Unit 22 - EXCRETION AND FLUID BALANCE
The excretory systems of the body function to remove metabolic wastes generated by the cells. The blood removes the waste products from the cells, and the excretory organs remove these wastes from the blood. In addition, many of the excretory organs play a role in balancing body fluids with regards to essential materials and consequently play a major role in homeostasis.
A. Organs of the excretory system ‑ Organs which have an excretory functions include the following.
l. Lungs ‑ These rid the body of carbon dioxide, one of the major excretory products of the cells.
2. Skin ‑ The sweat glands of the skin function primarily in temperature regulation, but in secreting perspiration to the skin surface, they also perform an excretory role as small amounts of salts and urea are contained in perspiration.
3. Digestive tract ‑ The breakdown products of hemoglobin are excreted via the liver bile into the intestinal tract. In addition, the digestive tract eliminates undigested food materials. This elimination is not considered physiological excretion because these materials have never been in the body proper.
4. Urinary system This is the major component of excretion. It consists of the kidneys and associated structures.
B. Materials excreted
l. End products of metabolism
a. Carbon dioxide.
b. Nitrogenous wastes ‑ These are the end products of protein, creatinine phosphate, and nucleic acid metabolism. They include the following products.
(1) Urea - This most abundant organic waste is produced from the breakdown of amino acids. Deamination leads to ammonia production. Ammonia is extremely toxic and is converted to urea by the liver. Urea is toxic, but not as toxic as ammonia.
(2) Creatinine - This is generated by skeletal muscle through the breakdown of creatinine phosphate for energy.
(3) Uric acid - This is derived from the nitrogenous bases of RNA molecles.
2. Any substance (even essential ones such as glucose) which is in excess of body needs.
C. Excretion and homeostasis ‑ Because even essential substances can be excreted if in too high a quantity, excretion plays a major role in maintaining the proper concentrations for the body fluids. Consequently is a major contributor to homeostasis.
D. Urinary system ‑ This is the major excretory organ system of the body and with the exception of carbon dioxide, it is responsible for the bulk of all other excretion. The organs of the urinary system include the following.
l. Kidneys ‑ These are paired organs which lie against the posterior wall of the abdominal cavity behind the peritoneum (retroperitoneal). The kidneys are the site of urine formation.
2. Ureters ‑ These are muscular tubes which transport urine from the kidneys to the bladder.
3. Bladder ‑ This is a hollow muscular structure which stores urine. It is capable of great distention and can contract with considerable force.
4. Urethra ‑ This is a muscular tube that transports urine from the bladder to the outside.
a. They regulate the composition and volume of the blood directly and the other body fluids indirectly.
b. They excrete wastes from the blood.
c. They regulate erythropoiesis by releasing erythropoietin.
d. They aid in regulating pH.
e. They help to regulate blood pressure by releasing renin which activates the angiotensin system.
f. Assist the liver in detoxification, deamination, and gluconeogenesis.
The kidney' role in erythropoiesis and blood pressure regulation have already been discussed. In this section its role in excretion and blood composition/volume regulation will be discussed. Its role in pH regulation will be discussed in the next section.
2. External anatomy ‑ The kidneys resemble large beans. Each has an indentation termed the hilus from which the ureter exits. There are three layers of tissue which surround the kidney.
a. Renal capsule ‑ This is the innermost layer and consists of a fibrous membrane which protects against infection.
b. Adipose capsule ‑ This surrounds the renal capsule and consists of a mass of fatty tissue. It cushions the kidney against shock and helps to hold it in place.
c. Renal fascia ‑ The outermost layer, it consists of a thin connective tissue membrane which anchors the kidney to the body wall. Sometimes the fascia pulls free from the body wall and the condition that results is termed a floating kidney.
3. Internal anatomy ‑ The kidney consists of two internal layers.
a. Medulla ‑ This is the innermost layer and is composed of 5 to l4 triangular shaped masses known as the renal pyramids. The broad bases of the pyramids face outward while the apex of each (termed renal papillae) are directed toward the center.
b. Cortex ‑ This is the outer layer of the kidney. It dips down between the medullary pyramids to form structures termed the renal columns.
c. Each renal pyramid empties into a funnel shaped space known as the minor calyx. The minor calyces become confluent to form even larger funnel shaped structures known as the major calyces. The major calyces eventually become confluent to form a large space known as the renal pelvis. The renal pelvis empties into the ureter.
