L4 Urinary System PDF

BIOL2012SEF The Urinary System Dr Emily Wong 1 Urinary System Consists of – – – – Kidneys Ureters Urinary bladder urethra 2 Kidneys A pair of bean-shaped organs located just below the rib cage and behind the peritoneum (retroperitoneal). To accommodate the size of the liver and the larger size of th...

BIOL2012SEF The Urinary System Dr Emily Wong 1 Urinary System Consists of – – – – Kidneys Ureters Urinary bladder urethra 2 Kidneys A pair of bean-shaped organs located just below the rib cage and behind the peritoneum (retroperitoneal). To accommodate the size of the liver and the larger size of the right kidney, the left kidney is located a little higher than the right kidney. The kidneys are surrounded by some adipose tissues for holding them in place as well as protecting them from physical damage. 3 Structure of the Kidney 4 Kidneys Two distinct regions: – the outer layer, the cortex, and – the inner region, the medulla Inside the medulla, there are structures called renal pyramids, which are made up of nephrons. The nephron is the basic functional unit of the kidneys. Each kidney contains approximately 1 million similar subunits, nephrons. It looks like a long coiled tube with a hairpin loop in the middle of its length. Each nephron comprises a filtering component called a renal corpuscle and a lengthy narrow tubule named a renal tubule. The primary function of the renal corpuscle is to filter the waste products from the plasma. 5 6 Kidneys In each renal corpuscle, there is the glomerulus, which a complex tuft of interconnected capillary loops, enclosed by the Bowman’s capsule. With such structure, a portion of blood can be efficiently but nonselectively filtered as it flows across the substantial surface area provided in the glomerulus whereas the remaining blood leaves the glomerulus via the efferent arteriole. After the glomerular filtration, the filtrate leaves the Bowman’s capsule, drains to the proximal tubule and further moves to the Loop of Henle, in which reabsorption of water and concentration of urine unremittingly take place. The countercurrent multiplier generated by the descending and ascending limbs of the Loop of Henle maintains a high salt concentration in the interior portion of the kidney, so that kidneys are capable of making concentrated urine. 7 Basic Structure of a Nephron 8 Kidneys As a result, osmotic gradient is formed in the nephron and enables the kidney to produce urine that is hyperosmotic to the blood. At the end of the filtration process, urine is passed to the collecting duct, emptied into the minor and major calyx of the medulla, and eventually transferred to the bladder for storage via the ureters. 9 Ureters The ureter is a long and thin duct for the urine produced in the kidney to be conveyed to the bladder for storage. In our body, there are two ureters because we have two kidneys; one ureter is connected to each kidney. The upper portion of the ureter is found in the abdomen and the lower part is located in the pelvic area. The urine descends from the renal pelvis of the kidney to the bladder along this fibromuscular structure by both gravity and contraction of the ureteral wall. 10 Ureters The wall of the ureter is made up of three tunics: tunica adventitia (the fibrous coat), tunica muscularis (the muscular coat) and tunica mucosa (the mucous coat). The fibrous coat is a general supporting layer of fibrous connective tissue. The muscular coat is formed from the inner circular and outer longitudinal muscles, and is mainly responsible for the propelling of urine. The mucous coat is actually a transitional epithelium that secretes mucus, and it provides shielding for the inner surface of the ureteral wall. Importantly, each ureter attaches to the bladder via a tunnel in the bladder wall and is angled to prevent any backflow of urine during the contraction of bladder muscles. 11 Bladder The urinary bladder is a muscular sac that is located in the pelvis area. Its primary function is to store the urine that is produced in the kidneys. The bladder is lined by layers of stretchy muscles for accommodating the storage of urine. When it is full, it signals to you the urge to urinate. Urine is released when your bladder muscles contract; the bladder holds on to urine when the bladder muscles relax. Meanwhile, the urethra is tightened so that leaking of urine is prevented. The normal holding capacity of the urinary bladder is approximately 400– 600 mL of urine in adults. The amount of urine produced and the frequency of urination really depends on the water level of our body, for instance, how much we drink. 12 Urethra The urethra is a narrowing tube for discharging urine out of our body from the bladder. However, there are some anatomical differences in the characteristics of the urethra between males and the females. In males, the urethra is linked with the reproductive system, so it runs through the penis and conveys semen besides urine. In females, the urethra is separated from the genital tract, but emerges from the opening of the vagina. The female urethra is much shorter than that of the male. Urethritis occurs more often in female than male. Nevertheless, urination, or the discharging of urine, in both males and females is voluntarily controlled by the urethral sphincter. 