26.1: Urinary System

Learning Objectives

By the end of this section, you will be able to:

• Recognize the anatomy of the urinary system
• Describe the process of urine formation
• Explain the physiological act of urination

A comprehensive understanding of the urinary system, urine formation, and the physiological act of urination is fundamental to appreciating the intricate mechanisms that contribute to maintaining the body’s internal balance. The anatomy of the urinary system, composed of the kidneys, ureters, bladder, and urethra, serves as the foundation for the elaborate processes involved. Exploring the process of urine formation unravels the intricate series of filtration, reabsorption, and secretion orchestrated by the kidneys. Meanwhile, understanding the physiological act of urination, from the micturition reflex to the stages of voiding, sheds light on the controlled release of urine. This knowledge is pivotal not only for comprehending the complexities of human physiology but also for recognizing the significance of these processes in sustaining overall health and homeostasis.

Anatomy of the Urinary System

The urinary system, a vital component of the human anatomy, is a sophisticated network of organs responsible for producing, storing, and eliminating urine from the body. Comprising the kidneys, ureters, bladder, and urethra, this intricately designed system orchestrates the filtration of blood, the formation of urine, and the controlled expulsion of waste from the body (Cleveland Clinic, 2023; Johns Hopkins, n.d.). The kidneys, positioned on either side of the spine, act as intricate filters, extracting excess fluids and waste products from the bloodstream. Nephrons, the microscopic functional units within the kidneys, play a central role in this process. The ureters transport the produced urine from the kidneys to the bladder, a muscular reservoir situated in the pelvic cavity. The urethra then serves as the exit pathway for urine, allowing for regulated voiding (Cleveland Clinic, 2023; Johns Hopkins, n.d.). This part of the chapter takes a closer look at each of these components.

Kidneys

A kidney is a bean-shaped organ located on either side of the spine, just below the rib cage, in the human body. Kidneys are essential components of the urinary system and play a critical role in maintaining overall health. The primary functions of the kidneys include filtering waste and excess substances from the blood to form urine, regulating fluid and electrolyte balance, managing acid-base equilibrium, contributing to erythropoiesis (the production of red blood cells), participating in detoxification by filtering drugs and toxins, aiding in the activation of vitamin D for calcium and phosphorus metabolism, and regulating blood pressure through the renin-angiotensin-aldosterone system (RAAS) (Cleveland Clinic, 2023; Johns Hopkins, n.d.). The kidneys are vital for maintaining physiological balance and supporting various bodily functions that contribute to overall well-being.

The size of the kidneys can vary among individuals, but on average, a typical adult human kidney is approximately 10 to 12 cm (about 4 to 5 in) in length and weighs about one-third of a pound (Preminger, 2022). The kidneys are encapsulated by a protective layer known as the renal capsule. Structurally, each kidney comprises an outer layer called the renal cortex and an inner region known as the renal medulla. A microscopic unit within the kidneys, called a nephron, is the functional units responsible for blood filtration and urine formation (Preminger, 2022). With approximately one million nephrons in each kidney, these structures play a pivotal role in maintaining the body’s fluid balance and regulating electrolytes (Preminger, 2022).

Blood is supplied to the kidneys through the renal artery, while the deoxygenated blood exits via the renal vein. As blood circulates through the nephrons, excess fluids, electrolytes, and waste, called urea, are meticulously filtered. The body generates urea during the breakdown of protein-containing foods. Subsequently, urea is transported through the bloodstream to the kidneys, where it, along with water and other waste products, is eliminated in the form of urine (Johns Hopkins, n.d.; Preminger, 2022). The resulting urine accumulates in the renal pelvis, a funnel-shaped structure that directs the urine into the ureter. The ureter, in turn, conveys the urine to the bladder for storage (Preminger, 2022). The location and anatomy of the kidneys are depicted in Figure 26.2.

