Question

Which of the following options has the correct pair of nephron parts that maintain pH and ionic balance of blood? (a) Proximal convoluted tubule and Henle's loop (b) Distal convoluted tubule and collecting duct (c) Proximal convoluted tubule and glomerulus (d) Collecting duct and Henle's loop

Short answer

Option (b) Distal convoluted tubule and collecting duct is the correct answer.

Step-by-step solution

Understanding the Function of Nephron Parts

To find the correct pair of nephron parts, it's vital to understand the role of each part of the nephron in maintaining blood pH and ionic balance. The nephron is the functional unit of the kidney, responsible for filtering blood and forming urine. Different parts of the nephron play specific roles in this process.

Analyzing Option (a) - Proximal Convoluted Tubule and Henle's Loop

The proximal convoluted tubule (PCT) primarily reabsorbs ions, water, and nutrients from the filtrate, thus contributing to ionic balance, but it's not the main site for pH regulation. Henle's loop also contributes to ionic balance through the countercurrent mechanism but is not the primary site for pH regulation.

Analyzing Option (b) - Distal Convoluted Tubule and Collecting Duct

The distal convoluted tubule (DCT) and collecting duct are the primary sites where the kidney exerts control over the electrolyte balance and pH of the blood. The DCT actively secretes ions, and the collecting duct adjusts the final composition of urine, which involves the regulation of pH by secreting H+ ions and reabsorbing bicarbonate.

Analyzing Options (c) and (d) - Proximal Convoluted Tubule and Glomerulus; Collecting Duct and Henle's Loop

Both options (c) and (d) can be ruled out because the glomerulus is primarily involved in filtering blood plasma rather than maintaining ionic balance or pH, and while Henle's loop is involved in concentrating urine and ionic balance, it does not have a primary role in pH regulation.

Choosing the Correct Option

Based on the functions of the nephron parts, option (b) is correct because the DCT and the collecting duct are the main sites responsible for pH and ionic balance of blood within the nephron.

Key concepts

Renal Physiology

Renal physiology is the study of how the kidneys function. As one of the body's vital organs, the kidneys perform several crucial tasks to keep the body in a state of homeostasis. The primary role of the kidneys is to filter waste and excess substances from the blood, which are then excreted as urine. This process is largely carried out by the nephrons, the functional units of the kidney.

Each kidney contains around a million nephrons, and each nephron has several components, including the glomerulus, the proximal tubule, the loop of Henle, the distal tubule, and the collecting duct. Blood enters the nephron via the glomerulus where it is filtered. The filtrate then passes through the proximal tubule, which reabsorbs water, ions, and nutrients back into the blood. The loop of Henle, on the other hand, helps create a concentration gradient in the medulla of the kidney, which assists in water reabsorption. The distal convoluted tubule continues the process of selectively reabsorbing substances and secreting others. Finally, the collecting duct adjusts the final urine composition by performing additional reabsorption and secretion—a critical aspect of maintaining the ionic balance and pH of the blood.

Maintaining Blood pH

Maintaining blood pH within a narrow range is essential for proper physiological function, as enzymes and other cellular processes rely on a stable pH. The kidneys contribute to this by managing the excretion of hydrogen ions (H+) and the reabsorption of bicarbonate (HCO3-) through the distal convoluted tubule and collecting duct. An increase in breathing rate can expel more carbon dioxide (which is in equilibrium with bicarbonate in the blood), and the kidneys can secrete more H+ and reabsorb more bicarbonate to raise blood pH if it becomes too acidic.

Role of the Distal Nephron in pH Regulation

Particularly within the distal nephron, which includes the distal convoluted tubule and the collecting duct, the final adjustments to urine composition are made here, which involves selectively secreting H+ ions into the urine, thereby removing acid from the blood. The reabsorption of bicarbonate also serves to buffer blood pH. This delicate balance must be maintained to prevent conditions like acidosis or alkalosis, which can lead to severe health issues.

Ionic Balance of the Blood

The ionic balance of the blood is another crucial aspect of homeostasis, involving the regulation of electrolytes such as sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-). The distal convoluted tubule and collecting duct in the nephron are largely responsible for this regulation. They actively secrete ions into the urine or reabsorb them into the blood according to the body's needs.

Key Ions Regulated by the Kidneys

For example, the kidneys help regulate potassium levels by controlling the amount secreted into the urine. A high potassium intake leads to increased secretion, while a low intake minimizes secretion to maintain appropriate blood levels. Similarly, sodium reabsorption through the actions of hormones like aldosterone ensures that blood volume and pressure remain stable. The active secretion and selective reabsorption of ions contribute to the ionic balance that is essential for nerve conduction, muscle function, and many other physiological processes.