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  2. PARTS OF URINARY SYSTEM The urinary system comprises- two kidneys, two ureters, urinary bladder and urethra. The kidneys remove waste products of metabolism, excess water and salts from blood and maintain the pH. The ureters convey urine from the kidneys to the urinary bladder. The urinary bladder is the muscular reservoir of urine and the urethra is the channel to the exterior.
  3. FUNCTIONS OF URINARY SYSTEM The kidneys maintain a stable internal environment by regulating the volume and composition of body fluids as well as by excreting the waste products and excess water. For maintaining homeostasis, kidneys do multiple functions as follows:  Excretion of metabolic waste products and chemicals, like urea, uric acid, creatinine and many drugs.  Regulation of body fluid volume and osmolality by excreting either dilute or concentrated urine.  Regulation of concentration of electrolytes and various ions.
  4.  Regulation of acid-base balance by excreting either excess acid or base.  Regulation of arterial blood pressure by adjusting Na* and water excretion.  Secretion and production of some hormones, like erythropoietin, 1,25-dihydroxycholecalciferol, renin, prostaglandins, etc.  Metabolism of various hormones, like insulin, glucagon, parathyroid hormone, etc.
  5. The urinary system is one of the four excretory systems in our body. The other three are the bowel, Iungs and the Skin. KIDNEYS (RENES; NEPHROS) The kidneys are a pair of excretory organs situated on the posterior abdominal wall, retroperitoneally, one on each side of the lumbar part of the vertebral column. The left kidney is slightly at a higher level than the right kidney since the massive liver occupies the right hypochondrium. Clinical Correlation Renal calculi also called nephrolithiasis is stone or calculi in kidney. It is characterized by pain in the flank, abdomen and groin area
  6. Shape and Size Each kidney is bean-shaped and measures 7.5-10 cm in length, 5 cm in width and is 2.5 cm thick. Each kidney weighs 150 g. It has two poles-upper and lower; two borders-medial and lateral; and two surfaces-anterior and posterior. 1. The upper pole is broad and is closely related to the suprarenal (adrenal) gland. The lower pole is pointed. The upper poles are closer to the median plane than the lower poles. 2. The lateral border is convex. 3. The medial border is concave. The middle part of the medial border is depressed and is known as the hilum (hilus). .
  7. Structures Present in the Hilum of Kidney The following structures are seen in the hilum of kidney (from anterior to posterior): • The renal vein • The renal artery • The renal pelvis-the upper extended portion of ureter. In addition to these three structures, the hilum also transmits nerves and lymphatics
  8. Coverings The kidney has three coverings:  Inner most fibrous capsule or true capsule. • Middle fatty capsule or perinephric fat-it is a collection of fatty tissue. It acts as a shock absorber and also helps to maintain the kidney in its position.  The false capsule is made of renal fascia. It has two layers-anterior and posterior. Superiorly, the two layers enclose the suprarenal gland and then merge with diaphragmatic fascia. (That is why the kidneys move with respiration.)  Pararenal fat-forms a cushion for the kidney.
  9. Organs Associated with Kidney As the kidneys lie on either side of the vertebral column, each is associated with a different group of structures. Right Kidney Anteriorly :the duodenum(second part),the hepatic flexure of the colon( colic area in the fig) and the right lobe of the liver. .
  10. Posteriorly: Diaphragm,muscles of the posterior abdominal wall (psoas major, quadratus lumborum and transverse abdomen is), and subcostal vessels gr nerves. Superiorly: The right adrenal gland
  11. Left Kidney • Anteriorly: The spleen and splenic vessels, jejunum and splenic flexure of the colon (colic area in the figure). pancreas and stomach. • Posteriorly: The diaphragm and muscles of the posterior abdominal wall (psoas major, quadratus lumborum and transversus abdominis), and subcostal vessels and nerves. Superiorly: The left adrenal gland
  12. Gross and Microscopic Structure of Kidney GROSS STRUCTURE The naked-eye examination of a coronal section shows the following features: Characteristic bean-shape  It has a convex lateral margin and a concave medial hilum (hilus).  The hilum leads into a space called renal sinus. This sinus is occupied by the upper expanded part of the ureter, which is called the renal pelvis (pelvis=basin).  Within the sinus, the renal pelvis divides into two or three major calyces (calyx=cup).  Each major calyx divides into minor calyces.
