testosterone 5 - which of the following is true of autoregulation in GFR: A-It has a wide regulation by prevents potentially great changes in GFR B-Increase in resistance of the afferent arterioles only C-it is extrinsic regulation of kidney D-Works when changes in blood pressure range from 75 to 160 mmHg. 6 - Constrict afferent arteriole and dilate efferent arteriole that are sign of : A-Increase in blood pressure B-Decrease in blood pressure 7 - If an increase in ABP, which one of these mechanism will occur? A-a decrease in resistance of the afferent arterioles B-Secrete angiotensin II C-decrease in renin release D-constrict efferent arteriole 8 - During myogenic mechanism , it will activate if there is changing in : A-Vascular resistance B-Oncotic pressure C-Blood pressure D- A & C Ans : 1

B, 2

B, 3

C, 4

A, 5

D, 6

A, 7

C. 8

D,