13 Functions of the Components of the Urinary System Ureters: transport urine from kidneys to bladder Bladder: stores urine until excreted from body Urethra: carries urine from bladder to the outside of the body 14 Renal Functions “renal” = “related to kidneys” Regulating the water concentration, inorganic ion composition, acid-base balance & fluid volume of the internal environment (e.g. blood volume) – Maintain the amount of water and inorganic ions within a narrow homeostatic range – E.g. increased consumption of salt  kidneys will increase the amount of salt excreted to match the intake Removal of metabolic waste products from the blood and their excretion in the urine Removal of foreign chemicals from the blood and their excretion in the urine Production of hormones/enzymes: – Erythropoietin, which controls erythrocyte production – Renin, an enzyme that controls the formation of angiotensin, which influences blood pressure and sodium balance – Conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D, which influences calcium balance 15 Nephron Nephrons are the structural and functional units of the kidney. Each kidney has over 1 million of these units. Each nephron consists of a renal corpuscle, which contains the glomerulus (which is a tuft of capillaries) and a renal tubule. The tubule forms a cup shape around the glomerulus called the glomerular capsule (Bowman’s capsule). 16 Basic Renal Processes 17 Glomerular Filtration Urine formation begins with the filtration of plasma within the Bowman’s capsule. The filtering process across the glomerular capillaries is termed glomerular filtration and driven by hydrostatic and osmotic pressure gradients. The filtrate at this stage is named glomerular filtrate. This filtrate is free of cells, lacks most plasma proteins such as albumin and globulin, but virtually mirrors the concentration of solutes in the plasma. The volume of fluid filtered from the glomeruli over time is calculated as the glomerular filtration rate (GFR). The GFR can be calculated in terms of the ability to clear off serum creatinine, which is a byproduct of muscle metabolism, with the formula as shown below. There is a coefficient factor of 0.85 for females. 18 Glomerular Filtration The GFR can be affected by the filtration pressure, the permeability of the corpuscular membranes and the surface area available for filtration that is age-, gender-, and race-dependent. The GFR is regulated by neural and hormonal inputs for altering the pressure of the afferent or efferent arteriole of the glomerulus. For example, constriction of the afferent arteriole or dilation of the efferent arteriole causes a decrease in GFR. In contrast, dilated afferent arteriole or constricted efferent arteriole can lead to an increase in GFR. For an average adult with a body weight (wt) of 70 kg, his/her estimated GFR should not be lower than 60 mL/min whilst the steady normal values are within the range of 90-140 mL/min. A value of less than 60 mL/min for 3 or more months implicates renal malfunctioning or is a sign of chronic kidney disease. 19 Glomerular Filtration If you see blood cells or protein in the urine (protinuria) then there is a problem with the filtration membrane. This is a common finding during diabetes and hypertension that signals that kidney damage has occurred. If untreated will progress to end stage renal disease and renal failure. 20 Glomerular Filtration 21 GFR and Sodium Loss 22 Tubular reabsorption Tubular reabsorption is a process that refers to the movement of filtered substances and water from the lumen of tubules back into the bloodstream. In general, only 1% of filtered substances are excreted from our body whereas the remaining 99% are reabsorbed into the blood. Tubular reabsorption is regulated by passive and active mechanisms: osmosis, passive diffusion and active transport. 23 Tubular reabsorption In the proximal tubule and the descending limb of the Loop of Henle, two thirds of the primary urine volume with electrolytes are reabsorbed. The reabsorption process is energy consuming and most of the energy consumed by the kidneys is used in the reabsorption of sodium ions as they are dissolved in the water of the filtrate. The reabsorption of other substances such as glucose, amino acids and bicarbonate ions is linked to the sodium reabsorption and proton secretion in the presence of some intracellular enzymes. With the movement of the solutes, water is reabsorbed by osmosis. 24 Tubular reabsorption Once the water and solutes enter the interstitial spaces, they can diffuse back to the tubular lumen via the epithelial tight junctions as the tight junctions between the epithelial cells are leaky. The remaining water and solutes are passed to the ascending limb of the Loop of Henle, which is arranged as a counter-flow of the descending limb. As a result, a high solute concentration can be generated deep in the renal medulla. In the ascending limb of the Loop of Henle, 30% of the filtered sodium is reabsorbed utilizing the sodium-potassium pumps. A few more sodium ions are further reabsorbed in the distal tubule. 25 Tubular reabsorption The permeability of the collecting ducts for water is high, so that it leads to a concentration of the urine up to the fivefold osmolarity of the plasma, and is regulated by the antidiuretic hormone (ADH). The amount of ADH in blood may be influenced by our body condition as well as the consumption of diuretics in our diet. 26 Tubular Reabsorption Tubular reabsorption begins as soon as filtrate enters the tubule cells. Paracellular transport occurs between cells (even though they have tight junctions) and is seen mainly with ions. Transport can be active (requires ATP) or passive (no ATP). 