  13. The kidney tissue, covered by a fibrous capsule or true capsule, consists of an outer cortex and inner medulla.  The outer cortex, which lies immediately below the capsule and outside the pyramid, is light in color.
  14.  The inner medulla is made up of triangular are as of renal tissue called renal pyramids and appears dark in color.  The apex of the papilla which fits into the minor calyx.  The bases of the pyramids are capped by renal cortex called cortical arches.  The parts of the renal cortex projecting between the pyramids are called the renal columns
  15. MICROSCOPIC STRUCTURE The kidney may be regarded as a collection of millions of uriniferous tubules. Each uriniferous tubule consists of an excretory part called the nephron and of a collecting tubule. There are 1-2 million nephrons in a kidney.
  16. Parts of a Nephron Nephron, the functional unit of kidney, has the following parts: A renal corpuscle or Malpighian corpuscle is a rounded structure consisting of a tuft of capillaries called glomerulus and a cup-like double- layered covering called the Bowman's capsule or glomerular capsule. The two layers of glomerular capsule are separated by the urinary space. The outer layer is lined by squamous cells. With the light microscope, the inner wall also appears to be lined by squamous cells, but the electron microscope (EM) shows that these cells, called podocytes, have a highly specialized structure
  17. The renal tubule (tubular part) is a long complicated tubule. It has different parts. These are:  Proximal convoluted tubule (PCT): The PCT is lined with cuboidal or low columnar epithelium. The cells are provided with microvilli and numerous mitochondria.  Loop of Henle: It has a descending limb, a loop and an ascending limb. The loop of Henle is lined with low cuboidal or squamous epithelium.  Distal convoluted tubule (DCT): The DCT is lined with cuboidal cells. These cells do not have microvilli. .
  18. The DCT ends by joining a collecting tubule.  The collecting tubules are lined with cuboidal or low columnar cells. The collecting tubules draining different nephrons join to form larger tubules called papillary ducts of Bellini, which open into a minor calyx at the apex of a renal papilla.  A minor calyx receives urine from papilla and delivers it to major calyx.  From the major calyces, urine drains into the renal pelvis and then out through the ureter to the urinary bladder  The renal artery after entering the kidney divides repeatedly
  19. Parts of a nephron Nephron Renal corpuscle Tubular part Glomerulus Bowman's capsule Proximal convoluted tubule Loop of Henle Distal convoluted tubule Key Points • The renal corpuscles and major parts of PCT and DCT are located in the cortex of the kidney. • The loops of Henle and the collecting ducts lie in the medulla
  20. Path of urine Nephron Collecting duct Papillary duct of Bellini Minor calyx Major calyx Renal pelvis Ureter Urinary bladder
  21. Functionally there are 2 sets of arterioles and capillaries.  • The first capillary system present in the glomeruli is concerned exclusively with the removal of waste products from blood.  It does not supply oxygen to the renal tissues.  The second set of capillaries present around the tubules is concerned with the exchange of gases
  23. Blood Supply About 25% of the total cardiac output flows through kidneys, ie., 1200 mL/minute at resting condition. However, kidney is the organ in our body possessing highest blood supply, i.e., about 400 mL/min/100 g. The blood supply to the kidney is through the renal artery. medullary nephrons are called vasa recta
  24. The renal artery (branch of abdominal aorta) branches progressively to form the interlobar arteries, the arcuate arteries, the cortical arteries or the interlobular arteries and the afferent arterioles. Each afferent arteriole breaks up into the glomerular capillary network which joins together to form the efferent arteriole which again divides into a second capillary network. The second capillary networks around the cortical nephrons are called peritubular capillaries and those surrounding juxta
  25. Flowchart : Path of blood in kidney.  Renal artery  Interlobar arteries  Arcuate arteries  Interlobular arteries  Afferent arterioles  Glomerular capillaries  Efferent arterioles  Peritubular capillaries/Vasa recta  Interlobular veins  Arcuate veins  Interlobar veins  Renal vein
  26. Both peritubular capillaries and vasa recta drain into the vessels of venous system running parallel to the arterial vessels and branch progressively to form the interlobular or cortical vein, the arcuate vein, the interlobar vein and finally the renal vein. The renal circulation possesses the feature of double capillary network, which is also known as portal circulation. Nerve Supply  Kidneys are innervated by autonomic nervous system.  Sympathetic fibers are derived from T10-L1 segments.  Parasympathetic fibers are derived from the vagus nerves. Lymphatics from the kidneys drain into the lateral aortic nodes.