4. Microscopic anatomy ‑ Each kidney consists of roughly one million structures termed nephrons. The nephron is the site of urine formation and consequently represents the functional unit of the kidney. Structurally, each nephron consists of a tubule and associated vascular components. Each one has the following components.
a. Glomerular (Bowman's) capsule ‑ This is the beginning of the kidney tubule. It is a double walled cup which surrounds a ball of specialized capillaries termed the glomerulus. These capillaries are fenestrated, making them very permeable. The outer layer of the cup (parietal layer) is composed of a squamous epithelium while the innermost layer (visceral layer) is formed by special epithelial cells termed podocytes. These cells have special filtration slits in which which the basement membrane is directly exposed. This makes the membrane very permeable. The visceral layer of the capsule, the basement membrane, and the endothelium of the glomerulus form the endothelial‑capsular membrane or filtration membrane. It is across this membrane that plasma is filtered.
b. Proximal convoluted tubule ‑ This is the highly twisted tubule which Bowman's capsule opens into.
c. Loop of Henle ‑ The proximal convoluted tubule straightens out to form a long hair pin shaped segment known as the loop of Henle. The first segment is termed the descending limb while the next segment is termed the ascending limb.
d. Distal convoluted tubule ‑ This is the last segment of the renal tubule. The ascending limb of the loop of Henle opens into this highly twisted segment.
e. Collecting ducts ‑ These are not part of the nephron tubule but the distal convoluted tubule empties into them. Each collecting duct may have a number of nephrons emptying into it.
f. Types of nephrons ‑ Two types of nephrons can be recognized.
(l) Cortical nephrons ‑ These have their glomeruli in the outer regions of the cortex. They lack a loop of Henle and usually do not penetrate into the medulla. About 85% of the nephrons are of this type.
(2) Juxtamedullary nephron ‑ The glomerulus usually lies in the cortex close to the border of the medulla. These nephrons have a loop of Henle which penetrates deep into the medulla. Major role is in water balance.
5. Circulation in the kidney ‑ A single renal artery enters each kidney and divides as follows.
Renal artery ‑ segmental arteries - lobar arteries - interlobar arteries ‑ arcuate arteries ‑ interlobular arteries ‑ afferent arterioles ‑ glomerulus - efferent arterioles ‑ peritubular capillary beds
The unique aspect of this pattern is the presence of two capillary beds (glomerulus and peritubular) connected by an arteriole (efferent). The peritubular capillary bed surrounds the nephron tubule. Juxtaglomerular nephrons have a long capillary loop termed the vasa recta which parallels the loop of Henle. The significance of this capillary arrangement will become clear when the function of the nephrons are discussed. The drainage of the peritubular capillary bed parallels the arterial supply.
6. Nephron function ‑ urine formation ‑ The formation of urine by the nephron involves three different processes, filtration, reabsorption, and secretion. Filtration occurs across the glomerular walls. Nutrient reabsorption occurs primarily in the proximal convoluted tubule. Secretion occurs primarily in the distal convoluted tubule. The loop of Henle and collecting duct regulate water and sodium loss in the urine.
a. Filtration ‑ This is where part of the plasma is transferred across the glomerular wall into Bowman's capsule. This is essentially the same process that occurs across every capillary wall, but it is much more efficient in the glomerulus for the following reasons.
(l) The glomerular capillary membrane is much more permeable than the average capillary. This is because it has completely open pores, where there are no cell, only the basement membrane.
(2) The surface area is very large, a total of about l.5 square meters per kidney
(3) The blood pressure in the glomerulus is high, about 60 mm Hg. The reason for this is that the efferent arteriole is smaller in diameter than the afferent arteriole. This pressure is opposed by pressure of fluid found in the space between the parietal and visceral layers of Bowman's capsule. This is about 20 mm Hg. A second opposing force is the osmotic pressure of the unfiltered blood which tends to pull fluid back into the glomerulus. It is about 30 mm Hg. Thus the total opposing force is equal to about 50 mm of Hg. which leaves a net filtering pressure of l0 mm Hg. which is about three times as high as the typical capillary bed.
(a) GFR (glomerular filtration rate) ‑ About 25% of the cardiac output flows through the kidneys. This is equal to l200 ml of whole blood or about 650 ml of plasma. Of this, about 20% is filtered across the glomerulus into the nephron tubule. This means that l25 ml of filtrate (initial urine) is formed per minute. This figures out to l80 liters per 24 hour day.
