II. TUBULAR REABSORPTION--returns substances from the filtrate to the blood. Normally 99% of the original filtrate is reabsorbed. Along with almost all of the water, solutes reabsorbed include:
Glucose (all) Lactic acid
Amino acids (all) Water soluble vitamins
Urea (some)
Small proteins (all)
Ions: Na+, K+, Ca++, Cl-, HCO3-, HPO4- (in varying amounts)
In all parts of the tubule, water is reabsorbed only passively by osmosis.
Solutes are reabsorbed by various active and passive processes. Epithelial cells that form the tubule carry on reabsorption.
A. PCT--these cells have microvilli (brush border) to increase the surface area and they do the majority of the reabsorbing. From the PCT on other parts are responsible for fine-tuning.
1. Sodium--large amounts of Na+ must be reabsorbed. In the PCT this is done by a form of active transport known as secondary active transport. Here is how it works:
Na+ is in fairly high concentration in the filtrate inside the tubule. Tubule cells have channels that make them permeable to Na+ so that it can move in by simple diffusion. Remember that simple diffusion can work only where there is a concentration gradient, and the steeper the gradient the better it works. If Na+ were allowed to build up in the tubule cells, simple diffusion would soon end.
To keep the Na+ moving out of the filtrate, it is necessary to quickly remove Na+ from the tubule cells into the extracellular fluid outside the tubule. This is done by a regular (primary) active transport mechanism, the Na+/K+ pump. The energy is all at the basal aspect of the tubule cells, but this active removal of Na+ maintains the steep concentration gradient required for diffusion to continue. This is secondary active transport.
The Na+ pumped into ECF diffuses into peritubular capillaries (back into the blood). As Na+ is reabsorbed actively, water follows passively by osmosis. As water leaves the filtrate, other ions become more concentrated, so K+, Cl-, and HCO3- can be reabsorbed by passive diffusion.
2. Nutrients--also reabsorbed in the PCT. 100% of these:
Small proteins--by pinocytosis
Glucose
Amino acids secondary active transport
Lactic acid
The active transport of these nutrients is still based on the Na+ gradient. This time the carrier is called a Na+ symporter because it transports the nutrient together with a Na+ ion into the tubule cell. (Symporters transport 2 or more substances in the same direction.) Glucose is transported as follows:
Two Na+ ions plus one glucose attach to a symporter. Glucose attached to a symporter this way can be pulled into the tubule cell AGAINST the glucose concentration gradient because the pull for the Na+ is so strong. Glucose leaves the tubule cell on the opposite side by facilitated diffusion and diffuses into peritubular capillaries. Other Na+ symporters specific to other nutrients plus Na+ work in a similar way.
Normally 100% of all nutrients reabsorbed this way are removed from the filtrate, but there is an upper limit to what can be transported in the time available (transport maximum). If blood concentration exceeds the amount the symporters can remove, the rest spills over into the urine--renal threshold is the term for the level at which this begins ( for glucose it is 200). When excess solute spills into the urine, water follows, so urine volume increases.
By the end of the PCT, the following have been reabsorbed:
100% filtered nutrients 65% filtered water
80-90% filtered HCO3- 50% “ Cl-
65% " Na+ 65% “ K+
B. Loop of Henle--this part reabsorbs:
30% filtered K+
20% " Na+ in ascending limb
35% " Cl-
15% " water--in descending limb
Symporters in the ascending limb absorb together by active transport (1 Na+, 1 K+, and 2 Cl-) as a group. Water DOES NOT follow out of the ascending limb because the cells of the ascending limb are impermeable to water.
C. DCT and collecting ducts--absorption of Na+, Cl- and water continue. Toward the ends of the DCTs and in the collecting ducts reabsorption is regulated by 2 hormones:
1. RAA (renin-angiotensin-aldosterone) system—decreased blood flow through the kidney results in secretion of the enzyme renin. This leads to the secretion of aldosterone by the adrenal cortex. W ithout aldosterone no Na+ is reabsorbed in the collecting ducts. As aldosterone level rises, more Na+ is reabsorbed by the principal cells.
