Urinary System Handout PDF

TCM 115 -- Basic Anatomy and Physiology --------------------------------------- URINARY SYSTEM - handout ------------------------ KIDNEYS ------- - Kidneys are paired organs, located retroperitoneally in the upper part of the abdominal cavity, attached directly to the posterior abdomina...

TCM 115 -- Basic Anatomy and Physiology --------------------------------------- URINARY SYSTEM - handout ------------------------ KIDNEYS ------- - Kidneys are paired organs, located retroperitoneally in the upper part of the abdominal cavity, attached directly to the posterior abdominal wall. - Generally, kidney's position corresponds to T12 -- L3 vertebral level. Ribs 11 and 12 are partially covering kidneys (upper poles) - Right kidney is slightly lover then the left one, which is caused by large right lobe of the liver. - Size: 10 cm (length) x 5 cm (width) x 2.5 cm (thickness) - Kidneys are found within the pockets that are created by the perirenal fascia and filled with fat, known as the perirenal fat, which ensures kidney's insulation as well as their minimal mobility. Perirenal fat continues at the kidney's hilum and also extends into the renal sinus. **[External anatomy of kidney:]** - Superior pole (where the suprarenal/ adrenal gland is found) - Inferior pole - Lateral (convex) surface - Medial (concave) surface - On this surface the **renal hilum** is located and it contains the renal artery, renal vein and renal pelvis. (**Renal pelvis** appears to be proximal dilation of the ureter, which collects urine from kidney's calices.) - Renal hilum opens into a large space within the kidney that is termed the **renal sinus**. Renal sinus contains the renal pelvis. Renal pelvis is a structure that collects urine. It originates after the fusion of renal calices (calyx -- singular) occurs. Minor calices are located at the tips of renal pyramids where the renal papillae open. After urine is collected from the renal papillae, minor calices unite to form 2 or 3 major calices, which in turn blend into a single common space -- the renal pelvis. Structure of kidneys -------------------- The external surface of kidneys is covered with shiny, red-brownish colored renal capsule. On a cross-section through the kidney (frontal plane section), it is possible to observe: - the cortex - peripherally situated - medulla - centrally positioned - The medulla is subdivided by renal columns (extensions of cortex) into smaller units that are termed the renal pyramids. Vascular supply of kidneys -------------------------- Arterial blood supply to kidneys -------------------------------- About 10% of the circulating blood goes through kidneys. The following illustrates kidney's arterial supply and venous drainage. **Renal arteries** arise directly from the abdominal aorta (L2 level). They enter the kidney through the hilum and within the hilum and renal pelvis they divide into: **2 or 3 lobar arteries** that pass towards the cortex, running between renal pyramids. Lobar arteries turn into **arcuate arteries** that are found passing between the cortex and medulla. Finally, arcuate arteries give rise to smaller **interlobular arterioles**. The following section of kidney's vascular supply is more interesting for the urinary system physiology, rather than anatomy itself. **Interlobular arterioles** (found within the cortex) divided to become: **afferent arteriole** that divide into **glomerular capillaries** where filtration takes place; blood from glomerulus is drained through **efferent arterioles** that ultimately form **peritubular capillaries** It is interesting that within the kidney we observe two different sets of capillaries, glomerular capillaries that allow filtration and formation of filtrate, and peritubular capillaries that are responsible for reabsorption. Venous drainage of kidneys -------------------------- **Peritubular capillaries** give rise to **interlobular veins** **arcuate veins** **interlobar veins**, which finally unite to create **renal vein**. Both left and right renal veins are direct tributaries to the inferior cava vein. URETERS ------- - Ureters are muscular tubes, approximately 25-30 cm long. They extend from the renal pelvis to the urinary bladder (vesica urinaria). Ureters have unremarkable external anatomy, but functionally we observe three constricted areas of the ureter (possible obstruction sites): a. Junction between the **ureter and renal pelvis** b. An area where the **ureter crosses into the pelvic inlet** c. Where the **ureter pierces the muscular wall of the urinary bladder.