Introduction
Many are familiar that the most important function of the kidneys is to get rid of the body’s waste materials that are ingested or produced as byproducts of body’s metabolism. Another function of the kidneys that are also of critical importance is to control the volume and composition of the body fluids (Guyton, 2006). The kidney maintains the balance between the intake of water and virtually all electrolytes (which could be either due to ingestion or metabolic production) with the output of the body (through excretion or metabolic consumption) (Berne, Levy, Koeppen, & Stanton, 2004). This regulatory function of the kidney enables the cells of the body to have a stable internal environment that is important for the cells to perform their various functions (Guyton, 2006).
The kidney performs its most vital functions by filtering the plasma and selectively removing some substances from the filtrate at variable rates, which depends on the need of the body. Basically, the kidney “clears” the unnecessary or the excess substances from the blood by excreting them through the urine. On the hand, the kidney “sequesters” back the substances to the blood that are needed by the body. In this paper, the author wishes to focus on the role of kidney in maintaining the concentrations of acid and bases in the blood, which is more commonly known as the acid-base balance (Guyton, 2006).
The Kidney
First, it is important to be familiar with the gross anatomy of the kidney and its components. The two kidneys lie on the posterior wall of the abdomen and are about the size of a clenched fist. The hilum is where the renal artery, renal vein, lymphatics, nerve supply, and ureter pass (Figure 1). If the kidney is bisected from top to bottom, the outer cortex and the inner medulla can be visualized. The medulla is divided into several masses of cone-shaped tissue called the renal pyramids, which all terminate in the renal papilla. Renal papilla projects into the space of the renal pelvis, which collects the urine from the papilla (Figure 1). The walls of the pelvis and ureter have muscular walls that push the urine towards the bladder, where the urine is stored temporarily until it is emptied by micturition or urination (Guyton, 2006).
A nephron is the kidney’s functional unit. Each human kidney contains about 1 million nephrons and each is capable of urine formation. Each nephron contains (1) glomerular capillaries called glomerulus where the blood is first filtered and (2) and a long series of tubules where the filtered fluid is processed to form the urine (Figure 2). The urine formed within the tubules will be emptied on the renal pelvis. The series of tubes where urine flow include, from the most proximal (nearest to glomerulus) to the most distal (farthest from glomerulus), proximal tubule; thin segment of the loop of Henle; thick segment of the loop of Henle; distal tubule; and collecting tubule (Figure 3).
Acid-Base Balance
Regulation of hydrogen ion (H+) balance is somehow similar with the regulation of other ions in the body. For example, to achieve constancy in the internal environment that is important to the cells, there must be a balance with the net input and removal of H+, which is also done by the kidney. In addition, there are also multiple acid-base buffering mechanisms that would prevent drastic changes in the pH of the body. An effective buffering mechanism is important because virtually all of the biochemical processes in the body involving enzymes are pH dependent (Berne, Levy, Koeppen, & Stanton, 2004). The buffering mechanisms requires the involvement blood, cells, and lungs to maintain normal H+ concentrations both inside (intracellularly) and outside (extracellularly) the cell. However, in this text the author will focus on the role of the kidney in controlling the acid-base balance (Guyton, 2006).
An acid is a molecule that is capable of releasing hydrogen ions in the solution. An example of an acid is hydrochloric acid (HCl), which gives of dissociates in the solution to release a hydrogen ion (H+) and chloride ion (Cl-). Moreover, carbonic acid (H2CO3) dissociates in water to for H+ and bicarbonate ion (HCO3-). On the other hand, a base is a molecule that can accept H+. For example HCO3- is considered a base because it can combine with H+ again to form carbonic acid (H2CO3) (Guyton, 2006; Murray, Granner, & Rodwell, 2006). The kidney controls the acid-base balance by the formation and excretion of either acidic or basic urine, depending on the situation. Excreting an acidic urine will reduce the concentration of acid in the extracellular (outside the cell) fluid and makes it more basic. On contrary, excreting basic urine removes the base from the extracellular fluid and makes it more acidic (Guyton, 2006).
