The Urinary System: Waste Product Elimination & Kidney Function

1. The Urinary System Chapter 18

2. Waste Products • Chemical reactions in the body result in waste products that may be potentially harmful. • Must be eliminated from the body. • Examples of metabolic waste products: • Urea • Salts • Water

3. The Urinary System • Most important route of waste-product removal in the body. • Removes nearly all the soluble waste products from blood and transports out of the body. • Removes excess water from the body.

4. The Urinary System Parts of urinary system: • Two kidneys • Make urine and carry out other vital functions. • Two ureters • Carry urine to the urinary bladder. • Urinary bladder • Collects, stores and releases urine. • Urethra • Carries urine from the body to the external environment.

5. Terminology • Nephrology- The study of the kidney. • Diuresis- Body has excess formation of urine • Oliguria- Little urine is formed and passed • Anuria- No urine is formed or passed • Dysuria- Difficult urination • Hematuria- Blood in urine • Polyuria- Increased urine volume. • Antidiuretic Hormone (ADH)- (may also be referred to as Vasopressin) promotes water conservation by reabsorbing urine from collecting ducts. • Aldosterone-mineralcorticoid hormone secreted by cortex of adrenal gland. Stimulates kidney to conserve sodium ions and water and eliminate potassium and hydrogen ions.

6. Kidney Function • Must maintain homeostasis through a variety of processes. • Blood filtration, reabsorption, and secretion • Blood is filtered, useful substances are returned to circulation (reabsorption) and waste products are secreted. • Fluid balance regulation • Makes sure body has enough water to maintain healthy environment. • Acid-base regulation • Ability to remove hydrogen and bicarbonate ions from blood. • Hormone production • Produce and regulate hormones. • ADH from posterior pituitary gland. • Kidneys produce erythropoietin- necessary for red blood cell production and some prostaglandins • Aldosterone- secreted by adrenal glands

7. Location of the kidneys • In dorsal part of abdomen • Ventral to and on either side of first few lumbar vertebrae. • In most domestic animals, right kidney is slightly more cranial than left. • Perirenal fat surrounds and helps protect from exertion from other organs. • Are retroperitoneal to the abdominal cavity and so are considered outside abdominal cavity. • Move somewhat with the movement of the diaphragm • Position may vary with species.

12. Gross Anatomy of the Kidney • Bean shaped and covered by fibrous connective tissue capsule. • Kidney bean • In horses right kidney becomes heart shaped • Reddish brown in color • Generally a smooth surface • Cattle have multiple lobed kidney which gives it a more lumpy appearance

13. Kidneys: Anatomy Dog Cow ~12 lobes

14. Gross Anatomy of the Kidney Continued • Hilus- where blood, lymph, vessels, nerves and ureters enter and leave the kidney. • What else has a hilus? • Renal pelvis- funnel shaped area inside the hilus which is urine collection chamber and beginning of the ureter. • Lined with transitional epithelium which is capable of stretching • Cortex- outer portion of the kidney. • Rough granular appearance • Reddish brown in color • Medulla- inner portion around the renal pelvis. • Smooth appearance • Dark purple outer area and gray-red inner area. • Varies with species can be multipyramidal (multilobar) or unipyramidal (unilobar). • Calyx- cuplike extension of the renal pelvis in which the medullary pyramids fit. • Acts as funnels to direct fluids to renal pelvis

15. Renal papilla

16. Microscopic Anatomy of the Kidney • Nephrons- basic functional unit of the kidney whose job is to filter, reabsorb and secrete. • Smallest part of the kidney • Number varies per kidney and species • Medium sized dog is about 700,000 • Composed of: • Renal Corpuscle • Proximal Convoluted Tubule (PCT) • Loop of Henle • Distal Convoluted Tubule (DCT) https://youtu.be/hiNEShg6JTI

18. Renal Corpuscle • Located in cortex of kidney. • Function is to filter blood in first stage of urine production. • This filtrate is called glomerular filtrate • Made up of Glomerulus-tuft of glomerular capillaries, and Bowman’s capsule- double walled capsule that surrounds the glomerulus. • Bowman’s capsule has inner visceral layer and outer parietal layer. • Space between these two layers is known as the capsular space and is continuous with the proximal convoluted tubule.

20. Proximal Convoluted Tubule (PCT) • Continuation of the capsular space of the Bowman’s capsule. • Longest part of the tubular system of the nephron. • Glomerular filtrate becomes tubular filtrate (primitive urine). • Epithelial cells are cuboidal and have a brush border on lumen side. • What does the brush border do?

