What is micturition?
The process by which the urinary bladder empties when it becomes filled.
Describe the two step process involved with micturition
- The bladder fills progressively until the tension in its walls rises above a threshold level. this tension elicits the second step
- The tension created from the filling of the bladder elicits the micturition reflex that empties the bladder or, if this fails, at least causes a conscious desire to urinate
- Although the micturition reflex is an autonomic spinal cord reflex, it can be inhibited or facilitated by centers in the cerebral cortex or brian stem
There are seven listed homeostatic functions of nephrons, name 4 of them.
- Get rid of waste materials
• Urea, creatinine, uric acid, bilirubin
- Regulate water and electrolyte balance
- Regulate body fluid osmolarity
- Regulate arterial pressure
• Long term
• Excrete variable amounts of sodium ion and water
• Short term
• Secrete hormones and vasoactive factors such as renin
- Regulate acid-base balance
• Excrete acids and regulate body fluid buffer stores
• Eliminate sulfuric and phosphoric acids (from protein metabolism)
- Secretion, metabolism, and excretion of hormones
• Active form of vitamin D
What are the 3 processes that determine the rates at which different substances are excreted in the urine?
How do the nephrons function to regulate arterial pressure in the long term?
They excrete variable amounts of sodium ions and water
How do the nephrons function to regulate arterial pressure short term?
They secrete hormones and vasoactive factors such as renin
What is the mathematical expression of the urinary excretion rate?
Urinary excretion rate = filtration rate - reabsorption rate + secretion rate
What is the first step in urine formation?
Name the 4 components of the glomerular filtrate.
What is the mathematical expression for filtration fraction?
Filtration fraction = GFR / Renal plasma flow
What is filtration fraction?
The fraction of renal plasma flow that is filtered ≈ 0.2 (i.e., 20% of plasma flowing through the kidney is filtered)
Describe the micturition reflex.
• Superimposed micturition contractions begin to appear as bladder fills.
• Sensory signals from bladder stretch receptors:
• Conducted to sacral region of spinal cord via pelvic nerves
• Conducted reflexively back to bladder via parasympathetic nerves
• Reflex contractions relax spontaneously when bladder is only partially filled.
• Once initiated, the micturition is self-regenerative.
• The self regenerative reflex fatigues after a few seconds and the bladder relaxes.
• As bladder continues to fill, micturition reflexes occur more often and are more powerful.
• When micturition reflex is powerful enough, it causes a second reflex:
• Passes through pudendal nerves to inhibit external sphincter.
• Higher brain centers (in pons) keep micturition partially inhibited except when micturition is desired.
• When it is time to urinate, the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur.
True or false, concentration of most substances, except for proteins, is the same in the filtrate and the plasma.
True or false, some low-molecular weight substances are not freely filtered because they are partially bound to proteins.
List and describe the layers of the filtration barrier.
• With fenestrae and negative charges
• With collagen and proteoglycan fibers and strong negative charges
• With negative charges
What is GFR determined by?
• Balance of hydrostatic and colloid osmotic forces acting across capillary membrane • Capillary filtration coefficient
• Product of permeability and filtering surface area of capillaries (Kf)
What is the normal GFR for an adult human?
What is the filterability of water?
Explain why albumin molecules do not filter out even though they are small enough.
Albumin molecules are slightly smaller than the filtration pores but have negative charges, thus they are repelled and not filtered out.
What are some of the diseases that lower glomerular capillary filtration coefficient?
• Chronic uncontrolled hypertension and diabetes mellitus
Define minimal change nephropathy
Loss of negative charges on the basement membrane
Distension and dilation of renal pelvis and calyces
What is the mathematical expresion of GFR?
GFR = Kf x Net filtration pressure
GFR = Kf x (Pg − Pb − πg + πb)
What is Kf?
• Capillary filtration coefficient =
• Product of permeability and filtering surface area of capillaries
What are some factors that influence glomerular capillary colloid osmotic pressure?
• Arterial plasma colloid osmotic pressure
• Filtration fraction
What increases glomerular colloid osmotic pressure?
increasing filtration fraction
How does an increase in arterial pressure affect GFR?
Increase → ↑P G → ↑GFR
How does an increase in afferent arteriolar resistance affect GFR?
Increase → ↓P G → ↓ GFR
How does an increase in efferent arteriolar resistance affect GFR?
Increase → ↑P G → ↑ GFR (slightly)
What factors determine renal blood flow?