** It creates functional valve that prevents retrograde movement of urine back to kidney. **[Urinary bladder (vesica urinaria)]** In an adult the urinary bladder is completely situated within the true pelvis, located posterior to the pubic symphysis. In infants the bladder is found within the greater pelvis / abdominal cavity, and it gradually descends into the true pelvis. The following are external landmarks of the urinary bladder: - - - - It is interesting to mention that the lining of the bladder is composed of ***transitional epithelium***. The wall is mostly composed of ***3 layers of muscle***, which combine and form **the detrussor muscle**. Fibers of the detrussor muscle converge on the bladder's neck to form the **sphincter of the bladder**. Internally, urinary bladder shows an area where the transitional epithelium tightly adheres to (tuan thu) the bladder -- **the trigone**. On two opposite side angles of the trigone, **ureteric openings** are observed. The third angle of the trigone shows the internal urethral orifice. Urethra ------- It is a part of the urinary system, which exhibits most differences when comparing urinary system of a male to urinary system of a female. Male urethra is approximately 15 cm long tube while in females it is only about 3-4 cm long. In males, it is divided into: preprostatic (within the neck of the bladder), prostatic, membranous (also known as the intermediate urethra) and spongy urethra, as it passes through the prostate gland, pelvic diaphragm and corpus spongiosum penis respectively. In males urethra opens at the tip of the glans penis. This opening is termed **the external urethral orifice**. Female urethra is significantly shorter, which predisposes females to acquire urinary infections more commonly. Female urethra is anterior and parallel to vagina. It passes through the pelvic diaphragm and the external urethral sphincter. The urethra, after its short course, opens into the **vestibule of the vagina**. **[Nephron]** - **Nephron** is a basic functional unit of the kidney. - Most of nephrons and their renal corpuscles (80-85%) are in renal cortex (outer portion of a kidney). They have shorter loops of Henle. - Additional 15-20% of nephrons are termed **juxtamedullary** nephrons because their corpuscles are located closer to the renal medulla. Their loops of Henle are quite long and they extend deep into renal medulla. **Nephron consists of** = **renal corpuscle** + **renal tubule** Where blood plasma where the filtered Is filtered fluid passes **Renal corpuscle** = **glomerulus** + **glomerular capsule** (capillaries) a.k.a Bowman's capsule Once the blood plasma is filtered through fenestrated glomerular capillaries, the filtered fluid is collected within the Bowman's capsule. From the Bowman's capsule, filtrate (too early to use the term urine) is directed to pass through the following structures: **Proximal convoluted tubules** **Loop of Henle** **Distal convoluted** **tubules** (located in cortex) (located in medulla) (located in cortex) Loop of Henle is composed of descending limb and ascending limb. Loop of Henle of juxtamedullary nephrons has a **thick** and **thin** portions of the ascending limb. This feature is used to produce dilute or concentrated urine. Distal convoluted tubules of several nephrons empty into **collecting duct** Collecting ducts converge to form **papillary duct** Papillary ducts empty their content into **small (minor) calyces** **Small calyces merge to become major calyces** **Major calyces ultimately form renal pelvis** **Histology of nephron and collecting duct** ============================================ **Glomerular capsule** is composed of visceral and parietal layers. - Visceral layer -- podocytes - Parietal layer -- simple squamous epithelium Between these two layers a cavity is found -- it is termed the capsular (Bowman) space where the filtrate is collected. **Proximal convoluted tubules** -- simple cuboidal cells with microvilli on the apical side -- to increase absorption or secretion. **Descending limb and thin ascending limb** -- simple squamous epithelium -- **Ascending limb (thick portion)** -- simple cuboidal and low columnar epithelium In the final part of thick portion, cells are packed tightly **-- macula densa,** which together with afferent arteriole create the **juxtaglomerular apparatus** (JGA). JGA regulates blood pressure within kidneys. Also, in the terminal section of distal convoluted tubules -- **principal cells** are present. Principal cells have receptors for ADH and aldosterone. Also present are the intercalated cells that play a role in regulation of blood's pH. **Basic renal physiology** ========================== There are three different processes that regulate and maintain blood's volume and its composition. 1. Glomerular filtration 2. Tubular reabsorption (about 99% of filtrate is reabsorbed through the peritubular capillaries and vasa recta) 3. Tubular secretion (removing different substances from blood) **Glomerular filtration** ========================= - Carried at rate of 16 --20% - It is equivalent of 150 -- 180 liters of filtrate per day but about 99% of this volume is reabsorbed, leaving only 1-2 l of urine per day to be eliminated. - Size of fenestration is 70 --100 nanometer (0.07 -- 0.1 micrometer) it allows almost every solute to be filtered except blood cells and proteins. - Podocytes (visceral layer of glomerular corpuscle) create filtration slits and effectively make the above-mentioned openings substantially smaller and only molecules of 6-7 nanometers are filtered. - Principal force that promotes filtration is blood hydrostatic pressure (BHP) **[Net filtration pressure (NFP)]** NFP = Glomerular blood hydrostatic pressure -- Capsular hydrostatic pressure -- Blood colloid osmotic pressure NFP = GBHP (55 mmHg) -- CHP (15 mmHg) -- BCOP (30 mmHg) = 10 mmHg ----------------------------------------------------------------- **Glomerular filtration rate** (GFR) = the amount of filtrate in all nephrons in both kidneys per minute. It is approximately 125 ml/min in males or 105 ml/min in females. *[If GBHP falls below 45 mmHg filtration ceases, but if the blood pressure rises sharply GFR isn't changing dramatically.]