Generally the kidney produces acidic or basic urine by continually filtering a large number of bicarbonate into a series of tubules. The bicarbonates are, therefore, removed from the blood if they are excreted through the urine. Likewise, large number of H+ is also secreted into the tubular lumen by the tubular epithelial cells lining the tubes. Through this, H+ ions are removed from the blood. In cases when H+ is secreted more than HCO3-, then there will be a net loss of acid from the extracellular fluid. On the other hand, when HCO3- is filtered more than H+, there will be a net loss of base (Guyton, 2006; Berne, Levy, Koeppen, & Stanton, 2004).
Each day, the human body produces an estimate of 80 milliequivalents (mEq) of non-volatile acids, mainly from protein metabolism. The acids are non-volatile and, thus, cannot be excreted through the lungs where volatile acids can escape. Therefore, the primary removal of the non-volatile acids is through renal excretion. Furthermore, the kidney should prevent the loss of bicarbonate ions in the urine, which is more important than the excretion of non-volatile acids because bicarbonate ions can also function as a buffer that would resist drastic changes in the pH of the body (Pitts, Ayer, & Schiess, 1946). However, each day, the kidneys filter an estimate of 4320 mEq of bicarbonate. Fortunately, under normal conditions, almost all this is reabsorbed from the tubules (Figure 4). In this way, the primary buffer system of the extracellular fluid is conserved (Guyton, 2006).
Basically the kidneys maintain extracellular fluid H+ concentration by three primary mechanisms: (1) secretion of H+, (2) reabsorption of filtered HCO3-, and (3) production of new HCO3- (Guyton, 2006; Pitts, Ayer, & Schiess, 1946).
Chronic renal failure causes acid-base disorder
When the kidney’s functioning deteriorates due to renal failure, a build-up of anions of weak acids will occur. This happens because the kidneys are already incompetent to excrete the excess anions of weak acids through the urine. Furthermore, the diminished rate of glomerular filtration also reduces the rate of excretion of phosphates and ammonia and accumulates in the body (Chertow, et al., 1997). This will cause changes in the pH that could possibly be fatal if not for the presence of the bicarbonates that buffers the extracellular fluid. However, because bicarbonates are used, their total free concentrations also lessen. The decrease in the concentration of bicarbonate base results in severe metabolic acidosis (Guyton, 2006).
To neutralize the excess acid, large amounts of sodium bicarbonate is usually administered to the patient. The sodium bicarbonate is absorbed from the intestine into the blood, which replenishes the bicarbonate concentration of the blood and restores the bicarbonate buffer system (Adrogue & Madias, 1999). Thus, the effect of sodium bicarbonate is to restore the pH of the extracellular fluid (Guyton, 2006).
Conclusion
The kidneys maintain the concentrations of all electrolytes by balancing the input with the output of the body (Berne, Levy, Koeppen, & Stanton, 2004) through plasma filtration; selective removal of unwanted substances; and reabsorption of the needed ones. This regulatory function of the kidney enables the cells of the body to have a stable internal environment that is important for the cells to perform their various functions (Guyton, 2006). The same is true with acids and bases. Basically the kidneys maintain extracellular fluid H+ concentration by three primary mechanisms: (1) secretion of H+, (2) reabsorption of filtered HCO3-, and (3) production of new HCO3- (Guyton, 2006; Pitts, Ayer, & Schiess, 1946). Incompetence of the kidney resulting from chronic kidney failure could lead to metabolic acidosis, which can be fatal when not treated (Guyton, 2006).
References
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- Berne, R., Levy, M., Koeppen, B., & Stanton, B. A. (Eds.). (2004). Physiology (5th ed.). St. Louis, Missouri, United States of America: Mosby-Elsvier.
- Chertow, G., Burke, S., Lazarus, J., Stenzel, K., Wombolt, D., Goldberg, D., et al. (1997). Poly[allylamine hydrochloride] (RenaGel): A noncalcemic phosphate binder for the treatment of hyperphosphatemia in chronic renal failure. American journal of kidney diseases , 29 (1), 66-71.
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- Murray, R. K., Granner, D. K., & Rodwell, V. W. (Eds.). (2006). Harper’s Illustrated Biochemistry. Singapore.
- Pitts, R., Ayer, J., & Schiess, W. (1946). The renal regulation of acid-base balance in man. III. the reabsorption and excretion of bicarbonate. American Journal of Physiology , 35-44.