Test: Renal System - 1 - MCAT MCQ

The nephron is the structural and functional unit of the kidney. It is responsible for filtering the blood, reabsorbing useful substances, and secreting waste products to form urine.

Antidiuretic hormone (ADH), also known as vasopressin, regulates water reabsorption in the kidney tubules. It acts on the collecting ducts to increase water permeability and reduce water loss in urine.

The proximal convoluted tubule (PCT) is responsible for the majority of water reabsorption in the nephron. It reabsorbs water along with various solutes, including glucose, amino acids, and ions.

The loop of Henle plays a critical role in reabsorbing water and sodium from the filtrate. It creates a concentration gradient in the interstitial fluid of the kidney, which allows for further water reabsorption in the collecting ducts.

Erythropoietin is a hormone produced by the kidneys in response to low oxygen levels. It stimulates the production of red blood cells in the bone marrow, helping to increase oxygen-carrying capacity.

The renal system is primarily responsible for regulating blood solute concentrations, blood pressure, and blood pH. It achieves these functions through processes such as filtration, reabsorption, and secretion in the nephrons. However, regulation of blood temperature is primarily carried out by other systems, such as the cardiovascular system and thermoregulatory mechanisms involving the skin and hypothalamus.

The proximal convoluted tubule (PCT) is the portion of the nephron that connects the Bowman's capsule to the loop of Henle. It is located immediately after the glomerulus within the renal corpuscle. The PCT is responsible for reabsorbing the majority of filtered substances back into the bloodstream, such as water, glucose, amino acids, and electrolytes. After leaving the PCT, the filtrate enters the loop of Henle, which plays a critical role in concentrating the urine.

The average adult human kidney contains approximately 1,000,000 nephrons. Nephrons are the functional units of the kidney responsible for filtering blood and producing urine. Each nephron consists of a glomerulus, which is a network of tiny blood vessels, and a tubule that carries the filtered fluid (urine) through various regions of the kidney for further processing. The large number of nephrons in each kidney allows for efficient filtration and processing of blood to maintain homeostasis.

The correct order in which urine is formed in the nephron is glomerular filtration, tubular reabsorption, and tubular secretion.

• Glomerular filtration: This is the first step in urine formation and occurs in the glomerulus of the nephron. Blood pressure forces fluid and small solutes from the glomerular capillaries into the Bowman's capsule, forming the filtrate. Larger molecules like proteins and blood cells are not filtered.
• Tubular reabsorption: After filtration, the filtrate enters the renal tubules, where selective reabsorption takes place. Essential substances such as water, glucose, amino acids, and ions are reabsorbed from the tubules into the surrounding capillaries. This process helps maintain the body's fluid balance and reabsorb valuable substances.
• Tubular secretion: As the filtrate passes through the tubules, additional waste products, toxins, and excess ions are actively secreted from the blood into the tubules. This process further modifies the composition of the filtrate before it becomes urine.

Overall, the three steps of glomerular filtration, tubular reabsorption, and tubular secretion work together to produce urine and regulate the balance of water, ions, and other substances in the body

In a healthy adult nephron, large molecular weight proteins are least likely to be present in the glomerular filtrate. The glomerular filtration process selectively allows the passage of small molecules and ions, while larger molecules such as proteins are usually excluded.

The glomerular filtration barrier consists of three layers: the fenestrated endothelium of the glomerular capillaries, the basement membrane, and the filtration slits formed by podocytes. This barrier acts as a sieve, allowing small molecules like water, ions, glucose, and amino acids to pass through, while preventing the passage of larger molecules, including most proteins.

Amino acids, glucose, and electrolytes are small molecules that are present in the glomerular filtrate. Amino acids are essential building blocks for protein synthesis, glucose is a vital energy source, and electrolytes are involved in various physiological processes.

Large molecular weight proteins, on the other hand, are normally retained within the bloodstream due to their size and charge. They are generally not filtered through the glomerular filtration barrier. Any presence of large proteins in the filtrate would indicate a malfunction or damage to the filtration barrier, which can occur in certain kidney diseases or conditions.

Therefore, option D, large molecular weight protein, is least likely to be present in the glomerular filtrate of a healthy adult nephron.