(b) This rate can be varied by regulating glomerular pressure. This is accomplished by vasoconstriction and vasodilation of both the afferent and efferent arterioles which receive sympathetic innervation.
(c) The composition of the initial filtrate is essentially the same as the plasma minus plasma proteins which are too large to cross the glomerulus.
b. Reabsorption ‑ This is the second major part of urine formation. While l80 liters of initial urine are formed each 24 hours, only about l liter of final urine is voided. The remainder is reabsorbed, back into the peritubular capillary bed. While filtration is non‑selective, reabsorption is a highly selective process.
(l) Most solutes are reabsorbed by active transport, cotransport, and diffusion. These processes are carried out by the epithelial cells that compose the nephron wall. The solutes are moved back into the blood in the peritubular capillary bed.
(2) Water is absorbed passively by osmosis. The water follows the concentration gradient created across the nephron tubule wall by the transport of the solutes, especially sodium.
(3) The nephron "knows" how much of each substance should be transported back into the blood. This is because most substances have a threshold value. This is the concentration in the plasma that must be exceeded before a substance will appear in the final urine. Another way of stating this is that if the concentration of a substance is below threshold, it will be completely reabsorbed back into the blood and none will appear in the urine. As a rule, the threshold value for nutrients is high while that for wastes is low.
(4) The threshold value equates to the tubular maximum (Tm) which is the maximum amount of a substance that a tubule can reabsorb. This maximum is established by the number of transport systems found in the membranes of the epithelial cells. If the concentration in the plasma being filtered exceeds the tubular maximum, then the transnport systems will be saturated and the substance will not be transported completely back into the blood. The excess (that above threshold) will pass out with the urine.
c. Secretion ‑ Here substances are actively secreted from the blood into the nephron tubule. While legions of materials can be reabsorbed from the filtrate, only a relatively few can be actively secreted into it. These include hydrogen ion, potassium, ammonia, creatinine, and certain drugs. Tubular secretion therefore rids the body of certain materials which cannot be removed efficiently enough by simple filtration, and also helps to regulate plasma pH.
d. Summary ‑ The processes of filtration, reabsorption, and secretion not only rid the body of metabolic wastes, but also maintain the proper range of essential nutrients such as glucose, amino acids, vitamins, and electrolytes. As such the kidney is the most significant organ in the body for the proper maintenance of body fluid homeostasis.
7. Composition of final urine ‑ The final volume of urine varies from about 600 ml to about 2500 ml per day, with l000 to l200 ml being about average. The variation in volume is largely a function of the amount of water present which is turn is a measure of the state of hydration of the body tissues. The organic waste levels are primarily a function of the metabolic activity of the body. Other constituents will vary with diet and the health of the body. A table on page 683 of your text lists some of the major constituents of urine. As the composition of urine is a major indicator of the metabolic state of the body, urine tests are powerful clinical diagnostic tools. Appendix E, page 1176 of your textbook lists urine constituents and their values.
F. Micturition - urination ‑ This is the elimination of urine from the bladder. Once a sufficient quantity of urine has accumulated in the bladder stretch receptors located in the bladder wall will become activated. These receptors can initiate an autonomic reflex (mediated by the parasympathetic division) which will result in bladder contraction along with relaxation of the internal sphincter (smooth muscle). There is however an external sphincter in the urethra which is composed of skeletal muscle and therefore under voluntary control. It must be relaxed by conscious thought before the urine can pass from the body.
G. Water and electrolyte balance ‑ One of the major homeostatic processes of the body is the maintenance of proper water and electrolyte levels. The levels of both are closely related and the kidney plays an important role in their regulation.
l. Water ‑ This is the single largest constituent of the body and accounts for 45 to 75 percent of the body weight. Water levels are dependent upon intake and output.
a. Water intake ‑ The sources of body water, on a per day basis, are as follows.
(l) Ingested liquids (l600 ml).
(2) Foods (700 ml).
(3) Metabolic water (200 ml).
b. Water output ‑ The following represent the major losses of water on a per day basis.
(l) Kidney (l500 ml).
(2) Skin ‑ perspiration (200 ml).
(3) Lungs ‑ respiration (300 ml).
(4) GI tract ‑ defecation (200 ml).
c. Normally, intake should equal output, but it is rare that both would be exactly the same. In order to avoid loss of homeostasis, some mechanism must be available to maintain constant water levels. This mechanism involves the kidney.
