Exam 4 - Urinary System Flashcards
Organs of urinary system
Two kidneys
Two ureters
Urinary bladder
Urethra
Main functions of kidneys
1) Excretion: Filters waste products from the blood, converting filtered fluid into urine while retaining large molecules like proteins.
2) Regulation of blood volume and pressure: Controls extracellular fluid volume and blood pressure by adjusting urine volume and concentration.
3) Regulation of blood solute concentrations: Maintains proper ion and solute levels (e.g., Na⁺, K⁺, Cl⁻, Ca²⁺, HCO₃⁻, HPO₄²⁻, urea) in the blood.
4) Regulation of extracellular fluid pH: Adjusts H⁺ secretion to maintain pH balance in extracellular fluid.
5) Regulation of red blood cell synthesis: Produces erythropoietin, a hormone that stimulates red blood cell production in bone marrow.
6) Regulation of vitamin D synthesis: Helps control blood Ca²⁺ levels by aiding in the synthesis of active vitamin D.
Location of kidneys
The kidneys are bean-shaped organs located in the nonmesenteric (retroperitoneal) region of the abdomen, meaning they lie behind the peritoneum.
Positioned on either side of the vertebral column, near the psoas major muscles.
They extend from the T12 to L3 vertebrae, with the right kidney slightly lower than the left due to the liver’s position above it.
External anatomy of kidney
Each kidney is about the size of a clenched fist:
11 cm long, 5 cm wide, 3 cm thick
Weighs ~130 g, roughly the weight of a cup of flour.
Outer layer: Renal capsule, a tough layer of connective tissue.
Surrounded by: A layer of adipose tissue for cushioning and protection.
Surrounded by: Renal fascia, a thin connective tissue layer that anchors the kidney to the abdominal wall.
Additional adipose tissue surrounds the renal fascia.
External anatomy of kidney: Hilum
The hilum is a small, indented region on the medial, concave side of the kidney that serves as the entry and exit point for several important structures. It leads into the renal sinus, a cavity filled with adipose and connective tissue. Through the hilum, the renal artery and nerves enter the kidney, while the renal vein, ureter, and lymphatic vessels exit. All these structures pass through the renal sinus, making the hilum a crucial gateway for vascular, nervous, and urinary connections.
Internal regions of kidney
The kidney is divided into two main regions: the renal cortex, which houses the blood-filtering structures, and the renal medulla, which surrounds the renal sinus. The medulla contains cone-shaped structures called renal pyramids, with their bases facing the cortex and their tips, or renal papillae, pointing toward the renal sinus. Projections of the pyramids into the cortex are called medullary rays, and the spaces between them contain renal columns, which are extensions of cortical tissue. Once urine is formed in the renal pyramids, it flows through ducts to the papillae and into minor calyces, small funnel-like chambers. Multiple minor calyces combine into major calyces, and all major calyces drain into the renal pelvis, a large funnel-shaped structure that leads to the ureter. The ureter then transports urine from the renal pelvis to the urinary bladder.
Structure and histology of nephron
The nephron is the structural and functional unit of the kidney, consisting of specialized structures called renal tubules. Each kidney contains about 1.3 million nephrons distributed throughout the cortex and medulla. A nephron is approximately 50–55 mm long and includes four main parts: the renal corpuscle, proximal convoluted tubule, nephron loop (loop of Henle), and distal convoluted tubule. These parts play distinct roles in urine formation: the renal corpuscle filters blood, the proximal convoluted tubule reabsorbs filtered substances, the nephron loop conserves water and solutes, and the distal convoluted tubule removes additional waste. The collecting ducts receive fluid from many nephrons and help regulate urine concentration as they carry urine toward the renal papilla. Near the papilla, several collecting ducts merge into a papillary duct, which then drains urine into a minor calyx.
Blood supply of kidney
1) The renal artery delivers about 21% of cardiac output to the kidneys. It gives rise to segmental arteries, which distribute blood to different kidney regions.
