25.2 Gross Anatomy of Urine Transport
3 min read•june 18, 2024
The urinary system is a complex network of organs that work together to remove waste and maintain fluid balance. From the transporting urine to the bladder for storage, to the expelling it from the body, each component plays a crucial role in this vital process.
Understanding the and neural control of urination is key to grasping how our bodies regulate this essential function. The interplay between involuntary reflexes and voluntary control highlights the sophisticated nature of our urinary system and its importance in maintaining overall health.
Urinary System Anatomy and Physiology
Structure and function of urinary organs
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- Ureters
- Paired muscular tubes transport urine from the to the (approximately 25-30 cm long)
- Contain smooth muscle layers undergo peristaltic contractions to propel urine toward the bladder
- Urinary bladder
- Hollow, muscular organ stores urine until expelled from the body
- Located in the pelvic cavity, posterior to the pubic symphysis
- Composed of four layers: mucosa, submucosa, , and adventitia/serosa
- contracts during urination to expel urine
- Bladder neck contains (involuntary smooth muscle) prevents urine leakage
- Urethra
- Tube carries urine from the urinary bladder to the outside of the body during urination
- Length and anatomy differ between males and females (male: 20 cm, female: 3-4 cm)
Male vs female urethras
- Male urethra
- Divided into three regions: prostatic, membranous, and spongy (penile) urethra
- Passes through the prostate gland, pelvic floor muscles, and penis
- Serves as a passage for both urine and semen (not simultaneously)
- Contains (voluntary skeletal muscle) in the controls urine flow
- Female urethra
- Passes from the bladder neck to the external urethral orifice in the vestibule of the vagina
- Serves only as a passage for urine
- Contains external urethral sphincter (voluntary skeletal muscle) along its length controls urine flow
Micturition reflex in urine elimination
- Micturition reflex ()
- Involuntary neural reflex controls the process of urination
- Involves coordination between the urinary bladder, internal and external urethral sphincters, and the nervous system
- Reflex arc components
- Receptors: Stretch receptors in the bladder wall detect increased bladder volume and wall tension
- Afferent pathway: Sensory neurons carry signals from stretch receptors to the sacral spinal cord (S2-S4) and higher brain centers
- Integration center: (PMC) in the brainstem coordinates the reflex
- Efferent pathway: from the sacral spinal cord (S2-S4) innervate the detrusor muscle and internal urethral sphincter; innervate the external urethral sphincter
- Reflex action
- As the bladder fills, stretch receptors send signals to the spinal cord and brain
- At a critical volume (300-400 mL), the PMC is activated and triggers the micturition reflex
- Parasympathetic stimulation contracts the detrusor muscle and relaxes the internal urethral sphincter
- Somatic input relaxes the external urethral sphincter, allowing urine to flow out of the body (urination)
Neural control of urination
- Involuntary urination (infants and young children)
- Micturition reflex is primarily a spinal reflex with limited higher brain center control
- Bladder filling and critical volume trigger the reflex, leading to automatic voiding
- Voluntary urination (older children and adults)
- Higher brain centers (cerebral cortex and PMC) exert control over the micturition reflex
- Allows for voluntary initiation or inhibition of urination
- Cerebral cortex can override the PMC and maintain even when the bladder reaches critical volume
- Conscious relaxation of the external urethral sphincter and contraction of abdominal muscles can initiate urination
- Neural pathways
- Afferent signals from stretch receptors travel via (parasympathetic) to the sacral spinal cord and higher brain centers
- Efferent signals to the detrusor muscle and internal urethral sphincter travel via pelvic nerves (parasympathetic)
- Efferent signals to the external urethral sphincter travel via (somatic)
Urinary Tract and Related Conditions
- : The system of organs responsible for urine production, storage, and elimination
- Includes kidneys, ureters, urinary bladder, and urethra
- Renal pelvis: Funnel-shaped structure at the center of the kidney that collects urine before it enters the ureter
- Continence: The ability to control urination and maintain a closed bladder until voluntary voiding
- : Involuntary leakage of urine, which can result from various factors affecting urinary tract function or neural control
Key Terms to Review (33)
Anatomical sphincter: An anatomical sphincter is a circular muscle that constricts or dilates to regulate the passage of substances through a bodily opening. In the context of the urinary system, it controls the release of urine from the bladder.