Figure 26.2 The kidneys are situated bilaterally on either side of the spine, below the rib cage and within the retroperitoneal space. These bean-shaped organs are essential components of the body’s urinary system. (credit: “Blausen 0592 KidneyAnatomy 01.png” by BruceBlaus/Wikimedia Commons, CC BY 3.0)

Ureters

Within the human body, there are two ureters, one connected to each kidney. Each ureter is a muscular tube that serves as a conduit for transporting urine from its kidney to the urinary bladder. The ureters’ primary function is to facilitate the unidirectional flow of urine, preventing backflow and ensuring the efficient drainage of urine from the renal pelvis to the bladder (Lescay et al., 2024). The unique structure and function of the ureters ensure the efficient and regulated transport of urine, safeguarding the urinary system from complications such as reflux (backflow) and obstruction (blockage).

Originating from the renal pelvis of each kidney, the ureters extend downward along the posterior abdominal wall before entering the pelvic cavity. Composed of smooth muscle and lined with specialized mucosa, the ureters exhibit rhythmic muscular movements called peristaltic contractions, which propel urine in a unidirectional flow toward the bladder. The ureters traverse the pelvic brim, passing behind the bladder before entering its posterior wall. This anatomical arrangement prevents the backflow of urine into the kidneys during bladder contraction (Lescay et al., 2024).

Bladder

The bladder, an essential component of the urinary system, acts as a storage reservoir for urine prior to its regulated expulsion from the body (Figure 26.3). Positioned within the pelvic cavity, this hollow and muscular organ possesses a flexible structure, enabling it to expand as it accumulates urine (Shermadou et al., 2023). In females, the bladder lies posterior to the pubic bone and anterior to the uterus and rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. In males, the anatomy is similar, minus the uterus and with the addition of the prostate inferior to the bladder.

Figure 26.3 The bladder’s triangular shape is characterized by three openings: two for the ureters and one for the urethra. (CC BY 4.0; Rice University & OpenStax)

As the bladder gradually fills, stretch receptors in its walls send signals to the central nervous system, indicating the need for voiding. The detrusor muscle, a layer of smooth muscle within the bladder wall, contracts under nervous system control, while the external urethral sphincter relaxes, facilitating the expulsion of urine through the urethra. The bladder’s triangular shape is characterized by three openings: two for the ureters and one for the urethra. The trigone, a triangular area between these openings, acts as a physiological valve, preventing reflux of urine into the ureters. The neck of the bladder connects to the urethra, through which urine exits the body during voiding (Shermadou et al., 2023). The bladder’s intricate anatomy and functional mechanisms enable it to store urine efficiently and facilitate controlled release, contributing to the overall regulation of the body’s fluid and waste balance.

Urethra

The urethra, the final component of the urinary system, serves as the conduit for the expulsion of urine from the body. The urethra is a tubular structure extending from the neck of the bladder to the external urethral orifice. The length of the urethra varies between males and females, with males typically having a longer urethra that also functions as a passageway for semen during ejaculation. The female urethra is shorter and solely dedicated to the elimination of urine. The urethra is lined with mucous membrane and surrounded by muscle, contributing to its role in the voluntary control of urine release. The external urethral sphincter, a ringlike muscle, aids in the regulation of urine flow (Cleveland Clinic, 2022).

Process of Urine Formation

The process of urine formation, a sophisticated series of physiological events coordinated by the kidneys, is essential for maintaining the body’s internal balance and eliminating waste products (Figure 26.4). This intricate journey begins with glomerular filtration, where blood entering the kidneys is meticulously filtered in the glomerulus to form an initial filtrate. Following filtration, tubular reabsorption occurs along the renal tubules, selectively reclaiming vital substances such as water, glucose, and ions from the filtrate back into the bloodstream. Simultaneously, tubular secretion involves the active transport of certain substances from the bloodstream into the renal tubules for further elimination. This complex interplay between glomerular filtration, tubular reabsorption, and tubular secretion culminates in the production of urine, a dynamic and essential function for sustaining internal homeostasis and overall bodily health.