2) Interlobar arteries travel between renal pyramids.
3) Arcuate arteries branch from the interlobar arteries and arch between the cortex and medulla.
4) Cortical radiate arteries extend from arcuate arteries into the renal cortex.
5) Afferent arterioles branch from the cortical radiate arteries and carry blood to the glomerular capillaries for filtration.
6) Glomerular capillaries are the site where blood filtration occurs.
7) Efferent arterioles carry filtered blood away from the glomerulus.
8) Peritubular capillaries, which branch from efferent arterioles, surround the renal tubules (proximal and distal convoluted tubules, and nephron loop) and are involved in reabsorption and secretion.
9) Vasa recta are straight capillaries that extend deep into the medulla and run alongside the nephron loops and collecting ducts, playing a key role in urine concentration.
3 processes necessary for urine production
1) Filtration: Blood pressure forces fluid and small molecules out of the glomerular capillaries into the nephron. This nonselective process creates filtrate by removing substances small enough to pass through the filtration membrane.
2) Tubular Reabsorption: Transport proteins in the renal tubules move useful substances like water and solutes from the filtrate back into the bloodstream, preventing their loss in urine.
3) Tubular Secretion: Additional waste solutes are moved from the blood into the filtrate by tubule cells, including substances that weren’t initially filtered, further refining urine composition.
Principal factors that influence filtration and how they affect the rate of filtrate formation
Glomerular Capillary Pressure (GCP):
GCP is the blood pressure inside glomerular capillaries and acts as an outward force, pushing fluid and solutes into the glomerular capsule. It is higher than in other capillaries (~50 mm Hg) due to the smaller diameter of the efferent arteriole, which increases resistance and pressure.
Capsular Hydrostatic Pressure (CHP):
CHP is an inward force created by the pressure of filtrate already in the glomerular capsule that resists further filtration. Its value is typically around 10 mm Hg.
Blood Colloid Osmotic Pressure (BCOP):
BCOP is another inward pressure caused by the osmotic pull of plasma proteins in the blood, drawing water back into the capillaries. It increases toward the end of the glomerulus and averages about 30 mm Hg.
How filtration is regulated
Two ways its regulated:
1) Intrinsic Mechanisms: Autoregulation: Autoregulation of kidney function is controlled by two mechanisms: the myogenic mechanism and tubuloglomerular feedback. The myogenic mechanism involves smooth muscle cells in the walls of afferent and efferent arterioles that respond to changes in blood pressure. When blood pressure rises, these cells stretch and contract, causing vasoconstriction of the afferent arteriole to reduce blood flow. When blood pressure drops, the muscle cells relax, leading to vasodilation, which increases blood flow. This mechanism helps maintain a stable glomerular filtration rate (GFR) despite fluctuations in systemic blood pressure.
2) Extrinsic Mechanisms: Hormones: In severe cases like hemorrhage or dehydration, blood pressure drops, triggering the sympathetic nervous system to reduce kidney blood flow and GFR in order to maintain overall blood pressure. Norepinephrine causes vasoconstriction of small arteries and afferent arterioles, decreasing renal perfusion and filtrate formation. While extreme stimulation (e.g., shock or heavy exercise) can significantly reduce filtration, minor sympathetic input has little effect on kidney function.
Role of proximal convoluted tubule in the process of reabsorption
The proximal convoluted tubule is the primary site for reabsorption in the kidney. Its cells are lined with microvilli to increase surface area for efficient reabsorption. Each cell has three distinct regions:
Basal membrane (outer wall),
Apical membrane (inner surface housing microvilli), and
Lateral surface (connecting adjacent cells).
Sodium (Na⁺) diffusion into tubule cells drives the reabsorption of most solutes due to a strong concentration gradient. Carrier proteins on the apical membrane co-transport Na⁺ with other molecules like glucose and amino acids into the cell. These molecules then exit into the interstitial fluid across the basal membrane.
