EXCRETION
MAJOR FUNCTIONS:
[see Figure] The paired kidneys receive about 25% of the resting cardiac output.
[Fig. 14-4] The NEPHRON is the functional unit of the kidney.
The nephrons empty into COLLECTING DUCTS, which empty into the RENAL PELVIS.
Each human kidney contains about one million nephrons.
[see Figure] The cortical nephrons have short loops of Henle, whereas those of juxtamedullary nephrons are long.
[Fig. 14-2]
The nephrons are richly supplied with blood via the AFFERENT ARTEROLE to the GLOMERULUS and EFFERENT ARTERIOLE from the GLOMERULUS to the PERITUBULAR CAPILLARIES surrounding the nephron tubules.
[Fig. 14-6] Urine formation involves three processes: FILTRATION, REABSORPTION, and SECRETION.
[Fig. 14-3]
The fluid in Bowman's capsule (CAPSULAR FILTRATE) is essentially filtered blood (i.e. plasma minus proteins). However, the composition of the final URINE is quite different.
[g filtered/day; g in urine/day]
Na+: [600; 6]
K+: [35; 2]
Ca2+: [5; 0.2]
Glucose: [200; trace]
Urea: [60; 35]
Water: [150-180 Liters; 1.5 Liters]
[see Figure]
[see Animation]
[Fig. 14-7]
[see Figure] A great deal of active and passive reabsorption occure in the proximal tubules; however, salt and water reabsorption also occurs further along the nephron.
[see Figure] GLOMERULAR FILTRATION depends on the balance between the hydrostatic and oncotic pressures. The GLOMERULAR FILTRATION RATE (GRFR) averages about 125 mL/min for an adult human male.
CLEARANCE
How much blood has substance X entirely removed from it in one minute?
C = (Ux • V)/Px
Where Ux = urinary [X] e.g. mg/mL, V = volume (rate of urine formation) e.g. mL/min, Px = plasma [X] e.g. mg/mL
Example:
Px = 0.8 mg/mL, Ux = 40 mg/mL, V = 2.5 mL/min
NOTE: Ux • V = E = amount excreted
E = 2.5 x 40 = 100 mg/min
C = 100/0.8 = 125 mL/min
INULIN is a substance which is freely filtered and which is neither secreted nor reabsorbed.
C
INULIN = GFR = mL plasma/minGFR • Px = amount filtered = F
For inulin, F = E
For other substances:
If F>E, there is net tubular reabsorption
If F<E, there is net tubular secretion
[see Figure] Inulin
[see Figure] Glucose
[see Figure] Urea
[see Figure] Penicillin
[see Figure] Glucose is almost entirely reabsorbed at low plasma concentrations; however, at plasma concentrations exceeding the reabsorption (renal) threshold, glucose appears in the urine (= GLUCOSURIA).
PAH is another substance useful for monitoring renal function. PAH is both filtered and secreted. At low concentrations, 90% of injected PAH is removed from the kidneys in a single passage.
C
PAH = effective renal plasma flow (ERPF)Total plasma flow = (RPF) = ERPF/0.9
Renal blood flow (RBF) = RPF/(1 - hematocrit)
Where hematocrit is expressed as a decimal fraction e.g. 45% = 0.45)
[see Figure] WATER BALANCE
The kidneys play a major role in maintaining balance by regulating water loss.
[see Figure] COUNTERCURRENT EXCHANGE
[see Figure] Countercurrent Multiplier
[Fig. 14-15] The ascending limb of the Loop of Henle is impermeable to water.
[see Figure] The water permeability of the collecting duct is low in the absence of and high in the presence of ADH.
[see Figure] Most of the water and salts removed from the Loop of Henle are recycled to the circulating blood via countercurrent exchange with the VASA RECTA. This maintains the steep concentration gradient in the renal medulla.
[see Figure] Human urine concentrations can range from about 100 to about 1200 mOsm. This depends on the hormone ADH from the posterior pituitary.
[Fig. 14-19] In addition to ADH the hormone ALDOSTERONE (from the adrenal cortex) affects the kidney by promoting the exchange of Na+ for H+ and K+ in the distal tubules.
The kidney itself produces RENIN, which converts ANGIOTENSINOGEN to ANGIOTENSIS, which, in turn, stimulates aldosterone secretion.
[Fig. 14-20] Renin is released by the renal JUXTAGLOMERULAR cells in (indirect) response to a decrease in blood volume. The resulting sodium retention acts to restore plasma volume via osmotic uptake.
[see Figure] The MACULA DENSA cells contribute to the mechanism for renin secretion.
[see Figure] The macula densa cells also contribute to regulation of the GFR.
The kidneys also produce the hormone ERYTHROPOIETIN (EPO), which stimulates the production of erythrocytes in the red bone marrow.
[see Figure] Acid - Base Balance
The kidney helps to maintain the proper blood pH by regulating the excretion of H+ and bicarbonate (HCO3-)
[see Figure]
[see Figure] NITROGENOUS WASTES
Most animals excrete most of their excess nitrogen as AMMONIA, UREA, or URIC ACID.
Ammonia is quite toxic to most animals (a concentration as low as 0.05 mM will kill most mammals). Also, ammonia is excreted by diffusion, and a large volume of water (i.e. about 0.5 liters per g of ammonia Nitrogen) is needed to keep the concentration in the excretory fluid below that in the body.
Urea is less toxic than ammonia and only 0.05 liters of water are needed to excrete a gram of urea nitrogen. However, converting ammonia to urea consumes ATP.
Uric acid has a low solubility and only 0.001 liters of water are needed to excreta a gram of uric acid nitrogen. Uric acid is excreted as a white pasty precipitate.
In general, water availability dictates nitrogenous waste excretion.
Aquatic animals normally excrete ammonia.
Frog tadpoles excrete ammonia, but convert to urea after metamorphosis. Mammals also produce urea.
Terrestrial arthropods, reptiles, and birds typically produce uric acid, often switching from ammonia excretion during embryonic development.
Most vertebrates synthesize urea via an ornithine - urea cycle. However, some animals use a uricolytic pathway (converting uric acid to urea).
Humans lack the uricolytic pathway. Some uric acid is still formed from the catabolism of nucleic acids. Uric acid crystals may precipitate in the blood if levels get too high, causing the painful condition known as GOUT.