2. Fluid compartments ‑ The body is divided by selectively permeable membranes into fluid compartments. There are two major compartments, the intracellular (fluid within the cells) and the extracellular(fluid outside of the cells). The extracellular can be further divided into the tissue fluid (interstitial fluid) and the plasma (within the circulatory system).
a. All of these compartments are separated by selectively permeable membranes through which water can move osmotically.
b. There is a dynamic balance between the compartments. If the plasma increases in solute concentration, water will move osmotically from the tissue fluid into the plasma. This in turn will increase the solute concentration of the tissue fluid and result in the osmotic movement of water out of the cells.
c. Because of these osmotic shifts between compartments, the kidney, which can only directly regulate water and solute levels in the plasma, can indirectly regulate fluid levels in the other compartments.
3. Sodium and water reabsorption
a. Relationship between sodium and water reabsorption ‑ Reabsorption of sodium from the filtrate in the nephron tubules is an active process which creates an osmotic gradient that water follows. Sodium is the most abundant cation in the body and it is sodium transport that is largely responsible for the bulk of water reabsorption. The bulk of sodium is reabsorbed in the proximal convoluted tubules and thus it is here that the bulk of water reabsorption also occurs.
b. Regulation of sodium reabsorption
(l) Aldosterone ‑ This is a steroid hormone produced by the adrenal cortex. It causes the kidney tubules, the distal portion specifically, to increase sodium transport and thus increases sodium reabsorption. This of course increases the osmotic gradient for water and hence more water is reabsorbed, increasing the volume of water in the body. Therefore, sodium retention always results in water retention.
(2) Atrial natriuretic factor ‑ This is a hormone released by the atrial myocardial cells in response to stretch. This factor has several different effects, all of which tend to reduce sodium and fluid volume.
(a) It inhibits the release of renin by the kidney and therefore modifies the activity of the angiotensin system. This results in reduced aldosterone, reduce sodium, reduced water retention, and reduced blood pressure.
(b) It directly inhibits the secretion of aldosterone from the adrenal cortex and therefore reduces sodium reabsorption.
(c) It also acts directly on the kidney to increase sodium excretion.
c. Water reabsorption ‑ It is rare for the exact intake and output of water from the body to be identical. Consequently there must be some method to compensate for deviations between output and input. This is accomplished by the kidney which is the major fluid balancing organ in the body. Evidence indicates that the kidney originally evolved as a water balancing organ and took on excretory functions secondarily in the transition to land living. If the body has an excess of water, the kidney will produce a copious, dilute urine which is hypotonic to the plasma and thereby eliminate excess water. If the body is dehydrated, the kidney will produce a low volume, concentrated urine which is hypertonic to the plasma. Finally, if body water is in balance, the kidney will produce a urine which is isotonic to the plasma and thereby will not effect the concentration of fluids in the body. The mechanism by which this is accomplished is as follows.
(l) 80% of water reabsorption occurs in the proximal tubules. This results in a 65 to 70% reduction in volume, but no change in concentration as water is simply following the osmotic gradient created by sodium transport and consequently the urine concentration remains isotonic with that of the plasma. It is the variation in the reabsorption of the remaining 20% which results in a hypo, hyper, or isotonic urine.
(2) Hairpin, counter‑current, multiplier system ‑ This is the mechanism by which the tonicity of the urine and thus the tonicity of the plasma are fine tuned. The microanatomy of the kidney is very important in this mechanism. The ascending and descending limb of the loop of Henle are arranged in such a manner that they lie parallel and very close the collecting duct. The vasa recta lies in close proximity to the loop of Henle. The loop of Henle establishes a concentration gradient in the interstitial fluid of the medulla through which the collecting duct must pass and the vasa recta maintains the concentration gradient. It is this concentration gradient which will be reabsorbed from the collecting ducts. The mechanism by which this is accomplished is as follows.
(a) Upon entrance into the descending limb, the urine is isotonic to the plasma with an osmotic concentration of 300 milliosmoles (mOsm).
(b) As the urine moves down the descending limb it becomes more concentrated. This is due to a loss of water to the interstitial fluid without a concurrent loss of solutes. This is because the descending limb is highly permeable to water but relatively impermeable to salts. Actually, small amounts of salts diffuse into the descending limb increasing its concentration. At the bottom of the loop, urea also diffuses inward. The results of this loss of water coupled with salt and urea gain pushes the tonicity of the urine at the bottom of the limb to about l200 mOsm.