Some solutes also diffuse between cells through lateral surfaces into the interstitial fluid. As solutes move out, water follows by osmosis, concentrating the remaining filtrate. By the end of the proximal tubule, about 65% of filtrate volume is reabsorbed, but the osmolarity remains nearly equal to that of the surrounding fluid (about 300 mOsm/kg) due to water permeability.
Role of nephron loop in the process of reabsorption
Descending Limb:
The thin segment of the descending limb is composed of simple squamous epithelium and is highly permeable to water, allowing water to exit by osmosis. It is also moderately permeable to ions like Na⁺ and Cl⁻ and urea. As water leaves the filtrate, it becomes more concentrated. By the end of the descending limb, the filtrate volume is reduced by about 15%, and its concentration increases to around 1200 mOsm/L.
Ascending Limb:
Initially, the ascending limb remains permeable to solutes but becomes impermeable to water, which helps lower filtrate concentration as solutes are reabsorbed. As the limb transitions into the thick segment, composed of simple cuboidal epithelium, it becomes impermeable to both water and solutes. Specialized transport proteins actively pump solutes like Na⁺ out of the filtrate into the interstitial fluid, playing a key role in the kidney’s ability to conserve water.
Role of distal convoluted tubule and collecting duct in the process of reabsorption
Reabsorption of certain solutes like K⁺ and H⁺ occurs later in the nephron, primarily in the distal convoluted tubule and collecting duct, and is regulated by hormones based on body needs. These regions are not always permeable to water, but the hormone ADH (antidiuretic hormone) increases their water permeability. When ADH is present, water is reabsorbed by osmosis due to the high solute concentration in the surrounding interstitial fluid, leading to concentrated urine. Without ADH, these tubules remain impermeable to water, resulting in the production of dilute urine.
Substances moved during tubular secretion and how they are moved
Proximal convoluted tubule:
1) Antioport: H+
2) Active transport: Hydroxybenzoates, PAra-aminohippuric acid, neurotransmitters (dopamine, acetylcholine, epinephrine), bile pigments, uric acid, drugs and toxins (penicillin, atropine, morphine)
3) Diffusion: Ammonia
Distal convoluted tubule:
1) Antiport: K+ and H+
2) Active transport: K+
How substances move across wall of tubule
1) Active transport: Actively secrete substances like H⁺, K⁺, penicillin, and para-aminohippuric acid (PAH) into the renal tubule.
2) Antiport system: Antiporters, such as the Na⁺/H⁺ antiporter, exchange Na⁺ into the tubule cell for H⁺ out into the filtrate.
3) Passive secretion/Diffusion: Substances like ammonia diffuse from epithelial cells into the tubule lumen.
Three mechanisms that explain kidney’s ability to concentrate urine
1) Countercurrent mechanisms
2) Medullary concentration gradient
3) Hormonal mechanisms
Countercurrent mechanisms
The kidneys use countercurrent mechanisms, where fluids in nearby structures flow in opposite directions, allowing for efficient exchange of materials. These mechanisms help regulate water and solute balance. There are two types:
Countercurrent Multiplier:
Located in the nephron loop, it creates a high concentration of solutes in the kidney’s medulla. This is made possible by changes in permeability along the nephron loop.
Countercurrent Exchanger:
Found in the vasa recta, it maintains the medulla’s high solute concentration. Because blood in the vasa recta flows slowly and at low pressure, it stays in equilibrium with the interstitial fluid, preventing solute loss and supporting the kidney’s water conservation.
Medullary Concentration Gradient
The medullary concentration gradient is the high solute concentration found in the kidney’s medulla, essential for water reabsorption and urine concentration. It is created and maintained by two key mechanisms:
- Countercurrent Multiplier (Nephron Loop):
In the descending limb, water leaves the filtrate by osmosis due to high solute concentration in the surrounding interstitial fluid.
In the ascending limb, solutes (but not water) diffuse or are actively pumped out into the interstitial fluid, further increasing medullary solute concentration.
