Urinary System PDF

Urinary System The kidneys - - - - Kidneys' other regulatory functions, : produce the enzyme renin, they help regulate blood pressure. The hormone erythropoietin, released by the kidneys, stimulates red blood cell production in bone marrow. Kidney cells convert vitamin D produce...

Urinary System The kidneys - - - - Kidneys' other regulatory functions, : produce the enzyme renin, they help regulate blood pressure. The hormone erythropoietin, released by the kidneys, stimulates red blood cell production in bone marrow. Kidney cells convert vitamin D produced in the skin to its active form These small, dark red organs with a kidney-bean shape lie against the dorsal body wall in a **retroperitoneal position** (behind the parietal peritoneum) in the superior lumbar region the right kidney is slightly lower than the left. **Kidney Structure** - - - Several structures, including the ureters, the renal blood vessels, and nerves, enter or exit the kidney at the hilum. Sitting atop each kidney is an **adrenal gland**, which is part of the endocrine system and is a separate organ. The kidney has three protective layers. Deep to superficial, they are as follows: A **transparent fibrous capsule** encloses each kidney and gives it a glistening appearance. A fatty mass, the **perirenal fat capsule,** surrounds each kidney and cushions it against blows. The **renal fascia**, the most superficial layer made of dense fibrous connective tissue, anchors the kidney and adrenal gland to surrounding structures If the amount of fatty tissue dwindles (as with rapid weight loss), the kidneys may drop to a lower position, a condition called **ptosis** (to9sis; "a fall"). Ptosis creates problems if the ureters, which drain urine from the kidneys, become kinked. When this happens, urine that can no longer pass through the ureters backs up and exerts pressure on the kidney tissue. This condition, called **hydronephrosis** can severely damage the kidney. When a kidney is cut lengthwise, three distinct regions become apparent The outer region, which is light in color, is the **renal cortex.** Deep to the cortex is a darker reddish brown area, the renal medulla. The medulla has triangular regions with a striped appearance, the renal pyramids, or medullary pyramids. The broad base of each pyramid faces toward the cortex; its tip, the apex, points toward the inner region of the kidney The pyramids are separated by extensions of cortex like tissue, called **renal columns** Lateral to the hilum is a flat, funnel-shaped tube, the **renal pelvis**. The pelvis is continuous with the ureter leaving the hilum **Calyces** - - - **Blood Supply** Approximately one-quarter of the total blood supply of the body passes through the kidneys each minute. **Renal Artery ---** The artery supplying each kidney. As the **renal artery** approaches the hilum, it divides into **segmental arteries**, each of which gives off several branches called **interlobar arteries**, which travel through the renal columns to reach the cortex. At the cortex-medulla junction, interlobar arteries give off the **arcuate arteries**, which arch over the medullary pyramids. **Small cortical radiate arteries** then branch off the arcuate arteries to supply the renal cortex. Venous blood draining from the kidney flows through veins that trace the pathway of the arterial supply but in a reverse direction--- **cortical radiate veins** to **arcuate veins** to **interlobar veins** to the **renal vein**, which emerges from the kidney hilum and empties into the inferior vena cava. **Nephrons** - - Nephron consists of two main structures: a **renal corpuscle** and a **renal tubule**. Each renal **corpuscle** consists of a **glomerulus**--- which is a knot of capillaries and a cup-shaped hollow structure that completely **surrounds the glomerulus** like a well-worn baseball glove encloses a ball. This portion of the renal corpuscle is called the **glomerular capsule** or **Bowman's capsule**. The inner (visceral) layer of the capsule is made up of highly modified octopus-like cells called **podocytes**. **Podocytes** have long branching extensions called **foot processes** that intertwine with one another and cling to the glomerulus. Openings called **filtration slits** between these processes allow the podocytes to form a porous, or "leaky," membrane around the glomerulus ideal for filtration. **renal tubule** - - - - Most nephrons are called **cortical nephrons** located almost entirely within the cortex. In a few cases, the nephrons are called **juxtamedullary** nephrons because they are situated close to the cortex-medulla junction,and their nephron loops dip deep into the medulla. **collecting ducts** Receives urine from many nephrons run downward through the medullary pyramids, giving the pyramids a striped appearance. They deliver the final