27 Tubular Secretion Substances such as hydrogen ion, potassium, and organic anions move from the peritubular capillaries into the tubular lumen. Tubular secretion is an important mechanism for: 1. disposing of drugs and drug metabolites. 2. eliminating undesired substances or end products that have reabsorbed by passive processes (urea and uric acid). 3. removing excess K+. 4. controlling blood pH. 28 Differential Handling in the Kidney 29 Tubular secretion In tubular secretion, substances move from the plasma peritubular capillaries into the renal tubules and become part of filtrate. Secretion at this stage is regulated by diffusion or transcellular mediated transport. The majority of substances secreted by the renal tubules are hydrogen, potassium and ammonium ions. Organ anions including choline and creatinine and chemicals like penicillin are also secreted within this region. 30 Tubular secretion Tubular secretion takes place in the epithelial cells which are located at the renal tubules as well as the collecting ducts. However, active secretion of a substance is driven by active transport and is often coupled to the reabsorption of sodium ions. The tubular secretion of hydrogen ions is an important mechanism in regulating the pH of our blood, which is within normal limits of 7.3 to 7.4. When the pH of the blood drops, the tubular secretion of hydrogen ions is accelerated. 31 “Division of Labor” in the Tubules The majority of the reabsorption is accomplished by the proximal tubule and the loop of Henle. Extensive reabsorption by the proximal tubule and Henle’s loop ensures that the masses of solutes and the volume of water entering the tubular segments beyond Henle’s loop are relatively small. These distal segments then do the fine-tuning for most substances, determining the final amounts excreted in the urine by adjusting their rates of reabsorption and, in a few cases, secretion. 32 Incontinence Incontinence is the involuntary release of urine. The most common types are stress incontinence (due to sneezing, coughing, or exercise) and urge incontinence (associated with the desire to urinate). Incontinence is more common in women. Medications (such as estrogen replacement therapy to improve vaginal tone) can often relieve stress incontinence. Severe cases may require surgery to improve vaginal support of the bladder and urethra. Any irritation to the bladder or urethra (e.g., with a bacterial infection) can cause urge incontinence. Urge incontinence can be treated with drugs such as tolterodine or oxybutin, which antagonize the effects of the parasympathetic nerves on the detrusor muscle. Because these drugs are anticholinergic, they can have side effects such as blurred vision, constipation, and increased heart rate. 33 Dilute Urine Urine becomes dilute as it moves up through the ascending limb of the loop of Henle due to the loss of Na+. Normally, the cortical collecting duct reabsorbs much of the water and concentrates the urine. 34 Concentrated Urine Vasopressin uses cAMP systems to cause the insertion of aquaporins into the membranes of the principle cells of the collecting ducts. So water flows out of the collecting ducts to be reabsorbed by the body. Urine can reach 1200 mOsm. 35 Renin-Angiotensis System 36 Renal Regulation of Potassium 37 Aldosterone and K+ Levels 38 Renal Regulation of Calcium and Phosphate Calcium reabsorption is increased by parathyroid hormone. Phosphate reabsorption is decreased by parathyroid hormone. 39 Summary – Division of Labor 40 Diuretics Diuretics are substances that promote the loss of Na+ and water. Alcohol acts like a diuretic by inhibiting the release of Vasopressin from the pituitary gland. Osmotic diuretics; carbohydrates that are filtered but not reabsorbed (ex. Mannitol). Loop diuretics (lasix, furosemide) are the most powerful diuretics because they inhibit the formation of the medullar gradient by inhibiting Na+ reabsorption. Hydrochlorithiazide acts on the distal collecting duct. Spironolactone is an aldosterone receptor antagonist. This is known as a K+ sparing diuretic. It acts because the K+ in the urine is from aldosterone-driven active tubular secretion into the late DCT and collecting ducts. 41 Kidney Disease Many diseases affect the kidneys including: – Bacterial infections – Hypertension – Diabetes End stage renal disease is one of the leading causes of death in the world and the leading cause of renal transplants. 42 Clinical Issues Infection of the renal pelvis occurs usually by bacteria and is called pyelitis. If it affects the whole kidney it is called pyelonephritis. In severe cases the kidney swells, abscesses form and the pelvis fills with pus. This can result in irreparable damage. Antibiotics are used to treat this condition. 43 Clinical Issues Abnormally low urine output (less the 50 mL/day) is called anuria. This may indicate that glomerular blood pressure is too low to cause filtration. Renal failure and anuria can result from any situation where the nephrons cease to function, including acute nephritis, transfusion reactions, and crush injuries. 44 Hemodialysis, Pertoneal Dialysis, and Transplantation 45