Only juxtamedullary nephrons, with loops extending deep into the medulla, significantly contribute to this gradient, although cortical nephrons also help by passing filtrate through collecting ducts.
Animals that produce highly concentrated urine (like desert mammals) have more juxtamedullary nephrons. - Countercurrent Exchanger (Vasa Recta):
The vasa recta preserve the medullary gradient by balancing solute and water exchange as blood flows down into the medulla and then back up toward the cortex.
The slow, low-pressure blood flow allows water and solutes to diffuse in and out efficiently without washing away the gradient.
As a result, the blood entering and leaving the vasa recta has nearly the same composition, maintaining the stability of the gradient. - Urea Recycling:
Urea contributes significantly to medullary osmolality.
It diffuses out of the collecting ducts into the interstitial fluid and is then reabsorbed into the descending limb of nephron loops.
Because the ascending limb and distal tubule are impermeable to urea, it cycles back into the collecting ducts, maintaining a high concentration of urea in the medulla.
Together, these processes enable the kidney to conserve water and produce concentrated urine by maintaining a stable and steep solute gradient in the medulla.
Hormonal mechanisms that allow kidney to concentrate urine: ADH
Antidiuretic Hormone (ADH):
ADH plays a key role in regulating water balance and urine concentration. It increases the permeability of the distal convoluted tubule and collecting duct to water by promoting the insertion of aquaporin-2 channels into their apical membranes. This allows water to move out of the tubules by osmosis into the hyperosmotic interstitial fluid of the medulla, producing concentrated urine with minimal water loss. In the absence of ADH, these tubules remain impermeable to water, resulting in the excretion of large volumes of dilute urine. ADH ensures that water is conserved when the body is dehydrated, and its effect is crucial in maintaining homeostasis in response to changes in blood osmolality.
Hormonal mechanisms that allow kidney to concentrate urine: Renin-Angiotensin-Aldosterone Mechanism
Renin-Angiotensin-Aldosterone Mechanism:
This hormonal system is primarily activated by low blood pressure or low sodium levels. The kidneys release renin, which leads to the formation of angiotensin II—a potent vasoconstrictor that also stimulates the release of aldosterone from the adrenal glands. Aldosterone promotes sodium reabsorption in the distal tubules and collecting ducts, which also draws water back into the bloodstream, increasing blood volume and pressure. It also enhances potassium secretion. When aldosterone levels drop, sodium and water reabsorption decrease, leading to increased urine volume and more dilute urine. This mechanism is essential for long-term blood pressure regulation.
Hormonal mechanisms that allow kidney to concentrate urine: ANH
Atrial Natriuretic Hormone (ANH):
ANH is secreted by the right atrium of the heart in response to increased blood volume and atrial stretch. It acts to lower blood pressure and volume by inhibiting sodium reabsorption in the kidney tubules and suppressing ADH secretion. This results in greater excretion of sodium and water, leading to increased urine output. ANH also causes vasodilation, which reduces peripheral resistance and enhances the return of blood to the heart. Overall, ANH serves as a counter-regulatory hormone that reduces the effects of the renin-angiotensin-aldosterone system and ADH when blood volume is too high.
Anatomy and histology of ureters and urinary bladder
Ureters: The ureters are muscular tubes that transport urine from the kidneys to the urinary bladder. They originate from the renal pelvis, exit the kidney at the renal hilum, and extend inferiorly and medially through the abdominal cavity to reach the bladder. The ureters enter the posterolateral surface of the bladder.
Urinary Bladder:
The urinary bladder is a hollow, muscular organ located in the pelvic cavity, just behind the symphysis pubis. In males, it is positioned anterior to the rectum; in females, it lies anterior to the vagina and below and in front of the uterus. The bladder’s volume expands and contracts based on the amount of urine it contains.