• Kidneys have 7X the blood flow of the brain but only 2X the oxygen consumption of the brain.
• Much of the oxygen consumed by the kidneys is related to the high rate of active sodium reabsorption.
• Tubular sodium reabsorption is closely related to GFR and rate of sodium filtered.
What is the mathematical expression for renal blood flow?
Renal Blood Flow = (Renal artery pressure − Renal vein pressure)/(Total vascular resistance)
True or false, all blood vessels of the kidney are richly innervated by sympathetic system.
What effect does strong activation of renal sympathetic nerves have on renal arteries, renal blood flow, and GFR?
• Constrict renal arterioles
• Decrease renal blood flow and GFR
True or false, moderate sympathetic activation has little effect on GFR.
What hormones control GFR consistency?
• Norepinephrine and epinephrine (from adrenal medulla):
• Parallel the sympathetic system
• Angiotensin II:
• Endothelial-derived NO
• Prostaglandins and bradykinin
Describe endothelin's role in controlling GFR.
• Released by damaged vascular endothelial cells of the kidneys and other tissues.
• May contribute to renal vasoconstriction leading to reduced GFR
• May contribute to hemostasis when a blood vessel is severed.
• Plasma levels increase in certain disease states associated with vascular injury:
• Toxemia of pregnancy
• Acute renal failure
• Chronic uremia
Describe angiotensin II's role in controlling GFR.
• Preferentially constricts efferent arterioles
• Formed usually in situations associated with decreased arterial pressure or volume depletion.
• Effects on the efferent arterioles will help to increase GFR
• Afferent arterioles seem to be protected against the effects of angiotensin II.
• Due to release of prostaglandins and nitric oxide which are vasodilators
Describe NO's role in controlling GFR?
• Derived from endothelial cells
• Basic level helps maintain renal vasodilation
Describe prostaglandins and bradykinin's role in controlling GFR?
• Vasodilators that may offset effects of sympathetic and angiotensin II vasoconstrictor effects (esp. on afferent arterioles)
What is autoregulation?
refers to the relative constancy of GFR and renal blood flow
What is the primary function of autoregulation?
• Maintain a relatively constant GFR
• Allow precise control of renal excretion of water and solutes.
• Prevent relatively large changes in GFR and renal excretion that would otherwise occur with changes in blood pressure.
What is the normal tubular reabsorption for an adult human?
What is the normal daily fluid excretion for an adult human?
True or false, without autoregulation, a slight increase in blood pressure could increase GFR up to 225 L/day.
What would an increase of GFR up to 225 L/day do to urine flow?
increase urine flow to 46.5 L/day
Describe the macula densa feedback mechanism for autoregulation of glomerular hydrostatic pressure and filtration.
Describe autoregulation via the JG Complex.
↓ GFR → slow flow rate in loop of Henle →
• ↑reabsorpNon of sodium and chloride ions in the ascending limb
• ↓ in sodium chloride at macula densa
↓ in [NaCl] results in a signal from macula densa →
• ↓resistance to blood in afferent arterioles
• ↑ renin release from JG cells (major storage site of renin)
• ↑angiotensin II
• ↑efferent arteriolar resistance
What is A?
What is B?
What is C?
What is D?
internal elastic lamina
What is E?
What is F?
What is G?
smooth muscle fiber
What is H?
What is I?
What are the two compononets of the tubuloglomerular feedback mechanism for autoregulation?
• An afferent arteriolar feedback mechanism
• An efferent arteriolar feedback mechanism
What are the two parts of the juxtaglomerular complex?
• Macula densa in distal tubule
• Juxtaglomerular cells in afferent and efferent arterioles
What is happening in A?
Substance is freely filtered but not reabsorbed
What is happening in B?
Substance is freely filtered but part is reabsorbed.
What is happening in C?
Substance is freely filtered but all is reabsorbed.
What is happening in D?
Substance is freely filtered, not reabsorbed, but is secreted.
What process is indicated by the number 1?
What process is indicated by the number 2?
What process is indicated by the number 3?
What process is indicated by the number 4?
What is A?
What is B?
What is C?
What is D?
What is E?
What is F?
What is G?
What is 1?
What is 2?
What is 3?
What is 4?
What is 5?
What is 6?
What is 7?
What is 8?
What is 9?
What is 10?
What is 11?
What is 12?
As you _________ effective molecular radius, you __________ relative filterability on dextran beads, regardless of electrical charge.