urine product into the calyces and renal pelvis Each and every nephron is associated with two **capillary beds**---the **glomerulus** and the **peritubular capillary bed** The glomerulus --- fed by the afferent arteriole, which arises from a cortical radiate artery. The efferent arteriole receives the blood as it leaves the glomerulus. The glomerulus, specialized for filtration, differs from any other capillary bed in the entire body because it is both fed and drained by arterioles. Also, the **afferent arteriole has a larger diameter than the efferent, resulting in a much** **higher blood pressure in the glomerular capillaries** This high pressure forces fluid and small solutes out of the blood into the glomerular capsule. Most of this filtrate (99 percent) is eventually reclaimed by the renal tubule cells and returned to the blood in the peritubular capillary beds. the **peritubular capillaries**, arises from the efferent arteriole that drains the glomerulus. these capillaries are low-pressure, porous vessels adapted for **absorption instead of filtration**. The peritubular capillaries drain into the cortical radiate veins, arcuate veins, and ultimately into the interlobar veins leaving the cortex. Urine Formation **Glomerular Filtration** nonselective, passive process fluid passes from the blood into the glomerular capsule part of the renal tubule. Once in the capsule, the fluid is called **filtrate---** blood plasma without blood proteins. As long as the systemic blood pressure is normal, filtrate will be formed. If arterial blood pressure drops too low, glomerular pressure becomes inadequate to force substances out of the blood into the tubules, and filtrate formation stops. **Oliguria** --- abnormally low urinary output if it is between 100 and 400 ml/day **anuria** if it is less than 100 ml/ day. Low urinary output usually indicates that glomerular blood pressure is too low to cause filtration, but anuria may also result from transfusion reactions and acute inflammation or from crushing injuries to the kidneys. **Tubular Reabsorption** the filtrate contains many useful substances (including water, glucose, amino acids, and ions), which must be reclaimed from the filtrate and returned to the blood. Tubular reabsorption begins as soon as the filtrate enters the proximal convoluted tubule The tubule cells are "**transporters**," taking up needed substances from the filtrate and then passing them out their posterior aspect into the extracellular space, from which they are absorbed into peritubular capillary blood. Some reabsorption is done passively (osmosis), but reabsorption of most substances depends on **active transport**. reabsorption occurs in the proximal convoluted tubules, but the distal convoluted tubule and the collecting duct are also active **Tubular Secretion** tubular reabsorption in reverse. Some substances, such as hydrogen and potassium ions (H1 and K1) and creatinine, also move from the blood of the peritubular capillaries through the tubule cells or from the tubule cells themselves into the filtrate to be eliminated in urine. This process seems to be important for getting rid of substances not already in the filtrate, such as certain drugs or excess potassium ions, or as an additional means for controlling blood pH Summary **Glomerular filtration**: Water and solutes smaller than proteins are forced through the capillary walls and pores of the glomerular capsule into the renal tubule. **Tubular reabsorption**: Water, glucose, amino acids, and needed ions are transported out of the filtrate into the tubule cells and then enter the capillary blood. **Tubular secretion**: H1, K1, creatinine, and drugs are removed from the peritubular blood and secreted by the tubule cells into the filtrate. **Nitrogenous Wastes** Nitrogenous waste products are poorly reabsorbed, if at all. Tubule cells have few membrane carriers to reabsorb these substances because we do not need them. They tend to remain in the filtrate and are found in high concentrations in urine excreted from the body. Various ions are reabsorbed or allowed to go out in the urine, according to what is needed at a particular time to maintain the proper pH and electrolyte (solute) composition of the blood. Common nitrogenous wastes include the following: **Urea** formed by the liver as an end product of protein breakdown when amino acids are used to produce energy **Uric acid**, released when nucleic acids are metabolized **Creatinine**, associated with creatine metabolism in muscle tissue **Characteristics of urine** kidneys filter some 150 to 180 liters of blood plasma through their glomeruli into the tubules, which process the filtrate by taking substances out of it (reabsorption) and adding substances to it (secretion). In the same 24 hours, only about 1.0 to 1.8 liters of urine are produced Filtrate contains