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Urinary System
Nursing (RLE70)
Capitol University
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Functions of the Urinary System
● Kidneys dispose of waste products in urine ○ Nitrogenous wastes ○ Toxins ○ Drugs ○ Excess ions ● Kidneys’ regulatory functions include: ○ Production of renin to maintain blood pressure ○ Production of erythropoietin to stimulate red blood cell production ○ Conversion of vitamin D to its active form
Organs of the Urinary System
● Kidneys ● Ureters ● Urinary bladder ● Urethra
Kidneys
● Location and structure ○ Retroperitoneal position (behind the parietal peritoneum) ○ Level of the T12 to L3 vertebrate ○ The right kidney is slightly lower than the left (because of position of the liver)
● Kidney structure
○ An adult kidney is about 12 centimeters (5 inches) long and 6 centimeters (2 inches) wide ○ Renal hilum ■ A medial indentation where several structures enter or exit the kidney (Ureters, renal blood vessels, and nerves) ○ An adrenal gland sits atop each kidney ○ Three protective layers enclose the kidney ■ Fibrous capsule encloses each kidney ■ Perirenal fat capsule surrounds the kidney and cushions against blows ■ Renal fascia is the most superficial layer that anchors the kidney and adrenal gland to surrounding structures ○ Three regions revealed in a longitudinal section 1. Renal cortex—outer region 2. Renal medulla—deeper region ● Renal (medullary) pyramids—triangular regions of tissue in the medulla ● Renal columns—extensions of cortex like material that separate thepyramids 3. Renal pelvis—medial region that is a flat, funnel- shaped tube ● Calyces form cup-shaped “drains” that enclose the renal pyramids
● Calyces collect urine and send it to the renal pelvis, on to the ureter, and to the urinary bladder for storage
● Blood supply
○ One-quarter of the total blood supply of the body passes through the kidneys each minute ○ Renal artery provides each kidney with arterial blood supply Renal artery divides into segmental arteries interlobar arteries arcuate arteries cortical radiate arteries
● Venous blood flow
Cortical radiate veins arcuate veins interlobar veins renal vein ○ There are no segmental veins ○ Renal vein returns blood to the inferior vena cava
Nephrons
● Structural and functional units of the kidneys ● Each kidney contains over a million nephrons ● Each nephron consists of two main structures 1. Renal corpuscle 2. Renal tubule
● Renal corpuscle consists of:
- Glomerulus, a knot of capillaries made of podocytes ■ Podocytes make up the inner (visceral) layer of the glomerular capsule ● Foot processes cling to the glomerulus ● Filtration slits create a porous membrane—ideal for filtration
- Glomerular (Bowman’s) capsule is a cup-shaped structure that surrounds the glomerulus ■ First part of the renal tubule
● Renal tubule
○ Extends from glomerular capsule and ends when it empties into the collecting duct ○ From the glomerular (Bowman’s) capsule, the subdivisions of the renal tubule are: 1. Proximal convoluted tubule (PCT) 2. Nephron loop (loop of Henle) 3. Distal convoluted tubule (DCT)
● Cortical nephrons ○ Located entirely in the cortex ○ Include most nephrons ● Juxtamedullary nephrons ○ Found at the cortex-medulla junction ○ Nephron loop dips deep into the medulla ● Collecting ducts collect urine from both types of nephrons, through the renal pyramids, to the calyces, and then to the renal pelvis
● Two capillary beds associated with each nephron
- Glomerulus
- Peritubular capillary bed
● Glomerulus ○ Fed and drained by arterioles ■ Afferent arteriole—arises from a cortical radiate artery and feeds the glomerulus ■ Efferent arteriole—receives blood that has passed through the glomerulus ○ Specialized for filtration ○ High pressure forces fluid and solutes out of blood and into the glomerular capsule
● Peritubular capillary bed
○ Arise from the efferent arteriole of the glomerulus ○ Low-pressure, porous capillaries ○ Adapted for absorption instead of filtration ○ Cling close to the renal tubule to receive solutes and water from tubule cells Drain into the cortical radiate veins arcuate veins interlobar veins
○ Sterile at the time of formation ○ Slightly aromatic, but smells like ammonia with time ○ Slightly acidic (pH of 6) ○ Specific gravity of 1 to 1.