* GFR is constant for BP between 80 -- 180 mmHg GFR is regulated by adjusting blood flow or by adjusting glomerular capillaries area available for filtration. This is accomplished through: 1. **Renal autoregulation** -- Myogenic -- rise of blood pressure stretches walls of afferent arterioles which in turn constrict and reduce blood flow through kidneys which puts back GFR to normal. 2. **Neural regulation** -- under the sympathetic stimulation, blood flow through kidneys is severely restricted due to vasoconstriction. It preserves blood volume, and it enables more blood to be diverted into other organs or tissues (hemorrhage is a good example when the sympathetic activity is accentuated). 3. **Hormonal regulation** -- Angiotensin II reduces GFR. Atrial natriuretic peptide (ANP) is released from atria of the heart when their walls become stretched. Presence of ANP increases GFR. **Tubular reabsorption and tubular secretion** ============================================== Other than water, tubules will reabsorb the following solutes: 1. glucose 2. amino acids 3. Na+, Cl-, K+, HCO3- and HPO4- ions 4. urea Most of the absorption occurs in the proximal convoluted tubules. Later, after the reabsorption of water and solutes is accomplished, the reabsorption is be directed to collect smaller molecules of proteins which is achieved through pinocytosis -- (cellular 'drinking'). Reabsorption occurs between adjacent tubular cells as they are not tightly connected **(paracellular reabsorption**) or directly through the tubular cells **(transcellular reabsorption).** Example: sodium ions diffus into apical portion of cell (low intracellular concentration of sodium creates concentration gradient), but at its basal part Na/K pump expels the Na from the cell into peritubular capillaries. Water is reabsorbed through the osmosis (it moves towards the higher concentration of solute). Example of symport: Na/glucose **Generating hydrogen ions - Antiport: Na/H ions** ================================================== H20 + CO2 = H2CO3 (carbonic acid) H2CO3 dissociates into H+ and HCO3- Finally, the secretion will affect the following substances: 1. H+, K+ and NH4+ ions 2. Creatinine 3. Drugs (such as penicillin) **[Proximal convoluted tubules]** **Reabsorption:** 1. Water 65% - osmosis 2. Na, K ions 65% 3. Glucose and amino acids -- 100% 4. HCO3 -- 80-90% 5. Urea 50% 6. Chlorine --50% 7. Ca and Mg -- variable **Secretion:** 1. H+ ions 2. Ammonia (NH4) 3. Urea 4. Creatinine At the end of PCT **tubular fluid is isotonic** to blood. **Loop of Henle** ================= **Reabsorption:** 1. Water -- 15% - via osmosis; it happens mostly in descending limb. Almost no water is reabsorbed in ascending limb. 2. Na and K ions -- 20-30% 3. Chlorine -- 35% 4. HCO3 -- 10-20% 5. Ca and Mg variable **Secretion:** 1. Urea At the end of loop of Henle **tubular fluid is hypotonic** as the ascending limb doesn't allow for further reabsorption of water. Solute is reabsorbed but water Is not. **[Distal convoluted tubules]** **Reabsorption:** 1. Water 10 -- 15% 2. Na -- 5% 3. Cl -- 5% 4. Ca -- variable (parathyroid hormone stimulated) **[Principal cells in late DCT and collecting duct]** **Reabsorption:** 1. Water 5 -- 9 % 2. Na 1 -- 4% 3. Urea (variable) **Secretion** ============= 1. Potassium Tubular fluid leaving the collecting duct is dilute (if ADH low) and concentrated (if ADH high). **Intercalated cells in late DCT and collecting ducts** ======================================================= **Reabsorption:** 1. HCO3 2. Urea **Secretion:** 1. **[Hormonal regulation of tubular reabsorption and secretion]** 1. Renin -- angiotensinogen -- angiotensin -- aldosterone 2. ADH (vasopressin) 3. Atrial natriuretic peptide (ANP) **Production of concentrated or dilute urine** ============================================== - Production of concentrated urine is mostly based on the amounts of reabsorbed solutes and water. - Initially in PCT fluid is isotonic, but as a significant water reabsorption occur within the descending limb of loop of Henle the osmolarity INCREASES. - In the ascending limb, little water is reabsorbed, but more active reabsorption of solutes occurs and it reduces osmolarity to HYPOTONIC. - From DCT reabsorption of solute continues, while water reabsorption will be pending on ADH. (low ADH -- urine becomes more dilute; high ADH, water is retained, and urine becomes concentrated). **[Micturition reflex]** The bladder is innervated through the parasympathetic division of the autonomic nervous system. Once the bladder is distended, it sends sensory signals towards the sacral segments of the spinal cord (S2 -- S4 segments), from which the sacral parasympathetic outflow originates. Motor parasympathetic fibers reach the bladder and stimulate contraction of the bladder's wall. Therefore, emptying of the bladder is a reflex activity. Later in life we learn how to suppress involuntary emptying of the urinary bladder when we achieve full control of voluntarily controlled muscles.