(c) As the urine moves upward in the ascending limb, it begins to lose its concentration. This is because chloride is actively transported outward and sodium passively follows it due to the electrical attraction. Water does not osmotically follow because the ascending limb is impermeable to water. The salt which is actively transported outward, along with urea which is reabsorbed from the collecting duct, is what establishes the concentration gradient. Small amounts diffuse into the descending limb are constantly recycled.
/l/ As the urine is most concentrated when it first enters the ascending limb, the most salt is transported outward here, and thus the interstitial fluid is most concentrated here. As the urine moves upward there is progressively less salt to transport outward and thus the interstitial fluid also becomes less concentrated.
/2/ The urea which diffused inward at the bottom of the loop stays in the urine until it passes into the bottom portion of the collecting ducts. This is because the ascending limb, distal convoluted tubule, and upper region of the collecting ducts are all impermeable to urea. When the urea reaches the lower portion of the collecting duct, some of it will diffuse out and be recycled there by increasing the concentration of the urine in the bottom of the loop. The degree to which urea contributes to the concentration gradient depends upon the amount of water being reabsorbed from the collecting duct. The greater the water reabsorption, the more urea will contribute(positive feedback).
/3/ By the time the urine reaches the top of the ascending loop it is about l00 mOsm, hypotonic to the plasma.
(d) Role of the vasa recta ‑ This loop in the peritubular capillary bed parallels the loop of Henle. Blood flow in the vasa is in the opposite direction of urine flow in the loop of Henle (countercurrent). It functions to maintain the medullary concentration gradient developed by the loop of Henle in the following ways.
/l/ Blood passing through the descending limb of the vasa recta picks up salts by diffusion and therefore parallels the concentration of the loop of Henle and the interstitial fluid. It does not carry these salts away because as the blood make the turn into the ascending limb, the countercurrent flow of blood permits the salts to diffuse right back out into the interstitial fluid and the descending limb of the vasa recta. Consequently, when blood reaches the top of the ascending limb, its concentration is only slighlty higher than it was when it entered, and the medullary concentration gradient is not disturbed by salt removal.
/2/ It removes the water reabsorbed from the collecting ducts. The osmotic pressure created by the plasma proteins which were not filtered draws in the water reabsorbed from the collecting ducts and thereby prevents this water from diluting the concentration gradient. While the osmotic pressure of blood leaving the vasa recta is only slightly higher than that of the blood entering, the volume of flow is greater, representing the water which as been reabsorbed.
(e) The last aspect of the process occurs in the collecting ducts. Flow in the tubule is counter to that of the ascending loop and thus the tubule passes through the concentration gradient established in the interstitial fluid. Several events occur in the collecting duct.
/l/ Water is reabsorbed back into the interstitial fluid and returned to the blood. When urine enters the collecting duct it is extremely dilute. If the collecting duct is freely permeable to water then water will osmotically leave the tube as the urine passes through the concentration gradient.If the tube is not permeable to water then the urine will pass through unchanged and a very hypotonic urine of large quantity will be produced. In reality, there is always some water reabsorbed from the urine.
/2/ Urea is concentrated during its passage and some may be reabsorbed to aid the development of the concentration gradient. Urea is not actively transported, but is concentrated in the tubule by the selective reabsorption of water. In the initial segment of the collecting duct large quantities of water are reabsorbed, but urea does not follow because this segment of the tube is impermeable to urea. This results in a concentrating of urea in the tube. Towards the end of the collecting duct, it does become permeable to urea and some will diffuse into the interstitial fluid and the bottom of the loop of Henle thereby enhancing the concentration gradient. The amount which diffuses out of the tubule largely depends upon the urea concentration in the tubule which in turn depends upon the amount of water leaving the tubule. Therefore, when water reabsorption is high, urea concentration is increased, more diffuses out, and the interstitial concentration gradient is increased thereby promoting further removal of water from the collecting duct.
/3/ Sodium is actively transported from the collecting duct to the interstitial fluid which can further reduce the concentration of the final urine.
(f) ADH (antidiuretic hormone) ‑ The amount of water reabsorbed from the collecting duct and there the final urine's volume and tonicity are dependent upon the permeability of the tubule to water. This in turn is controlled by ADH. ADH is released by the neurohypophysis, and endocrine gland located at the base of the brain, in response to changes in the blood's osmotic pressure.
