Introduction to the Urinary System

Transcription

1 Unit 7 Urinary System Session 22 Introduction to the Urinary System Session Outline Introduction 22.1 Organs of the urinary system 22.2 Basic structure of the urinary system 22.3 Blood and nerve supply 22.4 Functions of the urinary system 22.5 Regulation of the functions of the urinary system Summary Learning Outcomes Review Questions References The urinary system plays and vital part in maintaining homeostasis of water and electrolyte concentrations within the body. Every day the kidneys filter nearly 200 liters of fluid from the bloodstream. It filters and removes toxins, waste products of metabolism, and excess ions. These waste products leave the body in the form of urine. The urine contains metabolic waste products, nitrogenous compounds (urea and uric acid), and excess ions. At the same time it filters the blood and forms urine, the kidney makes the body retain much needed substances in the blood. Therefore the kidney s function is similar to a water purification plant that keeps the water supply to a city clean and disposes of its waste products. Often we don t value the function of the kidneys, until the kidneys stop functioning and the body fluids become contaminated. When the function of the kidney is impaired it becomes a danger to life. The lungs and skin also participate in excretion, but the kidneys are the major excretory organs in the body. Learning outcomes After studying this section you should be able to; Identify the organs associated with the kidneys Briefly explain the other structures related to the kidney Outline the gross structure of the kidneys Describe the structure of a nephron Explain the processes involved in the formation of urine Explain how body water and electrolyte balance is maintained

2 22.1 Organs of the urinary system The Urinary System consists of the following structures. 2 Kidneys - filter plasma and formation of urine 2 Ureters - convey the urine formed in the kidneys to the urinary bladder Urinary bladder collects urine and temporally stores urine Urethra - expels urine from the urinary bladder to the outside Figure 22.1 shows an overview of the urinary system Figure 22.1 The main organs of the urinary system 22.2 Basic structure of the organs of the urinary system Kidneys The kidneys are reddish organs shaped like kidney beans. We have two kidneys. They are located just above the waist, posterior to the peritoneum in the abdomen. Since the kidneys are located posterior to the peritoneum, it is known as a retroperitoneal organ. Other retroperitoneal structures in the body include the ureters and adrenal (suprarenal) glands. In relation to the vertebral column, the kidneys are situated laterally between the 12 th thoracic and 3 rd lumbar vertebrae. The right kidney is placed slightly lower than the left because the liver occupies a large area on the right side.

3 The hilum of the kidney is the concave medial border of the kidney. Renal blood vessels and lymph vessels, the ureter and nerves enter the kidney through the hilum. If we cut a section through the kidney longitudinally (fig. 22.2) we can see three main areas of tissue. The outer most layer is the fibrous capsule. It surrounds the kidney. The second outer layer is the cortex. It is a reddish- brown layer of tissue immediately just interior to the capsule and outside the pyramids. The third layer is the medulla. It is the inner most layer. The medulla is composed of conical-shaped striations. These striations are pale and are known as the renal pyramids due to its shape. The innermost tip of the medulla is the papilla. The papilla empties into pouches called calyces. The calyces communicate with the renal pelvis. The renal pelvis is the funnelshaped structure. It collects the urine formed by the kidney. The urine formed by each kidney drains from the pelvis into the ureter. It is transported along the ureter to the bladder. The bladder stores urine and eliminates it at regular intervals. The walls of the pelvis contain smooth muscles. The pelvis is lined with transitional epithelium. Urine reaches the bladder through the pelvis. Peristalsis of the smooth muscle in the walls of the calyces propel urine through the pelvis and ureters to the bladder. Urine is stored in the bladder and excreted by the process of micturition. We will discuss the process of micturition in a later session. Figure 22.2 Logitudinal cross section through the right kidney 2.1 Self Assessment Questions Can you list the organs of the urinary system? Can you briefly outline the structure of each of these organs? What are the retroperitoneal organs in the body?

