Urinary system part #3 Flashcards
Renin - Angiotensin - Aldosterone Systems Revisited -
Granular cells are stimulated to release renin when - Blood pressure is low ( glanular cells recieves less streatch) , Macula densa cells sense decreased solutes concentrations and signal granular cells to release renin. Sympathetic stimulation increases and neurons direclty stimulates granular cells to secrete renin. Renin converts angiotensinogen to angiotensin I. ACE ( angiotensin converting enzyme) converts angiotensin I to angiotensin II which has 4 effects on renal physiology:
1. Angiotensin II results in constriction of systemic arterioles ( increasing peripheral resistance). 2. Stimulates ADH release. 3. Stimulates the adrenal cortex to release aldosterone which increases the reabsorption of sodium ( and usually water). 4. Stimulates thirst.
Production of Dilute and Concentrated Urine -
One of the functions of the urinary system is to maintain the blood osmolarity (osmolality) at about 300 mOsmoles/liter and it does this by regulating the concentration and volume of the urine.
Osmolality - the amount of solute that is dissolved in a liter of water (not just any solution)
Countercurrent mechanism - substances (water and salt) are flowing in opposite directions. This flow of water and salt maintains a difference in osmolarity between the different limbs of the nephron loop that is always about 200 mOsmoles.
Factors that contribute to the countercurrent mechanism: 1. The geometry of the tubule - the fact that it has descending and ascending limbs. 2. In the descending limb of the nephron loop, not many solutes flow ou of the lumen but water can move freely. This concentrates the filtrate in the tubule. 3. In the ascending limb of the nephron loop, water can’t flow out of the lumen but sodium is actively transported into the surrounding interstitial fluid. This results in a build up of sodium chloride in the space in between the limbs.
4. The longer the loop, the more concentrated the filtrate becomes, especially in the juxtamedullary nephrons. The nephron loop acts a a countercurrent multiplier. 5. The vasa recta are very permeable to salt and water thus acting as a countercurrent exchanger because they exchange both water and solutes. The vasa recta will reach equilibrium with the interstitial space to preserve the concentration of solutes in the blood. The result of the counter current mechanism is that the descending limb concentrates the filtrate and the ascending limb dilutes the filtrate.
Formation of dilute urine -
When the osmolality of the filtrate is about the same as plasma or even lower, dilute urine should be produced to maintain solutes. Dilute urine : Accomplished because the principal cells are impermeable to water unless ADH is present and the ADH level should be low if the osmolarity of the blood is normal because high osmolarity is a stimulus for ADH release.
Formation of concentrated urine -
When the osmolarity of the filtrate is high ( low fluid intake or prefuse sweating), concentrate urine should be produced to remove excess solutes. Concentrated urine: results because of an increase in the permeability of the principle cells to waste because of the presence of ADH. This results in more reabsorption of water in order to conserve water and maintain blood volume. Diuretics - work by decreasing the reabsorption of sodium and therefore water,resultiung in a increased urine flow rate.
Urinanlysis -
Analysis of urine is an indicator of renal function. Normal urine: volume: 1-2 liter per day. Color: yellow to deep yellow depending upon diet (urochrome a breakdown product of hemoglobin provides yellow color). Turdity: clear to start. Odor: mild odor to start, bacteria increase the ammonia odor as it stands pH: 4.6 to 8.0 (6.0 average). Specific gravity: slightly higher density than water 1.001 - 1.035.
Abnormal urine : Hematuria - RBC’s in the urine. Pyuria - WBC’s in the urine. Casts - solutes that take the shape of the lumen of the tubule,WBC casts, RBC casts or epithelial casts. Glycosuria - (aka glucosuria) glucose in the urine (usually due to uncontrolled diabetes mellitus). Ketonuria - ketone bodies in the urine. Albuminuria - albumin - filtration slits are allowing albumin through, could be due to high blood pressure or disease. Microbes - with a “ clean catch” (swabbing with an antiseptic, and catching midstream urine) urine should be sterile, common microbes that casuse urinary tract infections include E.coli or Staphylococcus.
Ureters -
Each renal pelvis empties into a ureter that transports urine to the posterior portion of the bladder. 10 - 12 inches long and internal diameter varies 1mm - 1cm. Located retoperitoneal. 3 Layers : Mucosa - made of transitional epithelium and a lamina propria. Muscularis - made of smooth muscle, the proximal 2/3 has an outer circular and inner longitudinal layer, the distal 1/3 has an additional outer longitudinal layer. Outer adventitia - areolar connective tissue.
Mechanisms for movement of urine: peristalsis, gravity, and hydrostatic pressure all play a role in the flow of urine to the bladder.
Urinary Bladder -
Trigone is the floor of the bladder, the ureteric orifices form two points of the triangle and the internal urethral orifice forms the third. 3 layers : Mucosa - made of transitional epithelium and a lamina propria with rugae. Muscularis aka the detrsor has the same three layers as the distal ureters. (longitudinal, circular, longitudinal). The adventitia is fibrious on the sides but the superior surface is the parietal peritoneum.
Urethra -
Single tube that provides exit route for the urine. Female urethra : 1.5 inches (4 cm) with an external urethral orifice located between the clitoris and the vaginal opening. Mucosa - epithelium changes from transitional to pseudostratified columnar to NKSSE. Circular smooth muscle, fibrous connective tissue.
Male urethra : 6-8 inches. Divides into 3 sections: Prostatic - passes through the prostate gland. Intermediate part - ( also known as membranous urethra) passes through the urogenital diaphram. Spongy (aka penile) passes through the penis.
Micturition -
Aka voiding or urination. For males and females, circular smooth muscle fibers from the internal urethral sphincter and skeletal muscle forms the external urethral sphincter which is voluntarily controlled.
Micturation reflex - When the volume in the bladder reaches about 200ml, streach receptors detect a pressure change and communicate that change via afferent nerve fibers. These signals travel to your cerebral cortex giving us the conscious awarness of a full bladder. The pons is also receiving afferent signals. The visceral efferent motor signals that would complete the reflex allowing us to void are inhibited by sympathetic input and somatic motor input. An increase in parasympathetic activity promotes the motor signals that lead to the detrusor contracting and the internal urethral sphincter relaxing. Whwn it is convenient, we can relax the external urethral sphincter.
Incontinence - Lack of control of the external urethral sphincter which results in urine leakage. Normal in children who are not yet “potty trained” and can develope later in life. “potty training” isnt possible until the pons is developed enough to inhibit the micturition reflex.
