Chapter 26: Urine Formation By the
Kidneys. I. Glomerular Filtration,
Renal Blood Flow and Their Control
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Multiple Functions of the Kidney
•
Excretion of Metabolic Waste Products, Foreign
Chemicals, Drugs, and Hormone Metabolites
a. Eliminate waste products of metabolism that are
no longer needed ( i.e. urea, creatinine, uric acid,
hb breakdown products)
b. Also rapidly eliminate most toxins, pesticides,
drugs, food additives
Functions (cont.)
•
Regulation of Water and Electrolyte Balances
a. For homeostatsis, excretion of electrolytes must
match intake
b. Governed mostly by eating habits
Functions (cont.)
•
Regulation of Arterial Pressure
a. By excreting variable amounts of sodium and water
b. Short-term-by secreting hormones and vasoactive
factors (i.e. renin)
•
Glucose Synthesis
a. Gluconeogenesis-synthesize glucose from amino
acids and other precursors during prolonged
fasting.
Functions (cont.)
•
Regulation of Acid-Base Balance
a. By excreting acids and by regulating the body fluid
buffer stores
b. Kidney is the only means of eliminating sulfuric and
phosphoric acids (generated by the metabolism of
proteins
Functions (cont.)
•
Regulation of Erythrocyte Production
a. Secretes erythropoietin which stimulates the
production of red blood cells
•
Regulation of 1,25-Dihydroxyvitamin D3 Production
a. Produces the active form (calcitriol)
b. Calcitriol is necessary for the normal calcium
deposition in bone and calcium reabsorption by
the gastrointestinal tract
Physiologic Anatomy of the Kidneys
• General Organization of the Kidneys and
Urinary Tract
Fig. 26.2 General Organization of the Kidneys and Urinary System
Physiologic Anatomy of the Kidneys
• Renal Blood Supply
Fig. 26.3 Renal Blood Supply
Physiologic Anatomy of the Kidneys
• The Nephron-functional unit of the kidney
a.
b.
c.
d.
e.
f.
g.
Each kidney contains 800,000-1,000,000
Decrease with age or injury (cannot be replaced)
Contains the glomerulus (Bowman’s capsule)
Proximal and distal convoluted tubules
Loop of Henle
Macula densa
Collecting ducts
Physiologic Anatomy of the Kidneys
Figure 26.4 Basic tubular segments of the nephron
Physiologic Anatomy of the Kidneys
• Regional Differences in Nephron Structure:
Cortical and Juxtaglomerular Nephrons
Figure 26.5 Cortical and juxtaglomerular nephrons
Physiologic Anatomy of Urinary Bladder
• Micturition-process by which the urinary bladder
empties when it becomes filled
a. The bladder fills progressively until the tension
within the walls rises above a threshold level
b. The micturition reflex empties the bladder or
stimulates a conscious desire to urinate
c. It is an autonomic reflex that can be altered by
centers in the cerebral cortex
Physiologic Anatomy of Urinary Bladder
• Bladder Anatomy-smooth muscle chamber composed of
two main parts:
a. Body-where the urine collects
b. Neck-funnel shaped extension which connects
with the urethra
The smooth muscle of the bladder is the detrusor
muscle and its contraction is a major step in emptying
the bladder.
Fig. 26.6 Anatomy of the urinary bladder in
males and females
• Innervation of the Bladder-pelvic nerves through the
saccral plexus
Fig. 26.7 Innervation of the urinary bladder
Transport of Urine
• No significant changes in urine composition from
the renal calyces to the ureters to the bladder
• Peristaltic contractions in the smooth muscle of
the ureter are enhanced by parasympathetic
stimulation and inhibited by sympathetic
stimulation
Micturition Reflex
Fig. 26.8 Normalcystometrogram showing acute pressure waves
caused by micturition reflexes
Micturition Reflex
• As the bladder fills, micturition contractions occur
• Initiated by stretch receptors in the bladder
• When partially filled, the contractions relax
spontaneously
• The reflex is “self-regenerative”
Micturition Reflex
• Reflex is a Single Complete Cycle
a. Progressive and rapid increase of pressure
b. A period of sustained pressure
c. Return of the pressure to the basal tone
Urine Formation
Fig. 26.9 Basic kidney processes that determine the composition of urine
Urine Formation
• Sum of three renal processes
A. The substance is freely filtered but
not reabsorbed
B. The substance is freely filtered but
part is reabsorbed
C. The substance is freely filtered but
not excreted because all is
reabsorbed.