Figure 26.4 The complex interplay between glomerular filtration, tubular reabsorption, and tubular secretion culminates in the production of urine. These processes also ensure the fine-tuning of the urine’s composition, regulating electrolyte levels, fluid balance, and the removal of metabolic waste. (credit: “Anatomy and physiology of animals Summary of the processes involved in the formation of urine.jpg” by Ruth Lawson/Wikipedia, CC BY 3.0)

Glomerular Filtration

The initial and crucial step in the process of urine formation is glomerular filtration. It takes place within the glomerulus, the intricate network of tiny blood vessels in the kidneys. As blood flows into the glomerulus through the afferent arteriole, the high pressure in this network forces water, electrolytes, and small molecules, including waste products, out of the blood and into the renal tubules. This resulting glomerular filtrate consists of essential substances required by the body and waste materials slated for elimination. It is noteworthy that the selective permeability of the glomerular membrane retains larger molecules like proteins and blood cells in the bloodstream. The rate at which glomerular filtration occurs is quantified as the glomerular filtration rate (GFR), a critical indicator of kidney function. The normal GFR ranges between 120 and 125 mL/min. Glomerular filtration establishes the foundation for subsequent phases in urine formation, setting the groundwork for reabsorption and secretion processes that ultimately lead to the production of urine (Ogobuiro & Tuma, 2023).

Tubular Reabsorption

Following glomerular filtration, the pivotal phase of tubular reabsorption occurs within the renal tubules of the kidneys. The glomerular filtrate, which initially contains water, electrolytes, and essential substances, moves through the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. Along this journey, the renal tubules selectively reabsorb critical components back into the bloodstream, ensuring the retention of substances vital for maintaining the body’s balance. This includes the reabsorption of water, glucose, ions, and other necessary molecules, effectively reclaiming them from the filtrate. This reabsorption process is highly regulated, responding to the body’s needs and adjusting the composition of the filtrate accordingly. Tubular reabsorption significantly concentrates the urine, conserving valuable water resources and aiding in the maintenance of electrolyte and solute balance within the body (Ogobuiro & Tuma, 2023).

Tubular Secretion

The final crucial phase in the process of urine formation, tubular secretion, occurs in the renal tubules of the kidneys following glomerular filtration and tubular reabsorption. This selective mechanism involves actively transporting certain substances, including excess ions, drugs, and metabolic by-products, from the peritubular capillaries surrounding the tubules into the renal tubules. These substances, not effectively filtered during glomerular filtration or requiring further elimination, are crucial for maintaining the body’s acid-base balance and controlling specific ion concentrations. For instance, the secretion of hydrogen ions actively regulates blood pH. Tubular secretion ensures the elimination of substances not initially cleared from the bloodstream, actively contributing to the final composition of urine (Ogobuiro & Tuma, 2023).

Act of Urination

The act of urination (also referred to as voiding or micturition) is a fundamental physiological process that reflects the intricate coordination between the nervous and muscular systems to eliminate urine from the bladder through the urethra. Centered within the urinary system, this multifaceted mechanism involves distinct stages (Flores et al., 2023). Understanding the act of urination delves into the anatomical and physiological intricacies that underlie the body’s ability to store, process, and eliminate urine. This exploration not only reveals the complexity of micturition but also underscores its critical role in maintaining the body’s internal equilibrium and overall well-being.

Life-Stage Context

Age-Related Changes with Urination

As individuals age, various physiological adjustments occur in the urinary system, influencing the act of urination. One notable change is a decrease in bladder capacity, leading to a more frequent urge to urinate and a reduced tolerance for larger fluid volumes. Additionally, the pelvic floor muscles, crucial for maintaining bladder control, tend to weaken with age, contributing to instances of urinary incontinence, particularly during activities that increase intra-abdominal pressure. Nocturnal urination, or nocturia, becomes more prevalent in older adults, disrupting sleep patterns and impacting overall sleep quality. The micturition reflex, responsible for coordinating the voiding of urine, may slow down, resulting in delays in initiating urination and potential issues with urgency. For males, an enlarged prostate (benign prostatic hyperplasia) can further complicate the act of urination by causing difficulties in initiating and maintaining a steady urine stream. Awareness of these age-related changes is crucial for healthcare providers to tailor interventions and support for older individuals, ensuring optimal bladder function and overall well-being.

