Excretion is the elimination of waste products from the body. We excrete substances mostly in the urine or the feces, but also by sweat, milk, other body fluids, and even in our hair. Excretion is important from an environmental viewpoint for two reasons: (1) It is a means by which the body eliminates toxic substances; and (2) excretory organs may themselves be susceptible to the action of toxic substances, damaging their ability to maintain homeostasis.
In this section we focus on urinary excretion, that is, on kidney function. The kidneys are one of the most fascinating organs from an engineering point of view. They are at the center of several important control mechanisms, and their excretory function illustrates several important principles of mass transfer. The kidneys have a wide variety of functions, like the liver, but less so. Only the first four of these functions have to do with excretion:
• Regulation of water and electrolyte balance, including blood pH.
• Removal of metabolic waste products, such as nitrogen wastes, from the blood.
• Removal of foreign chemicals from the blood.
• Control of blood pressure and volume through the secretion of renin, which ultimately affects water and sodium excretion (Section 9.5.2).
• Control of red blood cell formation through the secretion of erythropoietin (Section 9.6).
• Control of calcium levels by formation of the active form of vitamin D.
• Conversion of amino acids into glucose during prolonged fasting.
Wastes from the metabolism of carbohydrates and fats produces CO2 and metabolic water, which do not need to be excreted by the kidney. However, the breakdown of nitrogenous compounds such as amino acids produces ammonia, which would be toxic if accumulated in the blood. The liver converts the ammonia to urea, which is relatively nontoxic. We excrete about 21 g of urea per day, plus small amounts of ammonia and uric acid. Some animals excrete nitrogen mostly as uric acid, which is insoluble. This further improves water conservation. The white material in bird droppings is mostly uric acid. We also produce about 1.8 g of creatinine per day for excretion, a breakdown product of creatine phosphate. Sodium, potassium, and chloride are lost with the urine and must be replaced in the diet. To conserve water, the kidneys concentrate these solutes to a total concentration over four times that of blood plasma. The kidney must remove these substances from the blood without losing vital solutes such as sugars and amino acids. The kidney concentrates solutes from the osmolality of blood plasma, about 300 mOsmol/L, by more than four times, to 1200 to 1400 mOsmol/L.
The kidney has three major regions, arranged in layers (Figure 9.13). The outer layer is the renal cortex, the middle layer is the renal medulla, and the innermost part is a cavity the renal pelvis. The fundamental unit of the kidney's mass transfer and urine production is a tiny tubule called a nephron, which is surrounded by blood vessels. The nephron starts in the cortex, passes into the medulla, and then back out to the cortex, where it connects to a collecting duct that conducts the urine into the pelvis. Each of the two kidneys is connected by a ureter to the bladder, which stores urine until a person is ready to release it by urination. The tube that drains the bladder to the outside of the body is called the urethra.
The nephron is about 50 mm long, 15 to 60 mm in diameter, and has four main parts. The ball-shaped glomerular capsule in the cortex contains a tuft of about 50 capillaries called the glomerulus. This connects to the proximal tubule, which, in turn, is connected to the loop of Henle, with a descending limb that passes down into the renal medulla and an ascending limb that goes back into the cortex. Finally, the distal tubule passes through more of the cortex before emptying into a collecting duct. Each of these four sections of the nephron play a role in forming urine using three distinct processes: (1) glomerular filtration, (2) tubular resorption, and (3) tubular secretion.
The capillaries of the glomerulus have pores with a diameter between 50 and 100 nm. In glomerular filtration, blood pressure forces plasma through these pores into the nephron. Blood cells and most blood proteins are retained, but the liquid, salts, and small organic molecules pass through to form a filtrate. The kidneys receive about 20 to 25% of the blood flow from the left ventricle of the heart. Of this, some 10%, about 125 mL/min, passes into the nephrons. This filtrate has a composition similar to plasma, except without the large proteins. The filtration rate is maintained at a fairly constant rate within the kidney but can be modified by hormones or by the autonomic nervous system. A disorder called glomerulonephritis can follow a severe infection of bacteria or viruses. The infection can produce a high concentration of antibody-antigen complex to circulate in the blood. The complexes plug the pores of the glomerulus, reducing the filtration rate and producing an inflammation of the renal cortex.
As the filtrate passes along the nephron, various materials are removed or added by diffusion, osmosis, or carrier-mediated transport such as facilitated transport active transport, cotransport, or countercurrent transport. Carrier-mediated transport has a limited capacity, and if the blood concentration of the transported compound exceeds a threshold, the excess will be lost in the urine. This is the fate of much of the water-soluble vitamins taken in high concentrations in pill form. After a high-sugar meal, your blood sugar may briefly exceed the threshold of 180 mg/dL, and some sugar will be lost. This is a chronic problem with diabetics.