22. Loop of Henle • Continued from PCT • Descends into medulla of the kidney and the ascends back to cortex. • Descending portion • Epithelial cells like PCT and brush border Ascending portion • Epithelial cells flatten to become simple squamous and lose brush border. • Lumen expands as loop ascends to cortex.

24. Distal Convoluted Tubule (DCT) • Continuation of ascending portion of Loop of Henle • Not as twisted as PCT. • Empties into a series of collecting ducts which carry tubular filtrate through medulla and eventually empty it into renal pelvis that becomes ureter. • Collecting ducts play important role in urine volume because are sight of ADH. • Potassium regulation and acid-base balance also occur in collecting ducts.

26. Nerve Supply to the Kidneys • Supplied by sympathetic portion of autonomic nervous system. • Is not essential for kidney function. • Why transplanted kidneys will work.

27. Blood Supply to the Kidneys • Kidneys have very large blood supply. • All circulating blood passes through kidney every 4 or 5 minutes. • Up to 25% of all blood that heart pumps goes to the kidneys. • Composed of: • Renal artery • Afferent glomerular arterioles • Glomerular capillaries • Efferent Glomerular arterioles • Peritubular capillaries • Renal vein

28. Blood Supply Continued • Composed of: • Renal artery • Branches off of aorta and enters kidney at hilus • Divides smaller and smaller until becomes afferent glomerular arterioles • Afferent glomerular arterioles • Carry blood into the glomerular capillaries of the renal corpuscle. • Glomerular capillaries • Continuation of afferent glomerular arterioles. • Filter plasma from bloodand in capsular space called glomerular filtrate. • Only place in body where capillary blood that enters and leaves capillaries is still oxygenated blood.

29. Blood supply continued • Efferent Glomerular arterioles • Divide into capillaries that surround nephron- called peritubular capillaries • Peritubular capillaries • Surround nephron and converge to form venules then become renal vein. • Oxygen transfer occurs here. • Usable substances are removed from tubular filtrate and reabsorbed. • Other waste substances are secreted into filtrate here as well. • Renal vein • Leaves kidney at hilus and joins abdominal portion of the caudal vena cava. • Purest blood in body. • Why?

30. Mechanisms of Renal Action • Three main mechanisms for waste elimination: • Filtrationof blood • Reabsorption of useful substances into bloodstream • Secretion of waste products from blood into tubules of nephron.

31. Terminology • Osmosis- passage of water from weaker to a stronger solution across a semi-permeable membrane. • Diffusion- passage of a substance from a high concentration to a low concentration. • Reabsorption-passage of a substance from the lumen of the renal tubules into the renal capillaries and so back into circulation; this is an active process and requires energy. • Secretion- passage of a chemical substance from the renal capillaries into the lumen of the tubules and out of the body in the urine; this is an active process and requires energy.

32. Filtration of the Blood • Occurs in renal corpuscle • Having high blood pressure forces out some plasma into capsular space. • Transfer is aided by fenestrations- or holes or pores, in the capillary endothelium. These allow more fluid to leave the bloodstream. • Glomerular filtratewhen it enters capsular space. • Similar to plasma except contains very little or no proteins. • Proteins in urine are an abnormality and may be due to capillary damage in the glomerulus.

33. Filtration of the Blood • Glomerular Filtration Rate (GFR) • Describes how fast plasma is filtered as it passes through the glomerulus. • Depends on rate of blood flow to the kidneys • Expressed in mL per minute • Not all that is filtered is produced as urine or would urinate constantly. • Reabsorption helps reduce volume of glomerular filtrate. • For every 100L of fluid filtered from the blood only 1 L is produced as urine. • 99% of original filtrate is reabsorbed back into blood.

34. Reabsorption • Reabsorption takes useful substances from the tubules back into the blood. • Most takes place in the PCT but may also take place in Loop of Henle, DCT, and collecting ducts. • Glomerular filtrate is considered outside the body. • Contains waste products that must be cleared from the body as well as substances the body needs back into bloodstream. • Sodium, potassium, calcium, magnesium, glucose, amino acids, chloride, bicarbonate and water.

35. Reabsorption Continued • Glomerular filtrate enters PCT and becomes tubular filtrate. • In PCT some substances may be passively or actively transported. • Path must travel is: • Out of tubular lumen • Through tubular epithelium • Into interstitial fluid • Into peritubular capillaries.

36. Sodium Reabsorption • Attaches to a carrier protein that carries it into the cytoplasm of the PCT epithelial cell. • Requires energy. • Sodium ions are reabsorbed in peritubular capillaries in ascending loop of Henle and DCT. • In DCT, sodium is exchanged for hydrogen, potassium ions and ammonium. • This process is influenced by aldosterone. • Glucose and amino acids “hitch a ride” on this carrier protein so that no extra energy has to be expended. • They follow sodium into epithelial cells by process called sodium cotransport.