everything that blood plasma does (except proteins) by the time it reaches the collecting ducts, the filtrate has lost most of its water and just about all of its nutrients and necessary ions. What remains, urine, contains nitrogenous wastes and unneeded or excess substances. Freshly voided urine is generally clear and pale to deep yellow. The normal yellow color is due to **urochrome** a pigment that results from the body's destruction of hemoglobin Urine pH is usually slightly acidic (around 6) a diet with large amounts of protein (eggs and cheese) and whole-wheat products cause urine to become quite acidic. A vegetarian diet makes urine quite alkaline as the kidneys excrete excess bases. Bacterial infection of the urinary tract also may cause the urine to be alkaline. specific gravity of urine usually ranges from 1.001 to 1.035 - - - - - - **Ureters** two slender tubes each 25 to 30 cm (10 to 12 inches) long and 6 mm (¼ inch) in diameter. Each ureter runs behind the peritoneum from the renal hilum to the posterior aspect of the bladder, which it enters at a slight angle The superior end of each ureter is continuous with the renal pelvis, and its mucosal lining is continuous with the mucosa lining the renal pelvis and the bladder inferiorly. The ureters carry urine from the kidneys to the bladder. Ureters would not transport urine while a person is lying down. Urine reaches the bladder even when you're standing on your head because the ureters do play an active role in urine transport Smooth muscle layers in their walls contract to propel urine by peristalsis. Once urine has entered the bladder, it is prevented from flowing back into the ureters by small valvelike folds of bladder mucosa that cover the ureter openings. When urine becomes extremely concentrated, solutes such as uric acid salts form crystals that precipitate in the renal pelvis. These crystals are called **renal** **calculi or kidney stones** Excruciating pain that radiates to the flank (lateral, posterior lower back) occurs when the ureter walls close in on the sharp calculi as they are being eased through the ureter by peristalsis or when the calculi become wedged in a ureter **urinary bladder** smooth, collapsible, muscular sac that stores urine temporarily. It is located retroperitoneally in the pelvis just posterior to the pubic symphysis. ---the two ureter openings (ureteral orifices), and the single opening of the urethra(the internal urethral orifice), which drains the bladder The smooth triangular region of the bladder base outlined by these three openings is called the **trigone** The trigone is important clinically because infections tend to persist in this region. In males, the prostate(part of the male reproductive system) surrounds the neck of the bladder where it empties into the urethra. The bladder wall contains three layers of smooth muscle, collectively called the **detrusor muscle**, and its mucosa is a special type of epithelium, transitional When the bladder is empty, it is collapsed, 5 to 7.5 cm (2 to 3 inches) long at most, and its walls are thick and thrown into folds. As urine accumulates, the bladder expands and rises superiorly in the abdominal cavity Its muscular wall stretches, and the transitional epithelial layer thins A moderately full bladder is about 12.5 cm (5 inches) long and holds about 500 ml (1 pint) of urine, but it is capable of holding more than twice that amount. **urethra** thin-walled tube that carries urine by peristalsis from the bladder to the outside of the body. At the bladder-urethra junction, a thickening of the smooth muscle forms the internal urethral sphincter---involuntary sphincter that keeps the urethra closed when urine is not being passed. A second sphincter, the external urethral sphincter, is formed by skeletal muscle as the urethra passes through the pelvic floor. This sphincter is **voluntarily** **controlled**. in men, the urethra is approximately 20 cm (8 inches) long and has three named regions the **prostatic, membranous, and spongy** (or penile) urethrae. The urethra opens at the tip of the penis after traveling down its length. The urethra of the male has a double function. It carries both urine and sperm (in semen) from the body, but never at the same time Thus, in males, the urethra is part of both the urinary and reproductive systems. In women, the urethra is about 3 to 4 cm (1½ inches) long, and its external orifice, or opening, lies anterior to the vaginal opening Its only function is to conduct urine from the bladder to the body exterior The female urinary orifice is close to the anal opening, so improper toileting habits (wiping from back to front rather than from front to back) can carry fecal bacteria into the urethra. And because the mucosa of the urethra is continuous with that of the rest of the urinary tract organs, inflammation of