  27. Juxtaglomerular Apparatus (JGA) • Juxtaglomerular apparatus is a specialized organ situated near the glomerulus of each nephron (juxta=near). The juxtaglomerular apparatus is formed by three different structures: 1. Juxtaglomerular cells 2. Macula densa 3. Lacis cells
  28. Parts Juxtaglomerular Cells  A part of the distal convoluted tubule (at the junction of its straight and convoluted parts) lies close to the vascular pole of the renal corpuscle, between the afferent and efferent arterioles  In this region, the muscle cells in the wall of the afferent arteriole are modified.  They are large and rounded (epithelioid) and have spherical nuclei.
  29.  Their cytoplasm contains granules that can be stained with special methods.  These are juxtaglomerular cells. They are innervated by unmyelinated adrenergic nerve fibers.  Juxtamedullary cells are regarded, by some, as highly modified myoepithelial cells as they contain contractile filaments in the cytoplasm.
  30. The granules of the juxtaglomerular cells are seen by EM to be membrane-bound secretory granules. They contain an enzyme called renin. The juxtaglomerular cells also probably act as baroreceptors reacting to a fall in blood pressure by release of renin. Secretion of renin is also stimulated by low sodium blood levels and by sympathetic stimulation.
  31. Note: In addition to renin, the kidney produces the hormone erythropoietin (which stimulates erythrocyte production). Some workers have claimed that erythropoietin is produced by juxtaglomerular cells, but the site of production of the hormone is uncertain
  32. Macula Densa  The wall of the distal convoluted tubule is also modified at the site of contact with the arteriole.  Here the cells lining it are densely packed together, and are columnar (rather than cuboidal as in the rest of the tubule). •  These cells form the macula densa. The cells of the macula densa lie in close contact with the juxtaglomerular cells.
  33.  In addition to the renin-producing cells, and the macula densa, the juxtaglomerular apparatus has a third component: these are lacis cells.  These cells are so called as they bear processes that form a lace-like network  They are located in the interval between the macula densa and the afferent and efferent arterioles.  The function of lacis cells is unknown
  34. Functions  The primary function of juxtaglomerular apparatus is the secretion of hormonal substances.  It also regulates the glomerular blood flow and glomerular filtration rate. Secretion of Renin The juxtaglomerular cells secrete renin. Renin is a peptide with 340 amino acids. Along with angiotensins, renin forms the renin-angiotensin system, which is a hormone system that plays an important role in the maintenance of blood pressure.
  35. Renin-Angiotensin System When renin is released into the blood, it acts on angiotensinogen and converts it into angiotensin I. Angiotensin I is converted into angiotensin II by the activity of angiotensin converting enzyme (ACE) secreted from lungs. Most of the conversion of angiotensin I into angiotensin II takes place in lungs. Angiotensin II has a short half-life of about 1-2 minutes. Then it is degraded into angiotensin III by angiotensinases, which are present in RBCS and vascular beds in many tissues. Finally, angiotensin III is converted into angiotensin IV
  36. Actions of Angiotensin Angiotensin I It is physiologically inactive and serves only as the precursor of angiotensin II. Angiotensin II Angiotensin II is the most active form. Its actions are: It increases arterial blood pressure by causing vasoconstriction and inhibiting baroreceptor reflex. It stimulates zona glomerulosa of adrenal cortex to secrete aldosterone. It regulates glomerular filtration. It increases sodium reabsorption from renal tubules. It increases water intake by stimulating the thirst center. It increases secretion of antidiuretic hormone (ADH) from hypothalamus. Secretion of Other Substances The extraglomerular mesangial cells of juxtaglomerular apparatus secrete prostaglandin. Flowchart : Conversion of angiotensinogen into angiotensin II. Renin ACE Angiotensinogen Angiotensin I Angiotensin II (ACE: angiotensin converting enzyme) Angiotensin IV Angiotensin III
  37. Regulation of Glomerular Blood Flow and Glomerular Filtration Rate Macula densa of juxtaglomerular apparatus plays an important role in the feedback mechanism called tubuloglomerular feedback mechanism, which regulates the renal blood low and glomerular filtration rate.