/l/ If the blood's osmotic pressure is increasing this is a signal that water levels in the body are too low. ADH levels will increase which in turn increase the permeability of the collecting ducts. A large quantity of water will be reabsorbed and a small volume, hypertonic urine will be produced.
/2/ If the blood's osmotic pressure begins to drop, indicating too much water, ADH secretion will decrease, the collecting ducts will become less permeable to water, and a large volume of hypotonic urine will be produced thereby ridding the body of the excess water.
d. Summary of water regulation
(l) 80% of the total water reabsorption occurs in the proximal convoluted tubules. The counter‑current‑ADH system is responsible for the remaining amount.
(2) The transport of chloride and sodium from the ascending loop creates an osmotic gradient in the interstitial fluid. This gradient is enhanced by the reabsorption of urea from the collecting ducts.
(3) The vasa recta maintains the concentration gradient established by removing the water reabsorbed from the collecting ducts.
(4) The collecting ducts pass through this osmotic gradient. ADH regulates the permeability of the collecting duct.
(5) If the tubule is permeable (high ADH) then water will be reabsorbed and the final urine will be hypertonic to the plasma.
(6) If the tubule is not permeable (low ADH) then water will not be reabsorbed and the urine produced will be hypotonic to the plasma and of copious volumes.
(7) The regulation of water levels in the plasma by the kidney will indirectly regulate the water concentration of the interstitial fluid and therefore the intracellular fluid. In this way the kidney is able to maintain proper water balance under normal circumstances.
2. Electrolytes ‑ The regulation of sodium was discussed along with water balance. Other important electrolytes and their regulation include the following.
a. Chloride ‑ This negative ion is regulated indirectly by aldosterone. Chloride usually follows sodium because of the electrical attraction and as aldosterone is the principal regulatory agent of sodium it also regulates chloride.
b. Potassium ‑ This is the most abundant cation in the intracellular fluid. Potassium levels are largely controlled by kidney excretion which in turn is regulated by aldosterone. Aldosterone has exactly the opposite effect on potassium that it does on sodium. It promotes the secretion of potassium, mainly at the distal tubules.
c. Calcium and phosphate ‑ These ions are regulated by several different endocrine hormones and their regulation is linked by these hormones much as are sodium and potassium. Their regulation will be discussed in detail in the section on the endocrine system.
l. Diabetes insipidus ‑ This is the condition whereby either ADH is absent or in low concentration. Little or no water is reabsorbed and individuals with extreme deficiency may excrete 24 liters of urine per 24 hour period. It can be controlled by giving an ADH inhalant.
2. Glomerulonephritis ‑ This is an inflammation of the glomeruli. It is frequently caused by the deposition of antigen‑antibody complexes in the glomeruli. Usually the antigen source is a streptococcal infection somewhere else in the body. The inflammatory response leads to blockage of many nephrons and increased permeability of the glomerulus. In severe cases total renal failure may occur.
3. Kidney stones ‑ These are crystals which form from several different substances in the renal pelvis. They may pass down the ureter to the bladder causing great pain and in some cases may block urine flow.
4. Cystitis ‑ This is inflammation of the bladder, usually due to an infection, although chemical and physical factors can also cause it. Urination is frequent and painful. Blood may also appear in the urine.
I. Diuretics ‑ These are substances that increase the volume of urine. They usually work by either blocking the release of ADH or by blocking its effect on the collecting duct. Alcohol depresses the nervous system and blocks the release of ADH which is manufactured by nerve cells. Caffeine blocks the effects of ADH upon the kidney tubules. Diuretics may be given to people who suffer from high blood pressure to reduce the total fluid volume and therefore the pressure.
J. Dialysis ‑ People who suffer total renal failure must have either dialysis or a transplant. In dialysis the blood is passed through a tube which permits the movement of urea and other small molecules out but blocks the passage of proteins and blood cells. The tube runs through a bath which contains the proper concentrations of vital plasma constituents such as glucose and vitamins. These prevent the diffusion outward of valuable plasma components.
K. Effects of aging ‑ With age there is a progressive decrease in the GFR. Renal blood flow decreases from l0 ml per minute at ages 20 ‑ 45 to roughly 5 ml per minute at ages 80 ‑89. There is also a decrease in the ability of the tubules to concentrate urine (decreased sensitivity to ADH) and the results are increased volumes of urine with increased frequency of micturition.