Transitional epithelium lines both ureters and urinary bladder. Rest of the walls of these structures consists of lamina propria, a muscular coat, and a fibrous adventitia. The wall of the urinary bladder is much thicker than wall of ureter because it consists of layers of primarily smooth muscle
Anatomy and histology of urethra
The urethra is the tube that transports urine from the urinary bladder to the outside of the body, exiting inferiorly and anteriorly. It is located at the base of the bladder and forms part of a triangular area known as the trigone, which lies between the openings of the two ureters and the urethral opening. The trigone is histologically distinct because it does not expand as the bladder fills, helping to funnel urine during bladder emptying. The urethra is lined with stratified or pseudostratified columnar epithelium, adapting to the flow of urine.
Has internal urethral sphincter that prevents urine leakage from urinary bladder
Has well-defined external urethral sphincter made of skeletal muscle. Allows person to voluntarily start or stop the flow of urine through the urethra
Flow of urine from nephron to urinary bladder
Urine formation begins in the nephron, where filtrate flows from the glomerular capsule through the renal tubule, progressively moving toward the renal pelvis. As it moves along the tubule, hydrostatic pressure decreases from about 10 mm Hg to 0 mm Hg, facilitating the flow into the renal pelvis. From there, urine enters the ureters, but since there is no pressure gradient to move urine forward, it is propelled by peristaltic contractions of smooth muscle in the ureter walls. These contractions are wave-like movements that occur every few seconds to minutes, pushing urine toward the urinary bladder at a speed of about 3 cm/s, generating pressures above 50 mm Hg.
The ureters penetrate the urinary bladder at an oblique angle through the trigone, forming a valvelike mechanism that prevents backflow when bladder pressure rises. Once inside the bladder, urine accumulates, and pressure gradually builds as volume increases—from 0 mm Hg when empty to 10 mm Hg at 100 mL, rising slowly until around 300 mL, then increasing more sharply above 400 mL, signaling the urge to void.
Micturition reflex
The micturition reflex is the process responsible for urination, involving both involuntary reflexes and voluntary control. As urine accumulates in the bladder, the wall stretches, and when volume exceeds around 400 mL, stretch receptors send signals to the brain, increasing the urge to urinate. The reflex is coordinated by the sacral region of the spinal cord and causes bladder contraction and relaxation of the internal urethral sphincter. Voluntary urination requires signals from the cerebrum to relax the external urethral sphincter and may be enhanced by abdominal muscle contraction, which increases bladder pressure. Typically, the urge to urinate is weak when bladder volume is under 300 mL, but pressure rises sharply beyond that point. Damage to the nervous system can disrupt this reflex. If the spinal cord is damaged above the sacral region, the reflex may recover without conscious control (automatic bladder). Damage within the sacral region can eliminate the reflex, causing the bladder to overfill and leak. In older adults or individuals with brain or spinal injuries, loss of inhibitory control can make the reflex hyperactive, leading to involuntary urination even with small volumes of urine.
Function of urinary system (NOTES)
The primary functions of the urinary system are carried out by the kidneys, involving three major processes: filtration, reabsorption, and secretion. The kidneys filter about 200 liters of blood daily, removing metabolic wastes, toxins, and excess ions. They regulate the volume and chemical composition of the blood and maintain proper water, salt, and acid-base balance in the body. During prolonged fasting, the kidneys perform gluconeogenesis to help maintain blood glucose levels. Additionally, they produce renin, which regulates blood pressure, and erythropoietin, which stimulates the formation of red blood cells. Another vital role of the kidneys is the activation of vitamin D, which is crucial for calcium absorption and bone health.
Outer anatomy of kidney (NOTES)
The kidneys are retroperitoneal organs located behind the peritoneum. Each kidney has a renal hilus where blood vessels, lymphatics, and nerves enter and exit. Three supportive tissue layers surround the kidneys: the renal capsule (a fibrous barrier that prevents infection), the adipose capsule (a fatty cushion that stabilizes the kidney’s position), and the renal fascia (a dense connective tissue layer that anchors the kidney).
