Question

The human kidney produces how much concentrated urine than the initial filtrate formed? (a) 2 times (b) 4 times (c) 6 times (d) 3 times

Short answer

None of the provided options are correct. The human kidney produces concentrated urine that is approximately 120 times more concentrated than the initial filtrate formed.

Step-by-step solution

Understand the Kidney Filtrate and Urine Production

The filtrate in the glomerulus of the kidney consists of all the normally filtered small molecules, which are in the same concentration as in plasma. The initial filtrate undergoes various processes like reabsorption and secretion, which results in the formation of urine.

Comparative Analysis

The final concentrated urine produced by the kidneys is much less than the volume of initial filtrate. The kidneys make up to 180 liters of filtrate daily, of which only approximately 1-2 liters are finally excreted as urine.

Find the Concentration Factor

Given the above volumes, we can find the concentrating factor. If the initial filtrate is 180 liters and the final urine is 1.5 liters on average, the approximating concentrating factor = 180/1.5 = 120 times.

Key concepts

Glomerulus

The glomerulus is a network of tiny blood vessels located at the beginning of the nephron in the kidney. It plays a crucial role in the kidney's filtration process. Blood that enters the glomerulus arises through the afferent arteriole and exits via the efferent arteriole.
In this stage, the blood pressure forces water and solutes through a filter of very small pores into the Bowman's capsule. This forms what is known as the 'glomerular filtrate'. This filtrate contains items such as wastes, nutrients, and ions in the same concentration as plasma, except for larger molecules like proteins which do not pass through the filter.

• Filtration in the glomerulus is the first step in the complex process of urine production.
• The health of the glomerulus is essential for the kidneys to perform their filtration functions effectively.
• Any damage or dysfunction in the glomerulus can significantly impact the filtering ability, potentially leading to medical issues such as proteinuria.

Urine Concentration

Urine concentration refers to the process by which the kidneys adjust the amount of water and electrolytes to maintain body homeostasis. This is achieved through the kidneys' remarkable ability to reabsorb most parts of the initial filtrate.
The glomerular filtrate, originally formed as dilute, is concentrated during its passage through various regions of the nephron. This is especially true in the loop of Henle, the distal tubule, and the collecting duct.

• The concentration process is fundamentally crucial because the body needs to preserve vital water and salts while getting rid of waste.
• The average human kidney can concentrate urine up to approximately 120 times more than the original filtrate, transforming roughly 180 liters per day of blood-derived filtrate into 1.5 liters of urine.
• This ability to concentrate urine allows humans to adapt to varying water availabilities in their environments.

Reabsorption and Secretion

Reabsorption and secretion are pivotal processes during urine formation in the kidney. After the glomerulus filters the blood, these processes chiefly occur in the tubules of the nephron.
During reabsorption, essential substances such as water, glucose, and salts are reabsorbed from the tubular fluid back into the bloodstream. This ensures that the body retains vital nutrients and balances fluid and electrolytes levels.

• Most reabsorption occurs in the proximal tubule, where the majority of glucose, amino acids, and electrolytes are reclaimed.
• The extent of reabsorption varies based on the body's current needs, controlled by hormones such as antidiuretic hormone (ADH) and aldosterone.

In contrast, secretion involves the addition of substances from the blood back into the tubular fluid. It deals with disposing of waste products, drugs, and excess ions, helping to maintain acid-base balance. This combination of reabsorption and secretion determines the final composition of urine.

• Secretion mainly occurs in the distal convoluted tubule and collecting duct.
• Together these processes significantly reduce the volume of filtrate, facilitating efficient urine production by the kidneys.