Table of Contents
Urinary Physiology
FORMATION OF URINE. The kidney produces urine containing metabolic wastes and regulates the composition of body fluids by three main processes: glomerular filtration tubular reabsorption tubular secretion.
A. Glomerular filtration
2. Mechanics of glomerular filtration
b.Two forces oppose glomerular hydrostatic pressure.
b.Two forces oppose glomerular hydrostatic pressure.
c.The effective filtration pressure (EFP) is the net driving force. It is the difference between the forces tending to force fluid out of the glomerulus into Bowman's capsule and the forces tending to move fluid into the glomerulus from Bowman's capsule.Ý
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3. The glomerular filtration rate (GFR) is the amount of filtrate formed per minute in all the nephrons of both kidneys. In males, the rate is about 125ml/min. or 180 L in 24 hours; In females, about 110ml/min.
4. Composition of the glomerular filtrate
Most of the filtrate (99%) is selectively reabsorbed in the kidney tubules. Approximately 85% of the sodium chloride and water and all of the glucose and amino acids in the glomerular filtrate are absorbed in the proximal convoluted tubule. Materials reabsorbed 1. Sodium ions(85%) 2. Chlorine ions and other negative ions
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C. Tubular secretion mechanisms are effective processes that transfer substances out of the blood in peritubular capillaries across the tubular cells into the tubular fluid for elimination in the urine.
1. Substances such as hydrogen,potassium, and ammonium ions, the metabolic end product creatinine and certain drugs (penicillin), are actively secreted into the tubules. 2.The selective tubular secretion of hydrogen and ammonium ions helps regulate plasma pH and the acid-base balance of the body fluids
Urine concentration and dilution mechanisms
B. The countercurrent multiplier system in the loop of Henle
2. As the iso-osmotic (same concentration) glomerular filtrate enters and passes through the loop of Henle, it becomes progressively more concentrated (hyperosmotic) at the bottom of the loop:
a.The descending limb of the loop of Henle is highly permeable to water and relatively impermeable to solutes such as Na and Cl
b.The ascending limb is impermeable to water but relatively (through active transport) permeable toNaCl. This increases the osmotic concentration of NaCl inthe interstitial fluid.
c. Due to the increased osmolarity of the interstitial fluid, water moves out of the descending limb and the loop into the peritubular interstitial fluid by osmosis. Volume of the filtrate decreases as water leaves. Osmotic concentration of the filtrate increases as it rounds the hairpin turn of the loop of Henle.
d. As the filtrate moves along the ascending limb, it looses sodium chloride. Because the ascending limb is impermeable to water, the water cannot follow, and the filtrate becomes progressively more dilute(hypo-osmotic) as it passes upward toward the cortex.That is, the volume of the filtrate does not change, but the concentration declines.
e.Some of the NaCl leaving the ascending limb moves by diffusion into the descending limb from the interstitial fluid, thereby increasing the solute concentration in the descending limb.
e. Conít. Also, new NaCl in the glomerular filtrate continuously enters the tubule inflow to be transported out of the ascending limb into the peritubular interstitial fluid. Thus, the recycling mechanism multiplies the concentration of NaCl.
f.The result is that the interstitial fluid surrounding the loop of Henle contains a high concentration of salt, as does the filtrate in the loop of Henle. A vertical concentration gradient from the cortex (iso-osmotic) to the medulla(hyperosmotic) is maintained.
3. The medullary recycling of urea helps maintain the vertical concentration gradient in the interstitial fluid and the loop of Henle.
a.Urea diffuses passively out of the collecting ducts into the medullary interstitial fluid. Some urea diffuses from the medullary fluid into the descending limb.
b.Thus, urea is recirculated between the collecting tubules and the descending limb. The high concentration of urea in the medullary interstitial fluid contributes to its osmolarity. This increases the osmotic movement of water out of the descending limb and increases the filtrate's concentration of NaCl in the descending limb.
C. The countercurrent exchanger mechanism in the blood vessels and the renal tubules aids the countercurrent multiplier mechanism.
1.The capillaries of the vasa recta function as countercurrent exchangers, because the direction of blood flow around the loop of Henle is opposite to the direction of filtrate flow around the loop.
2.The walls of the vasa recta are permeable to NaCl and water. As blood flows down the descending vessel of the vasa recta, which is parallel to the ascending limb of the tubule, it becomes hyperosmotic as it picks up sodium and chlorine ions and loses some water. At the bottom of the capillary loop, the plasma osmolarity is identical to that of the surrounding interstitial fluid.
3.As the blood flows back up the ascending vessel of the vasa recta, which is parallel to the descending limb of the tubule, salt diffuses back out of the capillary and the water reenters the vessel. The osmolarity of the blood decreases as It flows toward the cortex.
4.Because of the passive exchange of salt and water between the vasa recta and the medullary interstitial fluid and the fact that the blood flow in the vasa recta Is relatively slow, the blood leaves the medulla (through the vasa recta) only slightly hyperosmotic to arterial blood.
1. The hypo-osmotic (dilute)filtrate in the ascending limb of Henle's loop enters the distal convoluted tubule and travels down the collecting duct toward the ureter. As a result of the countercurrent systems,the interstitial fluid surrounding the collecting ducts is hyperosmotic and the necessary concentration gradient for osmosis of water out of the ducts is established.
2. The collecting ducts are impermeable to water in the absence of ADH. Water will leave the collecting ducts by osmosis in the presence of ADH.
E. Excretion of a dilute urine. When ADH is not present, the collecting ducts become almost impervious to water. The dilute urine enters the kidney pelvis to be excreted.
Micturition (urination) Is dependent on parasympathetic and sympathetic innervation as well as voluntary nerve impulses. The expulsion of urine requires the active contraction of the detrusor muscle.
1. The micturition reflex is initiated when distention of the bladder by 300 ml to 400 ml of urine stimulates stretch receptors in the bladder wall.
a. The micturition reflex causes contraction of the detrusor muscle and relaxation of the internal and external sphincters to result in emptying of the bladder.
2. Inhibition of the micturition reflex through voluntary control of the external sphincter is a learned response.
A. Cystitisis inflammation of the urinary bladder. It may be due to a bacterial infection (usually Escherichia coli)that has spread from the urethra or maybe an allergic reaction or the result of mechanical irritation of the bladder.
B. Glomerulonephritis is inflammation of the nephrons, especially of the glomeruli.
C. Kidney stones (urinary calculi) are formed from precipitated salts of calcium,magnesium, uric acid, or cysteine. Small stones may be passed in the urine; larger stones may become lodged in the ureter and cause intense pain (renal colic), which radiates from the kidney area to the groin.
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