The distribution of K+ within the body compartments and its importance are discussed. The hormones and factors that regulate plasma K+ levels and the significance of this regulation are explained. The mechanism of K+ transport by the various segments of the nephron and its impact on K+ excretion in urine is examined. The importance of the distal tubule and collecting duct in regulating K+ excretion is emphasized. The influence of plasma K+ levels, aldosterone, vasopressin, tubular fluid flow rate, and acid-base balance on K+ excretion is discussed. The physiological importance of calcium (Ca++) and inorganic phosphate (Pi) is explored, along with the mechanisms that maintain Ca++ and Pi homeostasis. The roles of the kidneys, intestinal tract, and bone in maintaining plasma Ca++ and Pi levels are outlined. The hormones and factors that regulate plasma Ca++ and Pi levels are identified. The cellular mechanisms responsible for Ca++ and Pi reabsorption along the nephron are described. The hormones that regulate renal Ca++ and Pi excretion are discussed, as well as the role of the calcium-sensing receptor (CaSR). Various clinical disorders of Ca++ and Pi homeostasis are mentioned. The involvement of the kidneys in the production of calcitriol, the active form of vitamin D, is explained. The effects of loop and thiazide diuretics on Ca++ excretion are examined. The critical role of K+ in numerous cell functions and its abundance in the body are highlighted. The maintenance of K+ homeostasis through regulatory mechanisms in the extracellular fluid (ECF) and renal K+ excretion is discussed. The total body concentration of K+ and its distribution within cells and the ECF are presented. The normal concentration of K+ in the ECF, as well as hyperkalemia and hypokalemia, are explained.
2.
What is the distribution of K+ within the body compartments? Why is this distribution important?
3.
What are the hormones and factors that regulate plasma K+ levels? Why is this regulation important?
4.
How do the various segments of the nephron transport K+ , and how does the mechanism of K+ transport by these segments determine how much K+ is excreted in the urine?
5.
Why are the distal tubule and collecting duct so important in regulating K+ excretion?
6.
How do plasma K+ levels, aldosterone, vasopressin, tubular fluid flow rate, and acid-base balance influence K+ excretion?
7.
What is the physiological importance of calcium (Ca++) and inorganic phosphate (Pi)?
8.
How does the body maintain Ca++ and Pi homeostasis?
9.
What roles do the kidneys, intestinal tract, and bone play in maintaining plasma Ca++ and Pi levels?
10.
What hormones and factors regulate plasma Ca++ and Pi levels?
11.
What are the cellular mechanisms responsible for Ca++ and Pi reabsorption along the nephron?
12.
What hormones regulate renal Ca++ and Pi excretion by the kidneys?
13.
What is the role of the calcium-sensing receptor (CaSR)?
14.
What are some of the more common clinical disorders of Ca++ and Pi homeostasis?
15.
What is the role of the kidneys in the production of calcitriol (active form of vitamin D)?
16.
What effects do loop and thiazide diuretics have on Ca++ excretion?
Potassium (K+) is one of the most abundant cations in the body, and it is critical for many cell functions, including regulation of cell volume, regulation of intracellular pH, synthesis of DNA and protein, growth, enzyme function, resting membrane potential, and cardiac and neuromuscular activity. Despite wide fluctuations in dietary K+ intake, [K+] in cells and extracellular fluid (ECF) remains remarkably constant. Two sets of regulatory mechanisms safeguard K+ homeostasis. First, several mechanisms regulate [K+] in the ECF. Second, other mechanisms maintain the amount of K+ in the body constant by adjusting renal K+ excretion to match dietary K+ intake. It is the kidneys that regulate excretion of K+ .
Total body [K+] is 50 mEq/kg of body weight, or 3600 mEq for a 70-kg individual. Ninety-eight percent of the K+ in the body is located within cells, where the average [K+] is 150 mEq/L. High intracellular [K+] is required for many cell functions, including cell growth and division and volume regulation. Only 2% of total body [K+] is located in the ECF, where its normal concentration is approximately 4 mEq/L. A [K+] in ECF that exceeds 5.0 mEq/L constitutes hyperkalemia. Conversely, a [K+] in ECF of less than 3.5 mEq/L constitutes hypokalemia.