67 Urinary functions and processes

Learning Objectives

After reading this section you should be able to-

  • Describe the major functions of the urinary system and which organs are responsible for those functions.
  • Describe the three processes that take place in the nephron (i.e., filtration, reabsorption, and secretion) and explain how the integration of these three processes determines the volume and composition of urine.
  • Compare and contrast blood plasma, glomerular filtrate, and urine.

Regulation of Osmolarity

Maintaining the right balance of solutes in our body fluids is crucial for cellular well-being and overall health. Osmolarity, which refers to the concentration of solutes in a solution, is closely monitored and regulated by the kidneys. This regulation is vital to prevent issues such as cell rupture or widespread edema, both of which can result from imbalances in solute concentrations. When our blood becomes excessively diluted (hypo-osmolarity), it can lead to problems like the rupture of blood cells or the accumulation of fluids in tissues, causing edema. On the other hand, severe hypertonic conditions, caused by factors like dehydration, vomiting, or uncontrolled diarrhea, can result in a solute imbalance. The kidneys play a pivotal role in managing osmolarity. If the glomeruli in the kidneys are compromised, large amounts of essential proteins may be lost in the urine, leading to a drop in serum osmolarity. Such a decline can trigger widespread edema and, if severe, may even result in harmful brain swelling.

Recovery of Electrolytes

Sodium, calcium, and potassium are vital electrolytes that play crucial roles in various physiological processes. Maintaining a delicate balance of these electrolytes is essential for nerve conduction, skeletal muscle function, and, most critically, cardiac muscle contraction and rhythm. The kidneys play a central role in the regulation of electrolytes, ensuring their concentrations are within narrow, optimal ranges. Sodium and calcium homeostasis have been extensively discussed, emphasizing the critical nature of their regulation. Failure in potassium regulation can have profound consequences on the functioning of nerves, muscles, and especially the heart. Proper pH regulation is another key aspect tied to electrolyte balance. The three-dimensional structure and function of enzymes, essential for various physiological processes, can be compromised if the pH deviates from the optimum. The kidneys, through their regulatory functions, contribute significantly to pH homeostasis, preventing deviations that could impact enzyme activity throughout the body. In essence, the kidneys are not only key players in maintaining electrolyte balance but also crucial contributors to overall physiological stability. Disruptions in electrolyte concentrations can have far-reaching consequences, underscoring the importance of the kidneys in upholding homeostasis and ensuring the seamless functioning of bodily processes.

pH Regulation

Blood pressure is a vital physiological parameter that influences the distribution of fluids and nutrients throughout the body. The kidneys, in collaboration with other organs such as the lungs, liver, and adrenal cortex, actively participate in maintaining blood pressure within optimal ranges. The intricate mechanism known as the renin-angiotensin-aldosterone system plays a central role in blood pressure regulation. When blood pressure is low, specialized cells in the kidneys release renin, initiating a cascade that converts angiotensinogen into angiotensin I. Further conversion by angiotensin-converting enzyme (ACE) in the lungs produces biologically active angiotensin II. One immediate effect of angiotensin II is vasoconstriction, leading to a short-term increase in blood pressure. Angiotensin II also stimulates the adrenal cortex to release aldosterone, a steroid hormone. Aldosterone acts on the kidneys, promoting the reabsorption of sodium and associated osmotic recovery of water. This extended effect helps raise and sustain blood pressure over a more prolonged period. The kidneys, through their influence on fluid and sodium balance, contribute significantly to blood pressure homeostasis. Disorders affecting renal function can lead to imbalances, resulting in hypertension or hypotension, with potential consequences such as stroke, heart attack, or aneurysm formation.

Blood Pressure Regulation

Blood pressure is a vital physiological parameter that influences the distribution of fluids and nutrients throughout the body. The kidneys, in collaboration with other organs such as the lungs, liver, and adrenal cortex, actively participate in maintaining blood pressure within optimal ranges. The intricate mechanism known as the renin-angiotensin-aldosterone system (Figure X.1) plays a central role in blood pressure regulation. When blood pressure is low, specialized cells in the kidneys release renin, initiating a cascade that converts angiotensinogen into angiotensin I. Further conversion by angiotensin-converting enzyme (ACE) in the lungs produces biologically active angiotensin II. One immediate effect of angiotensin II is vasoconstriction, leading to a short-term increase in blood pressure. Angiotensin II also stimulates the adrenal cortex to release aldosterone, a steroid hormone. Aldosterone acts on the kidneys, promoting the reabsorption of sodium and associated osmotic recovery of water. This extended effect helps raise and sustain blood pressure over a more prolonged period. The kidneys, through their influence on fluid and sodium balance, contribute significantly to blood pressure homeostasis. Disorders affecting renal function can lead to imbalances, resulting in hypertension or hypotension, with potential consequences such as stroke, heart attack, or aneurysm formation.

Figure X.1 The Enzyme Renin Converts the Pro-enzyme Angiotensin.

The glomerulus produce a simple filtrate of the blood and the remainder of the nephron works to modify the filtrate into urine. You will discover that different parts of the nephron utilize three specific processes to produce urine: filtration, reabsorption, and secretion. You will learn how each of these processes works and where they occur along the nephron and collecting ducts. The physiologic goal is to modify the composition of the plasma and, in doing so, produce the waste product urine.

Glomerular Filtration, Tubular Reabsorption & Tubular Secretion

Within the nephron, the tubular reabsorption process is a finely orchestrated mechanism essential for maintaining the body’s fluid and electrolyte balance. As the filtrate, initially produced during glomerular filtration, travels through the renal tubules, specialized cells along the tubule selectively and actively reclaim crucial substances from the filtrate and reintroduce them into the bloodstream. The glomerulus serves as a highly permeable filter, allowing various substances, including water, sodium, chloride, bicarbonate, glucose, and amino acids, to enter the filtrate. However, to prevent the loss of vital components, tubule cells diligently reabsorb these substances as the filtrate progresses along the nephron. Remarkably efficient, these tubule cells can recover nearly all glucose and amino acids and up to 99% of water and important ions that were initially lost through glomerular filtration. This intricate process ensures that essential elements are returned to the bloodstream, preventing their unnecessary elimination in urine. At the base of the collecting duct, the modified filtrate, having undergone tubular reabsorption, transforms into urine. Tubular reabsorption is a pivotal aspect of kidney function, contributing significantly to the regulation of fluid and electrolyte balance, and ultimately, the composition of urine. Understanding this process provides insights into how the kidneys actively participate in maintaining the body’s internal environment.

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 25

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