  38. Mode of Action of Juxtaglomerular Apparatus The juxtaglomerular apparatus is a mechanism that controls the degree of reabsorption of ions by the renal tubule. It appears that cells of the macula densa monitor the ionic constitution of the fluid passing across them (within the tubule). The cells of the macula densa appear to influence the release of renin by the juxtaglomerular cells.
  39. Functions of Kidney The functions of kidney are the same as that of urinary system and also summarized in Flowchart
  40. PHYSIOLOGY OF URINE FORMATION  Key Points Formation of urine involves three basic processes:  1. Glomerular filtration  2. Tubular reabsorption  3. Tubular secretion
  41.  Glomerulusis responsible for filtration and tubular parts of nephron are responsible for reabsorption and secretion. About 1800 L/day (1.2 L/min) of blood flows through both the kidneys. Of this, about 180 L/day is filtered through the glomeruli. But only less than 1% of the filtered water and variable amounts of many of the solutes are excreted in the urine. This is made possible by the processes of tubular reabsorption and secretion. Thus, the renal tubules precisely control the volume, composition and pH of the body fluids.
  42. GLOMERULAR FILTRATION It means the ultrafiltration of plasma by the glomerulus. The first step in the formation of urine is filtration of plasma through the glomerular capillaries. The fluid that enters the capsular space is called the glomerular fltrate (ultraflltrate). The composition of filtrate is similar to plasma except that it contains no proteins. The fraction of blood plasma in the afferent arterioles, which becomes glomerular filtrate, is the flltration fraction. The filtration fraction is 0.16-0.20 (16-20%). Filtration occurs across the glomerular filtration membrane or barrier
  43. . Glomerular Filtration Barrier The capillary endothelium, basement membrane and the foot processes of podocytes form the glomerular flltration membrane or fltration barrier. The endothelium is fenestrated with many pores, through which small solutes and water can easily pass. Also the pores are lined by sialoproteins, which provide a negative charge to them.
  44. The basement membrane or basal lamina is also porous. It is a layer of a cellular material formed by type-IV collagen, proteoglycan and other proteins found between the endothelium and podocytes. The podocytes of the visceral layer of Bowman's capsule have long foot-like processes that encircle the outer surface of the capillaries. In between these foot processes, there are filtration sites that are lined by negatively charged glycoproteins. Because of the negative charges lining the endothelium, basement membrane and filtration slits, the plasma proteins are repelled and their filtration is prevented.
  45.  All Solutes Up to 4 Nm Size Can Freely Pass Through The filtering Membrane.  Solutes with Size More Than 8 NM Are Not Filtered. Among The Charged Ions, Cations Are More Easily filtered Than Neutral Ions, And Then Anions. The Cellular Elements in the Blood Are Not Filtered.  The Total Area of the Filtering Membrane of Both Kidneys is 0.8 Square Meter.
  46. Glomerular Filtration Rate Definition : Glomerular Filtration Rate (GFR) is defined as the amount of filtrate formed in all the nephrons of both kidneys in one minute. The Total Quantity of Ultrafiltrate formed In all the nephrons of both Kidneys per Minute Is Termed as Glomerular filtration rate or GFR. It is the sum of the filtration rate of individual nephrons and hence gives an index of Kidney Function. Normal GFR IS About 125 ml / min or 180 l / day. The Number of Functioning Nephrons Decreases as The Age Advances and, Hence, The GFR Deceases in Old Age
  47. Measurement of GFR  The IDEAL Substance to Measure GFR is Inulin, Which is Fructose Polymer. Inulin Is Preferred Because of The Following Reasons  It is freely Filtered by The Glomerulus  * It is not reabsorbed Or secreted by Renal Tubules  * It is not Synthesized, Metabolized Or Stored in the Kidney  * It is Nontoxic Substance, Soluble in Plasma  Its concentration in Plasma and Urine Can Be Easily Measured.