Continence: Continence refers to the ability to control bodily functions, specifically the voluntary ability to retain urine and feces until an appropriate time for elimination. This concept is crucial for understanding how the urinary and gastrointestinal systems work together to maintain bodily functions and overall health.
Detrusor muscle: The detrusor muscle is a smooth muscle layer that forms the wall of the bladder, responsible for contracting to expel urine during urination. It relaxes to allow the bladder to store urine and contracts during urination to release it.
Detrusor Muscle: The detrusor muscle is a smooth muscle layer that forms the wall of the urinary bladder. It is responsible for the contraction and relaxation of the bladder, which allows for the storage and elimination of urine.
External Urethral Sphincter: The external urethral sphincter is a circular muscle that surrounds the urethra and helps control the release of urine from the bladder. It is an important component of the urinary system and plays a crucial role in maintaining continence.
External urinary sphincter: The external urinary sphincter is a circular muscle that controls the passage of urine out of the bladder and through the urethra. It is voluntarily controlled, enabling the decision to start or stop urination.
Incontinence: Incontinence is the involuntary loss of urine from the bladder, indicating an inability to control urination. It arises from issues within parts of the urinary system responsible for storing and releasing urine.
Internal Urethral Sphincter: The internal urethral sphincter is a smooth muscle valve located at the base of the bladder that controls the flow of urine from the bladder into the urethra. It is an essential component of the urinary system's mechanism for storing and releasing urine.
Internal urinary sphincter: The internal urinary sphincter is a ring of smooth muscle located at the junction between the bladder and urethra that involuntarily controls the flow of urine. It automatically contracts to retain urine in the bladder and relaxes when the bladder contracts during urination.
Membranous urethra: The membranous urethra is a segment of the male urethra that is located between the prostatic urethra and the spongy (penile) urethra. This part of the urethra is the shortest and most vulnerable section, as it passes through the urogenital diaphragm, which consists of muscle and connective tissue. Its role is critical in urine transport and is also involved in the passage of semen during ejaculation.
Micturition Reflex: The micturition reflex is a physiological mechanism that controls the process of urination, or the emptying of the bladder. It involves the coordinated contraction of the bladder muscles and the relaxation of the urethral sphincter to facilitate the expulsion of urine from the body.
Parasympathetic Neurons: Parasympathetic neurons are a division of the autonomic nervous system that is responsible for the body's rest-and-digest functions. These neurons work to conserve and restore energy, promoting activities such as digestion, urination, and sexual arousal.
Pelvic Nerves: The pelvic nerves are a group of nerves that originate from the sacral plexus and innervate the pelvic organs, including the bladder, urethra, and reproductive organs. These nerves play a crucial role in the gross anatomy of urine transport by facilitating the control and function of the urinary system.
Peristalsis: Peristalsis is a series of wave-like muscle contractions that moves food through the digestive tract. It involves the rhythmic contraction and relaxation of muscles in the organ walls to push contents forward.
Peristalsis: Peristalsis is the coordinated wave-like muscle contractions that propel food and other materials through the digestive tract. It is a fundamental process that enables the movement of contents through the esophagus, stomach, and intestines, ensuring the efficient digestion and elimination of waste.
Physiological sphincter: A physiological sphincter is a region where the muscles constrict to control the flow of substances through an organ or duct without an anatomical sphincter structure. It functions through the coordinated action of smooth muscle or, in some cases, skeletal muscle to regulate passage, such as preventing the backflow of urine.
Pontine Micturition Center: The pontine micturition center is a collection of neurons located in the pons region of the brainstem that play a crucial role in the coordination and control of the urinary bladder and urethral sphincter muscles during the process of urination, or micturition.
Prostatic Urethra: The prostatic urethra is the portion of the male urethra that runs through the prostate gland, measuring approximately 3 cm in length. This section is significant because it serves as a conduit for both urine and seminal fluid, playing a vital role in the male reproductive and urinary systems. As urine and semen pass through this segment, various structures and functions are closely related, influencing overall urinary transport and reproductive health.