Inner anatomy of kidney (NOTES)
Internally, the kidney is divided into three main regions. The renal cortex is the outermost layer, appearing light-colored and granular, and contains the glomeruli. The renal medulla consists of renal pyramids and renal columns. The pyramids are bundles of collecting tubules that drain into the papillae. The renal pelvis, located centrally, collects urine from the minor and major calyces, which in turn receive it from the papillae.
Structure of nephron (NOTES)
The nephron is the structural and functional unit of the kidney. There are two types: cortical nephrons, which have short loops of Henle and reside primarily in the cortex, and juxtamedullary nephrons, which have long loops of Henle extending deep into the medulla. Blood enters the glomerulus through the afferent arteriole (which has a larger diameter) and exits through the efferent arteriole (smaller diameter). This configuration creates high blood pressure in the glomerulus, which facilitates the formation of filtrate. The peritubular capillaries, arising from efferent arterioles, are low-pressure vessels that wrap around renal tubules and are adapted for reabsorption. In juxtamedullary nephrons, the vasa recta are specialized long straight vessels that play a role in concentrating urine.
Renal corpuscle of nephron (NOTES)
The renal corpuscle of the nephron consists of the glomerulus and the Bowman’s capsule. The glomerular epithelium is fenestrated, allowing the passage of solute-rich, protein-free filtrate. The visceral layer of the capsule contains podocytes with filtration slits that regulate the filtration process. The renal tubule includes the proximal convoluted tubule (PCT), the loop of Henle (with thin and thick segments), and the distal convoluted tubule (DCT). The PCT reabsorbs nutrients, ions, and water, while the loop of Henle differentially reabsorbs water and solutes. The DCT is mainly involved in secretion and is composed of principal cells that lack microvilli.
Juxtaglomerular apparatus and blood supply (NOTES)
The juxtaglomerular apparatus (JGA) is located where the DCT touches the afferent arteriole. It includes juxtaglomerular (JG) cells, which are mechanoreceptors containing renin, and the macula densa, which are chemoreceptors detecting NaCl concentration. Mesangial cells connect these components via gap junctions and mediate communication.
Blood pressure decreases along the renal circulation. It is highest in the glomerulus, facilitating a strong outward pressure for filtration. The filtration membrane between the blood and the filtrate consists of the fenestrated endothelium, a gel-like basement membrane, and the podocyte filtration slits.
Mechanisms of urine formation (NOTES)
Urine formation involves the entire plasma volume being filtered by the kidneys. Initially, the filtrate includes all plasma components except proteins. As it passes through the nephron, useful substances such as water and nutrients are reabsorbed, leaving behind waste and unneeded substances to form urine. The three key steps are glomerular filtration, tubular reabsorption, and tubular secretion.
Mechanisms of urine formation: Glomerular filtration (NOTES)
Glomerular filtration is highly efficient due to the large surface area and permeability of the filtration membrane, as well as high glomerular blood pressure. Net filtration pressure (NFP), which drives this process, is calculated as the glomerular hydrostatic pressure minus the sum of the osmotic pressure of glomerular blood and the capsular hydrostatic pressure. The glomerular filtration rate (GFR), the volume of filtrate formed per minute, depends on the surface area available for filtration, the permeability of the membrane, and the net filtration pressure.
Regulation of glomerular filtration rate: Intrinsic controls (NOTES)
GFR is tightly regulated through intrinsic and extrinsic mechanisms. Intrinsic controls (renal autoregulation) maintain a constant GFR when mean arterial pressure is between 80–180 mmHg. Myogenic mechanisms constrict or dilate the afferent arteriole in response to changes in blood pressure. Tubuloglomerular feedback involves the macula densa sensing NaCl concentration and adjusting arteriole tone accordingly.
Regulation of glomerular filtration rate: Extrinsic controls (NOTES)
GFR is tightly regulated through intrinsic and extrinsic mechanisms. Extrinsic controls include neural and hormonal responses. During stress, sympathetic activation causes vasoconstriction of afferent arterioles, reducing filtration and triggering renin release. The renin-angiotensin-aldosterone system (RAAS) then increases blood volume and pressure by promoting sodium and water reabsorption.