D. The substance is freely filtered but
is not reabsorbed but secreted from
the peritubular capillaries into the
renal tubules
Fig. 26.10
Glomerular Filtration
• Composition of the Filtrate
a. Protein free and cell free
b. Salts and organic molecules are similar to
those in plasma
c. Does not include calcium and fatty acids that
are bound to proteins
Glomerular Filtration (cont.)
• GFR (Glomerular Filtration Rate)
a. Determined by (1) the balance of hydrostatic and
colloid osmotic forces acting on the capillary
membrane, and (2) the product of the permeability
and filtering surface area of the capillaries
b.
Glomerular Capillary Membrane
• Consists of Three Major Layers
a. Endothelium of the capillary
b. Basement membrane
c. Layer of epithelial cells (podocytes)
Fig. 26.11 A: Basic ultrastructure of the glomerular capillaries;
B: Cross-section of the capillary membrane and its
major components
Membrane (cont.)
• Filterability of Solutes is Inversely Related to
Their Size
Table 26.1 Filterability of substances by glomerular capillaries based on molecular weight
Substance
Molecular Weight
Filterability
Water
18
1.0
Sodium
23
1.0
Glucose
180
1.0
Inulin
5,500
1.0
Myoglobin
17,000
0.75
Albumin
69,000
0.005
Membrane (cont.)
• Negatively Charged Large Molecules Are Filtered Less
Easily Than Positively Charged Molecules of
Equal Molecular Size
Fig. 26.12 Effect of molecular radius and electrical charge of dextran
on its filterability by glomerular capillaries
Determinants of the GFR
• GFR is determined by (1) sum of the hydrostatic and
colloid forces across the glomerular membrane (net
filtration pressure), and (2) the glomerular capillary
filtration coefficient.
Fig. 26.13
Determinants of the GFR
• Increased glomerular capillary filtration
coefficient increases GFR
• Increased Bowman’s capsule hydrostatic
pressure decreases GFR
• Increased glomerular capillary colloid osmotic
pressure decreases GFR
Fig. 26.14 Increase in colloid osmotic pressure in plasma flowing
through the glomerular capillary.
Determinants of the GFR (cont.)
• Increased glomerular
capillary hydrostatic
pressure increases GFR
Fig. 26.15 Effect of change in afferent or efferent
arteriole resistance on GFR and
renal flow.
Renal Blood Flow
• Renal Blood Flow and Oxygen Consumption
Fig. 26.16 Relationship between sodium reabsorption and oxygen consumption
Renal Blood Flow (cont.)
• Determinants of Renal Blood Flow
Table 26.3 Approximate pressures and vascular resistances in the circulation of a normal kidney
Beginning
End
Percentage of Total
Renal Vascular
Resistance
100
100
0
Approx. 100
85
16
Afferent arteriole
85
60
26
Glomerular capill.
60
59
1
Efferent arteriole
59
18
43
Peritubular capill.
18
8
10
Interlobar, arcuate,
and interlobular veins
8
4
4
Renal vein
4
Approx. 4
0
Vessel
Renal Artery
Interlobar, arcuate,
and interlobular
arteries
Physiologic Control of GFR and Renal Blood Flow
• Sympathetic Nervous System Activation
Decreases GFR
• Hormonal and Autacoid Control of Renal Circulation
Hormone or Autacoid
Effect on GFR
Norepinephrine
Decreases
Epinephrine
Decreases
Endothelin
Decreases
Angiotensin II
Constricts efferent arterioles
Endothelian NO
Increases
Prostaglandins
Increases
Autoregulation of GFR and Renal Blood Flow
Fig. 26.17 Autoregulation of renal blood flow and GFR but lack of
autoregulation of urine flow during changes in renal
arterial pressure
Tubuloglomerular Feedback and Autoregulation of GFR
• Links changes in NaCl concentration at the
macula densa with the control of renal
arteriolar resistance
• Two components
a. An afferent arteriolar feedback mechanism
b. An efferent arteriolar feedback mechanism
Fig. 26.18 Structure of the juxtaglomerular apparatus, demonstrating its possible
feedback role in the control of nephron function.
•
Decreased macula densa NaCl causes dilation of
afferent arterioles and increased renin release
Fig. 26.19
• Myogenic Autoregulation of Renal Blood Flow and GFR
a. Ability of blood vessels to resist stretching during
increased arterial pressure
b. Stretch allows the release of calcium from ECF into the
cells causing them to contract
•
High protein intake and increased blood glucose also
increase renal blood flow