Micturition Reflex

The act of micturition involves a complex physiological process governed by the micturition reflex. This reflex is generated by the nervous system, primarily the autonomic nervous system and the spinal cord. As the bladder fills with urine, stretch receptors in the bladder wall are activated, sending signals to the spinal cord. The parasympathetic nervous system is then stimulated, causing the detrusor muscle in the bladder wall to contract while simultaneously relaxing the internal urethral sphincter. The sensation of a full bladder is conveyed to the brain, and when an individual voluntarily decides to empty the bladder, the external urethral sphincter, under conscious control, relaxes. This coordinated relaxation of the sphincters and contraction of the detrusor muscle result in the expulsion of urine through the urethra. The micturition reflex is a finely tuned mechanism that ensures controlled and voluntary release of urine while preventing involuntary leakage. Dysfunction in this reflex can lead to issues such as urinary retention (the inability to empty the bladder fully) or urinary incontinence (involuntary loss of bladder control, leading to the unintentional release of urine) (Flores et al., 2023).

Stages of Micturition

The stages of micturition encompass a series of precisely coordinated events within the urinary system. From the resting and filling stages, in which the bladder gradually accommodates accumulating urine, to the pivotal voiding stage, in which conscious and involuntary mechanisms converge to facilitate controlled urination, each stage plays a crucial role in maintaining continence and ensuring efficient waste elimination. Understanding these stages not only unveils the physiological intricacies of the micturition process but also sheds light on the intricate coordination between nervous system responses and voluntary muscle actions. This exploration into the stages of micturition is essential for comprehending the dynamic interplay of factors that contribute to the regulated release of urine, a fundamental aspect of the body’s homeostatic mechanisms.

Resting and Filling Stages

During the resting stage, the bladder is in a relatively inactive state. As urine accumulates, stretch receptors in the bladder wall are activated, generally sending signals when there is at least 50 to 100 mL, signaling to the nervous system that the bladder is filling and prompting urination. The parasympathetic nervous system is gradually activated, leading to a mild contraction of the detrusor muscle while maintaining closure of the internal and external urethral sphincters. This allows the bladder to gradually expand to accommodate the increasing volume of urine without initiating the urge to void (Flores et al., 2023).

During this filling stage, the bladder continues to receive urine. The stretch receptors continue to send signals to the spinal cord and brain, providing information about the increasing volume of urine. Despite ongoing contractions of the detrusor muscle, the urethral sphincters remain contracted, preventing the involuntary release of urine. The individual typically remains unaware of the filling process during this stage, as the bladder’s main function is to store urine without generating a strong urge to void (Flores et al., 2023).

These early stages are essential for maintaining continence (the ability to control and maintain bladder functions) and preventing the constant need for urination. The transition to the next stage, the voiding or emptying stage, is initiated when the bladder reaches a certain level of distension, triggering the micturition reflex and the conscious sensation of needing to urinate.

Voiding Stage

The voiding stage, also known as the emptying stage, is a crucial phase in the micturition process. It follows the resting and filling stages and involves a coordinated series of events controlled by the micturition reflex and culminating in the release of urine stored in the bladder. The parasympathetic nervous system becomes more active, causing the detrusor muscle in the bladder wall to contract more forcefully. Simultaneously, the internal urethral sphincter relaxes involuntarily, allowing urine to flow from the bladder into the urethra. The conscious component of this stage involves the individual’s decision to voluntarily relax the external urethral sphincter, allowing urine to exit the body. This conscious control is crucial for maintaining continence and preventing involuntary urine release (Flores et al., 2023).

The voiding stage is a dynamic process, and the coordination between involuntary and voluntary muscle actions ensures efficient and controlled urination. Dysfunction in this stage can lead to issues such as urinary retention or incontinence. Overall, the voiding stage represents the culmination of the micturition process, facilitating the expulsion of urine from the body while maintaining voluntary control over the process.