2. ADH (antidiuretic hormone)—released by posterior pituitary—increases the water permeability of the principal cells. With no ADH they are impermeable to water—the more ADH released, the more permeable they become, so water can follow reabsorbed Na+.
3. Atrial natriuretic peptide—effect is to decrease reabsorption of Na+ and water directly and also to suppress secretion of aldosterone and ADH. This increases excretion of Na+ and water in the urine.
Obligatory water reabsorption--90% of filtered water is reabsorbed tied to the reabsorption of solutes. This occurs in the PCT, descending limb and first part of the DCT and cannot be regulated.
Facultative water reabsorption--the remaining 10% occurs in the last part of the DCT and collecting duct and is regulated mainly by ADH.
III. TUBULAR SECRETION--removes materials from the blood and adds them to the filtrate, eliminating certain substances and helping control blood pH. Things that can be secreted:
K+ ions
H+ ions
Ammonium ions (NH4+)
Creatinine
Penicillin and certain other drugs
Tubular secretion is an active process.
A. K+ ions--most of these are automatically reabsorbed from the filtrate, but if excess K+ is present it is secreted into the collecting ducts by the principal cells. Excess K+ causes cardiac arrhythmias, so an effective way to remove K+ is necessary.
B. H+ ions--we must maintain normal blood pH of 7.35-7.45 in spite of the fact that normal metabolic reactions produce more acids than bases. Cells of renal tubules can act in 3 ways to raise the blood pH.
1. Secretion of H+ ions into the filtrate by intercalated cells
2. Reabsorption of filtered HCO3- to buffer the H+ ions in the blood
3. Production of new HCO3- ions
Should the blood shift to the alkaline side, less HCO3 ions are reabsorbed and H+ secretion slows.
C. NH3 and NH4+ ---(ammonia & ammonium ions)--these are waste products of protein breakdown. Most ammonia is immediately converted to less toxic urea by the liver. Any ammonia in the blood needs to be quickly removed and secretion is the way. Both ammonia and urea are excreted in urine.
FIGURE 26.20 P.
1020 SUMMARY OF FILTRATION, REABSORPTION & SECRETION
DILUTION AND CONCENTRATION OF URINE
The amount of water excreted in urine and the concentration of solutes in urine varies greatly according to conditions at any given time.
Urine dilution—urine contains more water and less solutes than blood—occurs when fluid intake exceeds need. Dilute urine is easily produced. Glomerular filtrate begins isotonic to plasma. As it passes through the tubule, more ions are reabsorbed than water, so filtrate entering the DCT is always dilute (hypotonic) compared to plasma. If ADH is low or absent, no further water is reabsorbed and the urine that is eliminated may be up to 4X more dilute than plasma (hypo-osmotic urine).
Urine concentration--when water intake is low the kidneys must still eliminate wastes and excess ions while conserving water. Under these conditions, a concentrated (hyperosmotic) urine is produced. This is a more complicated process. Production of hyperosmotic urine requires a high concentration of solutes in the interstitial fluid of the medulla of the kidney (remember that water can leave the tubule ONLY by osmosis--moving from higher WATER conc. to lower WATER conc.).
The conc. of solutes in the tissues of the medulla is about 4X the conc. of solutes in plasma, so this provides the ability to produce urine 4X as concentrated as plasma. This high concentration of solutes in the medulla is achieved by 2 mechanisms:
1. Countercurrent multiplier mechanism---
Loop of Henle:
2 tubes parallel to each other
with flow in opposite directions
Since the descending limb is impermeable to ions, none of them can leave the filtrate. However, this portion is quite permeable to water, so water moves out. The filtrate becomes extremely high in solute concentration at the bottom of the loop. Then, as the filtrate goes up the ascending limb, the higher the solute concentration the more the active transport mechanisms in this portion can pump out. (The more there is, the more you can pump.) This puts large amounts of ions into the renal medulla, raising the solute concentration of the ECF there. These active transport pumps are called symporters because they can only pump Na-K-2Cls together as a group. Water cannot follow because the ascending limb is impermeable to water (always).