the urethra, or **urethritis**, can easily ascend the tract to cause bladder inflammation **(cystitis)** or even kidney inflammation (**pyelonephritis, or pyelitis**). Symptoms of urinary tract infection (UTI) include **dysuria (painful urination**), urinary urgency and frequency, fever, and sometimes cloudy or blood-tinged urine. When the kidneys are involved, back pain and a severe headache are common. **Micturition** or **voiding**, is the act of emptying the bladder. As noted, two sphincters, or valves---the internal urethral sphincter and the external urethral sphincter control the flow of urine from the bladder Ordinarily, the bladder continues to collect urine until about 200 ml have accumulated. At this point, stretching of the bladder wall activates stretch receptors. Impulses transmitted to the sacral region of the spinal cord and then back to the bladder via the **pelvic splanchnic nerves** cause the bladder to go into reflex contractions. As the contractions become stronger, stored urine is forced past the internal urethral sphincter into the upper part of the urethra. The person will then feel the urge to void. Because the lower external sphincter is skeletal muscle and is controlled voluntarily, we can choose to keep it closed and postpone bladder emptying temporarily. However, if it is convenient, the external sphincter can be relaxed so that urine is flushed from the body. When a person chooses not to void, the reflex contractions of the bladder stop within a minute or so, and urine collection continues. After 200 to 300 ml more have been collected, the micturition reflex occurs again. **Incontinence** Incontinence occurs when a person is unable to voluntarily control the external sphincter **Urinary Retention** essentially the opposite of incontinence. In this condition, the bladder is unable to expel its contained urine **hyperplasia** of the prostate, which surrounds the neck of the bladder. As the prostate gland enlarges, it narrows the urethra, making voiding very difficult. When urinary retention is prolonged, a slender flexible drainage tube called a **catheter must** be inserted through the urethra to drain the urine Water occupies three main locations within the body, referred to as fluid compartments About two-thirds of body fluid, the so-called **intracellular fluid (ICF**), is contained within the living cells. **extracellular fluid (ECF**), includes all body fluids located outside the cells. ECF includes blood plasma, interstitial fluid (IF) between cells, lymph, and transcellular fluid (in chambers lined with epithelium), which includes cerebrospinal and serous fluids, the humors of the eye, and others The **thirst mechanism** is the driving force for water intake. An increase in plasma solute content of only 2 to 3 percent excites highly sensitive cells in the hypothalamus called osmoreceptors which in turn activate the hypothalamic thirst center nerve impulses are sent to the posterior pituitary which then releases **antidiuretic hormone (ADH).** (The term antidiuretic is derived from diuresis which means "flow of urine from the kidney," and anti, which means "against.") this hormone prevents excessive water loss in the urine. Diabetes Insipidus ---When ADH is not released (perhaps because of injury or destruction of the hypothalamus or posterior pituitary gland), huge amounts of very dilute urine (up to 25 liters/day) flush from the body day after day. Besides ADH, aldosterone is a second hormone that helps to regulate blood composition and blood volume by acting on the kidney. **Aldosterone** is the major factor regulating sodium ion content of the ECF and in the process helps regulate the concentration of other ions and magnesium as well. Sodium ions are the electrolytes most responsible for osmotic water flow. When too few sodium ions are in the blood, water will leave the blood and enter tissues, causing **edema**. If serious, this situation can cause the circulatory system to shut down. Regardless of whether aldosterone is present or not, about 80 percent of the sodium ions in the filtrate are reabsorbed in the proximal convoluted tubules of the kidneys. When the aldosterone concentration is high, most of the remaining sodium ions are actively reabsorbed in the distal convoluted tubules and the collecting ducts. the most important trigger for aldosterone release is the **renin-angiotensin mechanism** mediated by the juxtaglomerular apparatus of the renal tubules. The juxtaglomerular apparatus consists of a complex of modified smooth muscle cells (JG cells) in the afferent arteriole plus some modified epithelial cells forming part of the distal convoluted tubule When the cells of the JG apparatus are stimulated by low blood pressure in the afferent arteriole or changes in solute content of the filtrate, they respond by releasing the enzyme **renin** into