  48. Net Filtration Pressure Filtration across the glomerular membrane depends on three main pressures-glomerular capillary hydrostatic pressure, colloidal osmotic pressure and hydrostatic pressure in Bowman's capsule .  The glomerular hydrostatic capillary pressure is the pressure exerted by blood in glomerular capillaries. It is = 55-60 mm Hg and the pressure favors GFR.  Colloidal osmotic pressure is the pressure exerted by plasma proteins in the glomeruli. It is = 25-30 mmHg and opposes GFR.  Hydrostatic pressure exerted by filtrate in Bowman's capsule. It is - 15 mmHg and opposes GFR. Net filtration pressure =Glomerular capillary pressure - (Hydrostatic pressure + Colloidal osmotic pressure) = 60-(15+25) = 10-20 mmHg
  49. Factors affecting GFR  Net filtration pressure: NFP from 10 to 20 mm is ideal for glomerular filtration.  Renal blood flow: GFR is directly proportional to renal blood flow.  Permeability of glomerular membrane: Substances with diameter less than 4 nm are freely filtered. Substances having diameter more than 8 nm will not be filtered. Substances having diameter between 4 and 8 nm are filtered depending on their charge.  Surface area of ​​filtering membrane: When the area of ​​the filtering membrane is decreased, less fluid is filtered.
  50. TUBULAR REABSORPTION Definition As filtered fluid flows through the renal tubules, the tubules reabsorb 99% of water and solutes. This is called tubular reabsorption About 180 liters of glomerular filtrate is formed per day. Only 1% of this volume is lost as urine. Large quantity of water (more than 99%), electrolytes and other substances are reabsorbed by the tubular epithelial cells. The absorbed substances move into the interstitial fluid of renal medulla. And, from here, the sub- stances move into the blood in peritubular capillaries.
  51.  Tubular reabsorption is known as selective reabsorption because the tubular cells reabsorb only the substances necessary for the body.  Substances reabsorbed by the renal tubules include water, glucose, amino acids and electrolytes. Reabsorption of substances occurs by active transport, passive transport and by osmosis.
  52. Hormones Influencing Selective Reabsorption summary of which is given in Table 2.1
  53. URINE Properties of Urine  Volume: 1000 to 1500ml per day  Reaction: Slightly acidic with pH of 4.5 to 6  Specific gravity: 1.010 to 1.025  Color: Normally, urine is straw-colored. Urine is clear and amber-colored due to the presence of urobilin, a bile pigment altered in the intestine, reabsorbed and then excreted by the kidneys.  Odor: Fresh urine has light aromatic odor. If stored for some time, the odor becomes stronger due to bacterial decomposition.
  54.  Composition: For composition of urine
  55. Role of ADH in the Concentration of Urine  Normally, the distal convoluted tubule and collecting duct are not permeable to water.  Antidiuretic hormone (ADH) from posterior pituitary makes the tubular cells of collecting duct permeable to water.  The stimulus for its secretion is the decreased body fluid volume and/or increased sodium concentration (hyperosmolarity).
  56.  ADH increases the water reabsorption from distal convoluted tubule and collecting duct resulting in concentration of urine.  But when the volume of body fluid increases or the osmolarity of body fluid decreases, ADH secretion stops.  So, water reabsorption from renal tubules does not take place, leading to excretion of large amount of water in urine making the urine dilute.  It brings back the normalcy of water content and osmolarity of body fluids.
  57. WATER BALANCE  The compositions of the ECF and ICF differ from each other and are maintained in a steady-state condition by a variety of regulatory processes called homeostatic mechanisms.  The composition of the ECF is maintained by the cardiovascular system, respiratory, renal, gastrointestinal, endocrine and nervous systems acting in a coordinated fashion.
  58.  The composition of the ICF is maintained by the cell membrane, which mediates the transport of material between the ICF and ECF. · We are discussing the major homeostatic mechanisms that operate, primarily through the kidneys and the lungs, to maintain the tonicity and the volume of body fluids.  Maintenance of tonicity: Normal plasma osmolality is: 280-295 mosm/L.  Maintenance of tonicity is primarily the function of: ADH (vasopressin), and Thirst mechanism.  The intensity of thirst and ADH secretion is directly proportional to the plasma osmolality.
  59. Body Fluid Volume Volume of the ECF is determined by two mechanisms:  1. By the plasma osmolality primarily, and  2. By control of water excretion through: ADH; Angiotensin II; and – Atrial Natriuretic Peptide (ANP) .