Pudendal Nerve: The pudendal nerve is a major peripheral nerve that originates from the sacral plexus and innervates the perineal region, including the external genitalia, rectum, and pelvic floor muscles. It is a critical component in the gross anatomy of urine transport, as it provides sensory and motor innervation to the structures involved in urinary function.
Renal pelvis: The renal pelvis is a funnel-shaped structure located within the kidney that collects urine from the collecting ducts and channels it into the ureter. This region plays a crucial role in urine transport, acting as the central hub for urine drainage from the kidney, before it enters the ureter for further movement towards the bladder. The renal pelvis is closely associated with the overall structure and function of the kidney and is vital for maintaining homeostasis within the urinary system.
Sacral micturition center: The sacral micturition center is a group of nerve cells located in the sacral region of the spinal cord that coordinates the process of urination. It sends signals to the bladder and sphincter muscles, controlling their contraction and relaxation for urine expulsion.
Somatic neurons: Somatic neurons are specialized nerve cells that transmit signals between the central nervous system and skeletal muscles, facilitating voluntary movement and sensory perception. They are a key component of the somatic nervous system, which controls activities that are under conscious control, such as walking or picking up objects. Somatic neurons play a vital role in connecting the brain and spinal cord to the muscles, allowing for coordinated motor functions.
Spongy Urethra: The spongy urethra is a section of the urethra that is surrounded by a layer of spongy erectile tissue. It is an important part of the urine transport system, allowing urine to flow from the bladder out of the body.
Transitional epithelium: Transitional epithelium is a type of epithelial tissue that can stretch and change shape, allowing organs like the bladder to expand and contract. It consists of multiple layers of cells that can slide over one another to accommodate organ movement.
Transitional Epithelium: Transitional epithelium is a specialized type of epithelial tissue found in the urinary system, particularly in the bladder and ureters. It is characterized by its ability to change shape and volume in response to the varying pressure and volume of the organs it lines.
Trigone: The trigone is a triangular area located in the bladder's base, formed by the openings of the two ureters and the urethra. It acts as an important landmark in the urinary system, being relatively fixed and less susceptible to stretching compared to other regions of the bladder.
Ureters: Ureters are muscular tubes that transport urine from the kidneys to the bladder. They play a crucial role in the urinary system by ensuring that urine is moved efficiently through peristaltic contractions, which are wave-like muscle movements. Ureters connect to the renal pelvis in the kidneys and enter the bladder at an angle, preventing backflow of urine and maintaining proper urinary flow.
Urethra: The urethra is a tube that connects the urinary bladder to the external body, allowing urine to be expelled from the body. In males, it also serves as a passageway for semen during ejaculation.
Urinary Bladder: The urinary bladder is a hollow, muscular organ that stores urine before it is eliminated from the body. It is an essential part of the urinary system, responsible for collecting and temporarily holding urine produced by the kidneys until it can be voluntarily expelled through the urethra.
Urinary Incontinence: Urinary incontinence is the involuntary loss of urine, which can occur due to a variety of factors affecting the urinary system. It is a common condition that can significantly impact a person's quality of life and is often associated with the gross anatomy of urine transport.
Urinary Tract: The urinary tract is the system of organs and structures involved in the production, transportation, and elimination of urine from the body. This complex system includes the kidneys, ureters, bladder, and urethra, working together to filter blood, remove waste, and maintain fluid balance within the body. Each part plays a specific role in ensuring that urine is efficiently produced and expelled, highlighting the importance of this system for overall health.
Urothelium: Urothelium is a specialized epithelial tissue that lines the urinary tract, including the renal pelvis, ureters, bladder, and part of the urethra. This unique layer plays a crucial role in urine transport, providing a barrier against toxic substances while allowing for the expansion and contraction of the urinary organs as they fill and empty.
Voiding Reflex: The voiding reflex is a physiological mechanism that controls the emptying of the bladder. It is a complex reflex arc involving the bladder, spinal cord, and brain, which coordinates the contraction of the bladder and the relaxation of the urinary sphincter to allow for the efficient elimination of urine from the body.