the blood an enzyme secreted by stomach glands. Renin initiates the series of reactions that produce angiotensin II. Angiotensin II in turn acts directly on the blood vessels to cause vasoconstriction (leading to an increase in peripheral resistance) and on the adrenal cortical cells to promote aldosterone release. As a result, blood volume and blood pressure increase. The renin-angiotensin mechanism is extremely important for regulating blood pressure. When pressure drops, baroreceptors in large blood vessels are also excited. These baroreceptors alert sympathetic nervous system centers of the brain to cause vasoconstriction (via release of epinephrine and norepinephrine) However, this neural mechanism's major focus is blood pressure regulation, not water or electrolyte balance. People with Addison's disease (hypoaldosteronism) have polyuria (excrete large volumes of urine) and so lose tremendous amounts of salt and water to urine. blood pH must be maintained between 7.35 and 7.45, a very narrow range. Whenever the pH of arterial blood rises above 7.45, a person is said to have **alkalosis**. A drop in arterial pH to below 7.35 results in **acidosis** Because a pH of 7.0 is neutral, any pH between 7.0 and 7.35 is called **physiological acidosis** **Blood Buffers** Chemical buffers are systems of one or two molecules that act to prevent dramatic changes in the hydrogen ion concentration when acids or bases are added. They do this by binding to hydrogen ions whenever the pH drops and by releasing hydrogen ions when the pH rises The three major chemical buffer systems of the body are the **bicarbonate, phosphate, and protein buffer systems** The **bicarbonate buffer system** is a mixture of carbonic acid (H2CO3) and its salt, sodium bicarbonate(NaHCO3). Carbonic acid is a weak acid, so it does not dissociate much in neutral or acidic solutions. Thus, when a strong acid such as hydrochloric acid is added, most of the carbonic acid remains intact. However, the bicarbonate ions (HCO3) of the salt act as bases to tie up the hydrogen ions released by the stronger acid, forming more carbonic acid: Respiratory Mechanisms The respiratory system eliminates carbon dioxide from the blood while it "loads" oxygen into the blood. When carbon dioxide (CO2) enters the blood from the tissue cells, most of it enters the red blood cells, where it is converted to bicarbonate ion **Renal Mechanisms** Chemical buffers can tie up excess acids or bases temporarily, but they cannot eliminate them from the body. And although the lungs can decrease carbonic acid levels by eliminating carbon dioxide, only the kidneys can rid the body of other acids generated during metabolism. Additionally, only the kidneys have the power to regulate blood levels of alkaline substances. The most important means by which the kidneys maintain acid-base balance of the blood are by excreting bicarbonate ions and reabsorbing or generating new bicarbonate ions. As blood pH rises, bicarbonate ions are excreted and hydrogen ions are retained by the tubule cells. Conversely, when blood pH falls, bicarbonate is reabsorbed and generated, and hydrogen ions are secreted. Urine pH varies from 4.5 to 8.0, which reflects the ability of the renal tubules to excrete basic or acidic ions to maintain blood pH homeostasis. **Adult polycystic** kidney disease is a degenerative condition that appears to run in families. One or both kidneys enlarge, sometimes to the size of a football, and have many blisterlike sacs (cysts) containing urine. These cysts interfere with renal function by obstructing but initially not stopping urine drainage Childhood streptococcalinfections, such as strep throat and scarlet fever, may cause inflammatory damage to the kidneys if the original infections are not treated promptly and properly. A common sequel to untreated childhood strep infections is glomerulonephritis, in which the glomerular filters become clogged with antigen-antibody complexes resulting from the strep infection Many types of bacteria may invade the urinary tract to cause urethritis, cystitis, or pyelonephritis. Escherichia coli bacteria are normal residents of the digestive tract and generally cause no problems there, but they act as pathogens (disease-causing agents) in the sterile environment of the urinary tract and account for 80 percent of urinary tract infections. Bacteria and viruses responsible for sexually transmitted infections (STIs), which are primarily reproductive tract infections, may also invade and cause inflammation in the urinary tract, Another consequence of aging is bladder shrinkage and loss of bladder tone, causing many elderly individuals to experience **urgency** (a feeling that it is necessary to void) and **frequency** (frequent voiding of small amounts of urine). **nocturi**a the need to get up during the night to urinate,