● Solutes normally found in urine ○ Sodium and potassium ions ○ Urea, uric acid, creatinine ○ Ammonia ○ Bicarbonate ions
● Solutes not normally found in urine ○ Glucose ○ Blood proteins ○ Red blood cells ○ Hemoglobin ○ WBCs (pus) ○ Bile
Ureters
● Slender tubes 25–30 centimeters (10–12 inches) attaching the kidney to the urinary bladder ○ Continuous with the renal pelvis ○ Enter the posterior aspect of the urinary bladder ○ Run behind the peritoneum ● Peristalsis aids gravity in urine transport
Micturition ● Micturition
○ Voiding, or emptying of the urinary bladder ○ Two sphincters control the release of urine, the internal urethral sphincter and external urethral sphincter ● Bladder collects urine to 200 milliliters ● Stretch receptors transmit impulses to the sacral region of the spinal cord ● Impulses travel back to the bladder via the pelvic splanchnic nerves to cause bladder contractions ● When contractions become stronger, urine is forced past the involuntary internal sphincter into the upper urethra ● Urge to void is felt ● The external sphincter is voluntarily controlled, so micturition can usually be delayed
Fluid, Electrolyte, and Acid-Base Balance
● Blood composition depends on three factors
- Diet
- Cellular metabolism
- Urine output
● Kidneys have four roles in maintaining blood composition
- Excreting nitrogen-containing wastes (previously discussed)
- Maintaining water balance of the blood
- Maintaining electrolyte balance of the blood
- Ensuring proper blood pH
Maintaining Water Balance of the Blood ● Normal amount of water in the human body ○ Young adult females = 50% ○ Young adult males = 60% ○ Babies = 75% ○ The elderly = 45% ● Water is necessary for many body functions, and levels must be maintained ● Water occupies three main fluid compartments
- Intracellular fluid (ICF) ■ Fluid inside cells ■ Accounts for two-thirds of body fluid
- Extracellular fluid (ECF) ■ Fluids outside cells; includes blood plasma, interstitial fluid (IF), lymph, and transcellular fluid
- Plasma (blood) is ECF, but accounts for 3Liters of total body water.
■ Links external and internal environments (Figure 15)
● The link between water and electrolytes ○ Electrolytes are charged particles (ions) that conduct electrical current in an aqueous solution ○ Sodium, potassium, and calcium ions are electrolytes
● Regulation of water intake and output ○ Water intake must equal water output if the body is to remain properly hydrated ○ Sources for water intake ■ Ingested foods and fluids ■ Water produced from metabolic processes (10%) ○ Thirst mechanism is the driving force for water intake
● Thirst mechanism
○ Osmoreceptors are sensitive cells in the hypothalamus that become more active in reaction to small changes in plasma solute concentration ○ When activated, the thirst center in the hypothalamus is notified ○ A dry mouth due to decreased saliva also promotes the thirst mechanism ○ Both reinforce the drive to drink
● Sources of water output ○ Lungs (insensible since we cannot sense the water leaving) ○ Perspiration ○ Feces ○ Urine ● Hormones are primarily responsible for reabsorption of water and electrolytes by the kidneys ○ Antidiuretic hormone (ADH) prevents excessive water loss in the urine and increases water reabsorption ○ ADH targets the kidney’s collecting ducts
Maintaining Electrolyte Balance
● Small changes in electrolyte concentrations cause water to move from one fluid compartment to another ● A second hormone, aldosterone, helps regulate blood composition and blood volume by acting on the kidney ○ For each sodium ion reabsorbed, a chloride ion follows, and a potassium ion is secreted into the filtrate ○ Water follows salt: when sodium is reabsorbed, water follows it passively back into the blood
Electrolyte Balance
● Renin-angiotensin mechanism ○ Most important trigger for aldosterone release ○ Mediated by the juxtaglomerular (JG) apparatus of the renal tubules ○ When cells of the JG apparatus are stimulated by low blood pressure, the enzyme renin is released into blood ○ Renin catalyzes reactions that produce angiotensin II ○ Angiotensin II causes vasoconstriction and aldosterone release ○ Result is increase in blood volume and blood pressure
Maintaining Acid-Base Balance of Blood
● Blood pH must remain between 7 and 7 to maintain homeostasis ○ Alkalosis—pH above 7. ○ Acidosis—pH below 7. ○ Physiological acidosis—pH between 7 and 7. ● Kidneys play greatest role in maintaining acid-base balance ● Other acid-base controlling systems ○ Blood buffers ○ Respiration
● Blood buffers
○ Acids are proton (H+) donors ■ Strong acids dissociate completely and liberate all of their H+in water ■ Weak acids, such as carbonic acid, dissociate only partially ○ Bases are proton (H+) acceptors ■ Strong bases dissociate easily in water and tie up H+ ■ Weak bases, such as bicarbonate ion and ammonia, are slower to accept H+ ● Molecules react to prevent dramatic changes in hydrogen concentrations ○ Bind to H+ when pH drops ○ Release H+ when pH rises
● Three major chemical buffer systems 1. Bicarbonate buffer system 2. Phosphate buffer system 3. Protein buffer system
● The bicarbonate buffer system ○ Mixture of carbonic acid (H2CO3) and sodium bicarbonate (NaHCO3) ■ Carbonic acid is a weak acid that does not dissociate much in neutral or acid solutions ■ Bicarbonate ions react (HCO3-) with strong acids to change them to weak
Urinary System
Course: Nursing (RLE70)
University: Capitol University
