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Regulation of Water and Electrolytes

pH Balance

Blood pH is largely regulated by the kidneys by modifying reabsorption and secretion rates of hydrogen ions and bicarbonate ions to maintain pH balance.

The normal pH of blood is between 7.35 and 7.4 on the pH scale, which ranges between 0 and 14, with 7 being neutral. When the slightly alkaline pH levels drop, blood becomes more acidic, and when the pH levels rise blood becomes more basic, or alkaline. Either scenario will result in disruptions in cellular processes and must be corrected. Deviations from normal blood pH can be corrected by the blood's buffer system, the respiratory system, and the urinary system. The concentration of hydrogen ions (H+) in the blood determines blood pH. An acid is a substance that lowers pH in the blood by increasing the levels of hydrogen ions. A base is a substance that increases blood pH by removing hydrogen ions. Chemical compounds that release H+ in an aqueous environment are considered acids, and those that release OH are bases. Blood pH decreases, becoming more acidic, from consuming excessive acid-producing foods such as proteins, lactic acid production during metabolism, a decreased ventilation rate, and the loss of excess OH from diarrhea. Blood can become more alkaline from an increased ventilation rate, consuming antacids, or the loss of hydrochloric acid from the stomach during vomiting.

Two main mechanisms by which pH balance is maintained include chemical buffering systems and physiological buffering systems. Chemical buffering systems work on a timescale of seconds, whereas physiological buffering systems take minutes to days. The respiratory system and kidneys are the primary systems that regulate pH.

A buffer is a solution that resists changes in blood pH. There are three major chemical buffers in the blood: bicarbonate, phosphate, and proteins. The bicarbonate buffer system is the primary buffering system in blood. It involves carbonic acid (a weak acid) and its salt form, sodium bicarbonate (a weak base). The strength of an acid or base is determined by its tendency to release or accept a hydrogen ion. For example, a strong acid will completely dissociate in water (e.g., HCl), and a weak acid will be found in dissociated and nondissociated forms. A strong acid is going to be more detrimental to the body than a weak acid because it releases more hydrogen ions. When blood becomes more acidic, carbonic acid does not release its hydrogen, and sodium bicarbonate dissociates, exchanging the sodium for the excess hydrogen ion. When hydrogen ion levels fall and blood becomes more alkaline, carbonic acid will give up its hydrogen, which combines with an OH to create water.

The phosphate buffer system plays a minor role in the blood but is an important buffer in intracellular fluid. It acts the same as the bicarbonate buffer system by converting strong bases to weak bases and strong acids to weak acids. The protein buffer system is at work in the blood plasma and in cells. Proteins can act as acids or bases depending on the pH of the environment. Proteins can give up H+ in an alkaline environment and accept H+ in an acidic environment.

In the lungs, carbonic acid is converted to water and carbon dioxide. Changes in ventilatory rates can generate alterations in blood pH by ultimately increasing or decreasing carbonic acid through the exchange of carbon dioxide with the environment. A respiratory or ventilatory imbalance can create an increase or decrease in blood pH.

The most important physiologic system in pH balance is the kidneys. Many acids are generated through metabolic processes and need to be removed from the body to prevent the blood from becoming more acidic. The urinary system is the only system that can carry this out. Tubule transport in the nephron can be modified based on the need to conserve or reduce particular chemical buffers in the bloodstream. Hydrogen ions are secreted in the PCT and the collecting duct. Bicarbonate ions can be reclaimed from the tubules or secreted to help adjust pH levels.

Role of the Respiratory and Urinary Systems in Acid/Base Balance

Maintaining a proper balance between acids and bases in the body is essential for the body to maintain homeostasis. The usual pH of blood is 7.4. The body uses a chemical buffering system, the respiratory system, and the urinary system to regulate pH levels.