2. Urea recycling---to further increase the concentration of solutes in the medulla, cells of the collecting ducts in the deep medulla area are permeable to urea, so some leaks out and a certain amount collects in the medulla area. (Most of the urea, a waste, is excreted).
If the need is to produce a concentrated urine, ADH is secreted and cells of the DCT and collecting duct become highly permeable to water. As the filtrate passes through the medullary portion of the collecting duct, water can move out by osmosis to produce a urine up to 4X as concentrated as plasma. This is possible because the high level of solute surrounding the tubule can draw that much water out by osmosis if the permeability allows.
If we need to produce a dilute urine, the cells of the DCT and collecting tubule can be made impermeable to water, so no water can leave the filtrate regardless of the condition of the medullary ECF.
EVALUATION OF KIDNEY FUNCTION
1. Blood urea nitrogen (BUN)--measures nitrogen in blood to determine how well the kidneys are eliminating nitrogenous waste. Normal value 8-26 mg/deciliter
2. Plasma creatinine--creatinine is an end product of energy
production in skeletal muscle that is also eliminated by the kidneys. Normal value 1.5 mg/deciliter
3. Renal plasma clearance--this shows how effectively the kidneys remove a substance from blood plasma.
4. Urinalysis—study of the volume and physical, chemical, & microscopic properties of urine. Normally, 1000 - 2000 ml of urine are produced daily. Urine is usually about 95% water with only tiny traces of proteins or nutrients present (or none at all). Waste products, excess ions, drugs, & hormones make up the remainder.
SEE TABLE 26.5 & 26.6
P. 1021
HEMODIALYSIS--if the kidneys are unable to function, an artificial device can take over. Dialysis means the separation of large molecules from small ones through a selectively permeable membrane. The artificial kidney machine performs hemodialysis by removing a portion of the patient's blood and bringing it in contact with a dialysis membrane. A fluid, the dialysate, on the other side of the membrane contains no waste and a normal conc. of all ions. Wastes and excess ions cross out of the blood. Drawbacks:
1. Blood cells can be damaged
2. Anticoagulants may cause bleeding problems
3. Time-consuming
URETERS--carry urine to the urinary bladder. Urine is not changed in content from the calyces on. It moves down the ureters by peristalsis, gravity and hydrostatic pressure. A layer of mucus prevents much contact with the walls, so diffusion does not occur.
The ureter enters the urinary bladder from the posterior aspect. Once the bladder begins to fill, pressure compresses the openings of the ureters and prevents backflow.
URINARY BLADDER--hollow muscular organ for the storage of urine. It is located in the pelvic cavity and held loosely by folds of peritoneum. It is anterior to the rectum in males and to the vagina in females.
On the lower aspect of the bladder, a small triangular area, the trigone, contains the openings of the ureters and the urethra.
The wall of the bladder is of 3 layers:
1. Mucosa (inner)--transitional epithelium specialized for stretching and a lamina propria
2. Detrusor muscle--3 layers of smooth muscle
As the urethra leaves it passes through 2 sphincters, an inner involuntary and an outer voluntary. Urine is expelled from the bladder by the act of micturition (urination). It occurs due to both voluntary and involuntary nerve impulses. When the bladder becomes about 1/4 to 1/2 full, stretch receptors send impulses to the spinal cord and the resulting micturition reflex causes contraction of the detrusor muscle and relaxation of the internal sphincter (this is parasympathetic). In adults the cortex must also direct relaxation of the external sphincter for urination to take place. If urination does not occur, the reflex ends after a few minutes and is repeated later.
URETHRA--carries urine from the urinary bladder to the outside. In females it is located between the symphysis pubis and the vagina, with a length of about 1 1/2 inches. This arrangement makes females prone to urinary tract infections. In males the urethra is longer (about 8") and passes through the penis.