37. Sodium Reabsorption continued • When sodium is pumped out of epithelial cell, electrical imbalance occurs. • Chloride is diffused into cell to correct imbalance. • When some substances leave tubular filtrate, some water moves into peritubular capillaries by osmosis, other substances may be passively reabsorbed at this point. • Urea- a non protein source of nitrogen produced by the liver; is one of the substances that is not completely filtered. • Measured by Blood Urea Nitrogen (BUN) blood enzyme.

38. Potassium Reabsorption • Diffuses out of tubular filtrate by moving between epithelial cells and into interstitial fluid before into peritubular capillaries. • Takes place in the PCT, ascending part of Loop of Henle, and DCT.

39. Reabsorption Continued • Calcium Reabsorption • Takes place in the PCT, the ascending Loop of Henle, and the DCT. • Magnesium Reabsorption • Takes place in the PCT, the ascending Loop of Henle, and the collecting duct. • PTH release increases the rate of absorption of magnesium.

41. Secretion • Whatever is not filtered is a true waste. product and must be eliminated from body. • These substances are transferred through system to tubular filtrate. This process is called tubular secretion. • Takes place in the DCT. • Eliminates Hydrogen, potassium, and ammonia. • Some drugs are eliminated this way as well. • When is this beneficial?

42. Urine Volume Regulation • Determined by amount of water contained in tubular filtrate when it reaches renal pelvis. • Regulated by two hormones: • Antidiuretic hormone (ADH)- released by posterior pituitary gland. • Most important role in regulating urine volume • Acts on DCT and collecting ducts to promote water reabsorption and prevent water loss. • If not present will result in polyuria. • Aldosterone- secreted by adrenal cortex. • Increases reabsorption of sodium into the bloodstream in the DCT and collecting duct. • Causes osmotic imbalance in which water follows sodium out of tubular filtrate into blood. • However, ADH must be sufficient to move water out of DCT.

43. Ureters • Tube that exits the kidney at the hilus and connects to the urinary bladder near the neck of the bladder at its caudal end. • Are a continuation of renal pelvis. • Composed of three layers: • Fibrous layer • Outer layer • Muscular layer • Middle layer, composed of smooth muscle. • Epithelial layer • Inner layer, lined with transitional epithelium.

44. Function of the Ureters • Continuously move urine from the kidneys to the urinary bladder. • Smooth muscle propels urine through peristaltic contractions. • Enables urine to be moved regardless of position of animal’s body • Ureters enter bladder at oblique angle, so that when bladder is full it will collapse to prevent urine from backing up into ureters. • This will not keep ureters from pushing urine into bladder however.

45. Urinary Bladder • Stores urine as it is produced and releases it periodically from the body. • Function is to store, collect and release urine. • Bladder keeps urine from constantly being released. • Has two parts: • Muscular sac • Position and size vary depending on amount of urine it contains. • Lined with transitional epithelium. • Lined with smooth muscle bundles that run in all directions. • As these muscles contract, urine is squeezed out into urethra. • Neck • Is a muscle sphincter of skeletal muscle fibers. • Under voluntary control and open and close to allow urine to leave bladder and go into urethra.

46. Urinary Bladder continued • Rests on pubic bones when empty. • Has thick wall lined with transitional epithelium. • In carnivores, bladder extends to abdominal cavity. • Walls become thinner, the more full the bladder becomes. • Urination (micturition or uresis) is the expulsion of urine from the urinary bladder into the urethra.

47. Process of Urination • Urine Accumulation • Urine is constantly accumulated in bladder. • When reaches a certain point, triggers stretch receptors. • Where have we seen these before? • Muscle contraction • When trigger point is reached, spinal reflex sends impulse to bladder muscles • Muscles in bladder contract and give sensation of needing to urinate. • Sphincter Muscle Control • Sphincter at neck of bladder provides temporary control of urine. • If enough pressure is exerted on sphincter, will eventually relax and release urine. • Rupture can occur if bladder is not emptied in timely fashion. • Use caution when manually expressing or palpating bladder • https://youtu.be/US0vNoxsW-k

50. Urethra • Continuation of the neck of the urinary bladder that runs through pelvic canal. • Carries urine from bladder to external environment. • Lined with transitional epithelium • Female • Short and straight urethra • Opens on the ventral portion of the vestibule of the vulva. • Only carries urine • Male • Long and curved urethra • Runs down center of penis • Also serves in reproductive role to carry semen, however during ejaculation, sphincter at urinary bladder closes to prevent urine from mixing with seminal fluid.