Tubular reabsorption (NOTES)
Tubular reabsorption selectively reclaims desirable substances from the filtrate. Although the body produces about 180 liters of filtrate daily, only 1.5 liters of urine is excreted. Most reabsorption occurs in the PCT, where ions (Na⁺, K⁺, Ca²⁺, Mg²⁺), water, and nutrients are transported across cell membranes. Sodium reabsorption is primarily active and drives the reabsorption of other substances and water. Hormonal regulation of reabsorption occurs in the DCT and collecting ducts, involving ADH (water), aldosterone (Na⁺), PTH (Ca²⁺), and ANP (Na⁺ inhibition).
Tubular secretion (NOTES)
Tubular secretion moves substances from the peritubular capillaries into the nephron tubules. It is vital for eliminating drugs, excess ions (like potassium), and maintaining acid-base balance. This process also removes substances bound to plasma proteins that were not filtered initially.
Urine concentration mechanisms (NOTES)
The default production is dilute urine, where collecting ducts are impermeable to water. Concentrated urine forms under the influence of ADH, which increases water permeability in the collecting ducts. The counter-current mechanism involves the loop of Henle and vasa recta. In the descending limb, water is reabsorbed due to high surrounding solute concentration, while the ascending limb actively transports out solutes but is impermeable to water. This interaction creates a concentration gradient in the medulla, aiding in water conservation.
Ureters (NOTES)
Ureters transport urine from the kidneys to the bladder through a trilayered wall: transitional epithelium (mucosa), smooth muscle (muscularis), and fibrous connective tissue (adventitia). Entry into the bladder occurs at the posterior wall, and increased bladder pressure closes the ureteral openings to prevent backflow.
Bladder (NOTES)
The bladder, a retroperitoneal organ, typically holds about 500 mL. It has three openings (two ureters and one urethra) forming the trigone area, which is prone to infection. The bladder wall consists of a transitional epithelium mucosa, a muscularis with detrusor muscle (circular, longitudinal, and oblique layers), and an outer adventitia.
Urethra (NOTES)
The urethra drains urine from the bladder and differs between sexes. It includes two sphincters: internal (involuntary) and external (voluntary). The male urethra has three sections: prostatic, membranous, and spongy.
Micturition reflex (NOTES)
Micturition, or urination, involves both voluntary and involuntary processes. Stretch receptors in the bladder wall send signals to the spinal cord, which can trigger the micturition reflex. Parasympathetic stimulation contracts the detrusor muscle and opens the internal sphincter, while voluntary control of the external sphincter is mediated by somatic motor neurons. Higher brain centers can override the reflex through the pontine micturition and storage centers.
Development of kidneys (NOTES)
Three sets of kidneys form during embryonic development, but only one remains functional by the fifth week. By the third fetal month, the kidneys produce urine. In infants, bladder capacity is small, and the kidneys cannot concentrate urine efficiently. Voluntary control over the external urethral sphincter typically develops around age two.
Aging of kidneys (NOTES)
With aging, kidney function gradually declines. The number of glomeruli and the ability to concentrate urine decrease. The kidneys become less responsive to hormones like ADH and aldosterone, and there is reduced vitamin D activation, contributing to calcium deficiency and osteoporosis. Incontinence becomes more common in the elderly.
Common disorders of urinary system (NOTES)
Urinary tract infections are commonly caused by E. coli. Sexually transmitted diseases can also inflame the urinary tract. Congenital disorders include horseshoe kidneys (fused kidneys), hypospadias (urethral opening in the wrong location), and polycystic kidney disease (cyst formation that can lead to renal failure).
Common diuretics of urinary system (NOTES)
Diuretics increase urine output and include various types. Osmotic diuretics, such as excess glucose in diabetics, are not reabsorbed. ADH inhibitors like alcohol reduce water reabsorption. Substances like caffeine and certain medications inhibit sodium reabsorption, thereby increasing water loss.