  60. Control of Water Excretion Role of ADH In general, increase in ECF volume inhibits ADH secretion, whereas decrease in ECF volume stimulates it. However, major stimuli for ADH secretions are: plasma hyper-osmolality and hypovolemia. Therefore: a. 1% to 2% increase in plasma osmolality stimulates osmoreceptors (located in the anterior hypothalamus); and b. 10% decrease in effective circulating blood volume + decreases firing from Baroreceptors (venous and arterial). (a) and (b) → ↑ ADH secretion → directly ↑ permeability of DCT and collecting tubules to ↑ water reabsorption Key Point Since the plasma (Na'] accounts for 95% of the effective osmotic pressure, therefore, in general, the plasma [Na] is the primary determinant of ADH secretion
  61. Role of Atrial Natriuretic Peptide (ANP) ANP refers to a group of polypeptides produced by the atrial muscle cells that increases the urinary excretion of sodium. ANP is secreted when NaCl intake is increased and/or increased in ECF volume. ↑ ECF volume → stimulate atrial stretch receptors in the right atrium ↑ Secretion of ANP > (a) Natriuresis (increased sodium excretion), and (b) Diuresis (increased water excretion). Note: ANP inhibits renin secretion. It also antagonizes the action of many vasoconstrictor agents and, thus, decreases arterial BP.
  62. URETERS The ureters are tubular structures which serve to conduct urine from the kidneys to the urinary bladder. They are approximately 25 cm in length with a diameter of 0.6 cm. Extent Each ureter starts from within the renal sinus as a funnel-shaped expanded part called the pelvis of ureter. The ureter proper runs downwards and medially on the psoas major, crosses the pelvic brim to enter the pelvic cavity; where it ends by opening into the lateral angles of the bladder.
  63. Parts For the purposes of description, the ureter is divided into two parts: 1. Abdominal part-from the site of origin to pelvic brim 2. Pelvic part-from pelvic brim to the entry into urinary bladder
  64. Constrictions There are three natural constrictions in the ureter: 1. At the pelviureteric junction 2. At the pelvic brim 3. Just before it enters the bladder The renal stones tend to get arrested at these sites.
  65. Blood Supply Ureter is supplied by branches of renal artery, abdominal aorta, gonadal artery, common iliac artery, internal iliac artery and inferior vesical artery. These branches form a continuous arterial chain. The veins correspond to the arteries. Nerve Supply The ureter is supplied by sympathetic from T10-L1 segments and parasympathetic from S2-S4 nerves.
  66. Applied Anatomy Ureteric calculi: Calculi may be located in the calyces of the kidneys, ureters or urinary bladder. * A kidney stone may pass from the kidney to the ureter, causing partial or complete obstruction. * It causes severe rhythmic pain called ureteric colic. * The ureteric colic is a sharp, stabbing pain, which passes inferolaterally from loin to groin. * Ureter can be injured or accidentally tied during surgeries of ovary or during hysterectomy (removal of uterus) Congenital anomalies:Bifid uterus or double ureters may be seen
  67. URINARY BLADDER The urinary bladder is a hollow, muscular organ, which functions as the reservoir for the urine received from the kidneys and to discharge it out periodically. Shape The empty bladder resembles a four-sided pyramid. It has: Four angles-apex, neck and two lateral angles .
  68. Four surfaces Base (posterior surface) Two inferolateral surfaces Superior surface When distended, it is ovoid in shape. Position Empty bladder in the adult is situated within the true pelvis. When distended, it rises up into the abdominal cavity and becomes an abdominopelvic organ. In the newborn, it is abdominal in position.
  69. Capacity The normal capacity is 200-300 cc. Relations The organs related to urinary bladder in males and females are shown in Figure 12.20
  70. Interior of Bladder  The mucous membrane is straw- colored and is thrown into folds. When bladder is distended, these folds disappear. •  The posterior wall shows a smooth triangular area called trigone.  There are no mucous folds in this region.
  71.  The mucosa is pink in color. It is richly innervated and highly sensitive.  At the upper lateral angles of the trigone are the ureteric openings.  At its inferior angle is the internal urethral orifice.