Using facilitated transport and cotransport, the proximal tubule removes 99% of organic nutrients such as sugars, amino acids, and vitamins, as well as drugs and toxins, from the urine. It is an important site for the removal of peptide hormones, such as insulin, from the blood. Many ions, such as sodium, potassium, magnesium, bicarbonate, phosphate, and sulfate, are actively transported out of the urine. Water follows the solutes by osmosis. Overall, the proximal tubule reduces the volume of the filtrate by 60 to 70%. Some secretion also occurs, as described below. The proximal tubule and the loop of Henle remove most of the calcium filtered by the glomerulus.
The osmolarity of the urine is still at the level of plasma, about 300 mOsmol/L.The urine now passes to the loop of Henle, where half the remaining water and two-thirds of the sodium and chloride ions are removed. The descending loop runs parallel to the ascending loop through the medulla. The thin descending loop is permeable to water but not to solutes. The thick ascending section is impermeable to both, but has active transport mechanisms that pump sodium and chloride out of the tubule and into the medulla. The collecting duct also passes through the medulla, and urea diffuses from the urine in the duct into the medulla. The sodium chloride and the urea increase the osmolarity of the medulla near the turn in the loop to about 1200 mOsmol/L. As a result, water passively diffuses out of the descending loop by osmosis. As the urine passes up the ascending loop, the removal of sodium chloride reduces the osmolarity to levels below that of plasma, as low as 100 mOsmol/L. This mechanism for the removal of water and salts involving opposite flow directions of the descending and ascending loops is called the countercurrent multiplier effect (Figure 9.14).
The total flow is now about 15 to 20% of the original filtrate, and continues on to the distal tubule. Both the proximal and distal tubules are active in tubular secretion. Drugs such as penicillin and phenolbarbitol are removed from the blood in this way. The urinary drug testing of athletes is possible because of tubular secretion. Sodium and chloride is removed from urine by active transport, but at the expense of two potassium ions for each three sodium ions. This is stimulated by the hormone aldosterone, which, as was noted above, conserves water in response to stress, but produces potassium loss. The distal tubule also secretes hydrogen ions and exchanges them for bicarbonate. This gives the kidneys some control over blood pH. Both the proximal and distal tubules produce and secrete ammonia as a way to remove hydrogen ions from the blood without decreasing urine pH excessively.
Contrary to what the name implies, the collecting ducts do more than just act as pipelines. They are critical for the final processing of urine and in the kidney's role in controlling blood pressure and volume. The walls of the ducts are permeable to water. As the duct passes into the high-salinity medulla, water is removed by osmosis until it is in equilibrium with the medulla at an osmolarity approaching 1200 mOsmol/L. At this point, its volume has been reduced to about 1% of the amount filtered in the glomerulus. The permeability of the distal tubule and the collecting ducts is controlled by the hormone ADH
(vasopressin). In the absense of the hormone, the ducts become impermeable to water. No water is absorbed from the distal tubule on, and the person secretes large amounts of dilute urine. This is what occurs in the disease diabetes insipidus (not to be confused with the insulin production disorder diabetes mellitus). Normal persons secrete ADH continuously to closely control water recovery. ADH is opposed by the hormone ANP that is produced by the heart (Section 9.5.2). ANP increases glomerular filtration, suppresses sodium absorption by the distal tubule, blocks release of ADH and aldosterone, and inhibits the response of the distal tubule and collecting ducts to ADH and aldosterone.
The water and solutes removed from the filtrate reenter blood vessels that are intimately associated with the nephron. These blood vessels, not shown in Figure 9.14, form a loop parallel to the loop of Henle after passing through the glomerulus. The blood picks up solutes as it passes down into the medulla, increasing its osmolarity to about 1200 mOsmol/L. It then loops back up to the cortex, absorbing water as it goes, until its osmolality returns to a normal 300 mOsmol/L.
The pH of urine is normally between 5.5 and 6.5, but may range from 4.6 to 8.0. The value is influenced by diet. Drinking milk can produce acidic urine with a pH about 6.0. A diet high in fruits and vegetables produces alkaline urine. The concentration of urea varies directly with nitrogen in the diet, particularly protein. Creatinine is excreted in proportion to a person's muscle mass.
Some of the normal constituents of urine may precipitate in the ureter or urethra, forming kidney stones. One-third of these are associated with alkaline urine or calcium problems and include Ca3(PO4)2, MgHN4PO4, CaCO3, or a mixture of these. About half of all kidney stones are calcium oxalate, caused by eating large amounts of spinach or rhubarb, which have high levels of oxalic acid. Stones may also be formed from organics such as uric acid.
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