Micturition and Glomerular Filtration Flashcards
(87 cards)
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
- Erythropoietin
- Active form of vitamin D
- Gluconeogenesis
What are the 3 processes that determine the rates at which different substances are excreted in the urine?
- Filtration
- Reabsorption
- Secretion
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?
Filtration
Name the 4 components of the glomerular filtrate.
- Water
- Ions
- Glucose
- Urea
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
True or false, some low-molecular weight substances are not freely filtered because they are partially bound to proteins.
true
List and describe the layers of the filtration barrier.
Endothelium
• With fenestrae and negative charges
Basement membrane
• With collagen and proteoglycan fibers and strong negative charges
Podocytes
• 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?
180 L/day
What is the filterability of water?
1.0
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
Define hydronephrosis
Distension and dilation of renal pelvis and calyces
What is the mathematical expresion of GFR?
GFR = Kf x Net filtration pressure
or
GFR = Kf x (Pg − Pb − πg + πb)
What is Kf?
- Capillary filtration coefficient =
- Product of permeability and filtering surface area of capillaries