  72. Histology of Urinary Bladder  • Outer fibroelastic coat  • The middle muscle coat made of smooth muscle fibers called detrusor muscle  .* Inner mucous coat is lined by transitional epithelium.It rests on lamina propria, made chiefly of collagen fibers
  73. Blood Supply  Branches of internal iliac artery The corresponding veins form a plexus and drain into the internal iliac vein. Nerve Supply  The vesical plexus, composed of sympathetic and parasympathetic fibers, innervate the bladder.  Parasympathetic fibers are derived from S2, S3 and S4 segments of spinal cord. Sympathetic fibers are derived from L1 segment.
  74. URETHRA The urethra is a tubular passage extending from the neck of the bladder to the external urethral meatus or orifice. Female Urethra  Female urethra is 3.75-4 cm long. It extends downward and forward, closely related to the anterior wall of the vagina.  It is surrounded by the sphincter urethrae muscle.  It ends at the external urethral orifice in the vestibule.  The mucosa is folded. There are a number of paraurethral glands in the submucosa which open by small ducts on the mucous membrane (These glands are said to be homologous to the prostate gland of a male).
  75. Male Urethra  The male urethra is 18-20 cm long, In the flaccid state of penis, the urethra is S-shaped. When penis is erect, it becomes J-shaped. The male urethra forms a part of the urinary system as well as. reproductive system. The male urethra is divided into three parts: 1. Prostatic part 2. Membranous part 3. Spongy or penile part
  76. Prostatic part is 3 cm long and passes through the prostate gland. It is the widest and most dilatable part of the male urethra. It receives the openings of ejaculatory ducts on a raised area called verumontanum. It also receives the openings of the glands of the prostate. Membranous part: About 1 cm in length, it is the narrowest and least distensible part. It passes through the urogenital diaphragm and is surrounded by the sphincter urethrae muscle. Spongy or penile part: About 15-16 cm in length, it passes through the bulb and corpus spongiosum of penis. It is narrow, with a diameter of 6 mm. It shows two dilatations. There are a number of openings of urethral and bulbourethral glands into the urethra.
  77. Applied Anatomy Catheterization of bladder: In some cases, the patient is unable to pass urine (retention of urine). In such cases, a rubber or metal tube (catheter) is introduced into the bladder through the urethra. While passing a metallic catheter in the males, the normal curvature shave to be considered. Forceful insertion may rupture the urethra. Catheterization of female urethra is easy because it is short and wide.
  78. Clinical Correlation Congenital anomalies: • Hypospadias-the urethra opens on the ventral surface of penis (normally opens at the tip) • Epispadias-urethra opens on the dorsal side of penis Infection of urethra is called urethritis Rupture of urethra can occur following a fracture of the pelvis. Usually, the membranous part is involved.
  79. Sphincters There are two sphincters in relation to the urethra: 1. Internal 2. External The internal sphincter is made up of smooth muscle fibers and is situated at the neck of the bladder. It is innervated by ANS and is involuntary. The external sphincter is made up of striated (skeletal) muscle surrounding the membranous part of urethra. It is supplied by pudendal nerve (S2, S3, S4) and it is voluntary. The urethra is lined internally by stratified columnar epithelium. Close to the external urethral orifice it is lined by stratified squamous epithelium.
  80. MICTURITION It is the process by which the urinary bladder empties when it becomes filled with urine. The urinary bladder fills progressively until the pressure inside it (called intravesical pressure) rises above a particular threshold level.
  81. Then it initiates the micturition reflex as follows:  Several stretch receptors are present in the bladder wall, which get stimulated when it is filled with urine.  They send signals to the "micturition center" in the spinal cord via the pelvic nerves and micturition contractions are initiated in the bladder.  Initial contractions will further stimulate the stretch receptors and so micturition contractions are said to be self- regenerative. This lasts for about a few seconds to one minute.  As the bladder becomes more and more filled, micturition reflexes occur more frequently and more powerfully, and urge to urination (micturition) occurs.
  82. The whole process of micturition can be summarized here: The micturition reflex is an autonomic spinal cord reflex, but it can be suppressed or facilitated voluntarily by several centers in the brain, including cerebral cortex, posterior hypothalamus and midbrain. They keep the micturition reflex partially inhibited except when it is desired. Micturition can also be initiated voluntarily by contraction of abdominal muscles. After micturition, female urethra empties by gravity and male urethra by contraction of