4 Microscopic structure of the kidney The kidney is composed of about 1 million functional units, known as the nephrons and lesser number of collecting ducts. The collecting ducts transport the formed urine through the pyramids to the renal pelvis. The nephron The nephron is formed by a cup- shaped structure known as the glomerular capsule (Bowman s capsule) and continues as a tube. The other end of the tube opens into a collecting tubule. The Bowmen s capsule is completely enclosed by a network of arterial capillaries, the glomerulus. Continuing from the glomerular capsule the reminder of the nephron is about 3 cm long. This part of the nephron consists of three sections known as the proximal convoluted tubule ( PCT), the medullary loop (loop of Henle)( LOH) and the distal convoluted tubule (DCT). The distal convoluted tubule leads into a collecting duct Blood and nerve supply The kidneys are supplied by the renal arteries which arise out of the aorta. There are two renal arteries, one for each kidney. These arteries divide into smaller arteries and arterioles in the kidneys. In the cortex of the kidney an arteriole, the afferent arteriole enters each glomerular capsule. Then it subdivides into a cluster of capillaries forming the glomerulus. The blood vessels leading away from the glomerulus is the efferent arteriole. Once the efferent arteriole leaves the glomerulus a set of capillary network is formed to supply oxygen and nutrients to the remainder of the nephron. Venous blood drained from this capillary bed eventually leaves the kidney in the renal vein which empties into the inferior vena cava. The blood pressure in the glomerulus is higher than in other capillaries because the diameter of the afferent arteriole is greater than that of the efferent arteriole. Figure 22.3 Parts of the Nephron and the associated blood vessels

5 The walls of the glomerulus and the glomerular capsule consists of a single layer of flattened epithelial cells which are permeable than other capillaries. The blood vessels of the kidney are supplied by both sympathetic and parasympathetic nerves. The presence of both branches of the autonomic nervous system permits control of the diameter of renal blood vessels. This allows the regulation of renal blood flow Functions of the urinary system Functions of the Kidneys The main function of the kidney is to filter blood and form the urine. The kidney receives about 25% of the cardiac output to perform the above function efficiently. It filters out the entire blood volume by about 60 times a day. The main functions of kidneys are: Formation of urine Regulate the volume and electrolyte composition of blood Maintain the balance between water and electrolytes in blood Maintain acid-base balance. These balances are essential to maintain life. Produce and secrete erythropoietin, the hormone responsible for stimulating the bone marrow to produce red blood cells. Produce and secrete renin, an important enzyme in the control of blood pressure. Metabolise vitamin D to its active form (1-25 dihydroxycholecalciferol) Formation of urine The kidneys form urine which passes through the ureters to the bladder for storage prior to excretion. There are three processes involved in the formation of urine.

6 1. Glomerular filtration 2. selective reabsorption in the renal tubules 3. secretion of substances in to the renal tubules Glomerular filtration Filtration takes place through the semipermeable walls of the glomerulus and glomerular capsule. Water, electrolytes and a large number of small molecules pass through, but blood cells, plasma proteins and other large molecules are unable to pass through and remain in the capillaries The filtrate in the glomerulus is very similar in competition to plasma except for the absence of plasma proteins. Filtration is assisted by the difference between the glomerular hydrostatic pressure and the hydrostatic pressure of the filtrate in the glomerular capsule. Because the diameter of the efferent arteriole is less than that of the afferent arteriole, a capillary hydrostatic pressure of about 55mmHg builds up in the glomerulus. This pressure is opposed by the osmotic pressure of the blood (30mmHg), and by the filtrate hydrostatic pressure of 15mmHg in the glomerular capsule. The net filtration pressure is therefore 10mmHg into the glomerular capsule: 55- (30+15) = 10 mmhg The glomerular filtration rate (GFR) is the volume of plasma filtered by both kidneys per each minute. In a healthy adult the GFR is about 125 ml/ min or 180 litres/ per day. Most of the filtrate is reabsorbed with less than 1%, i.e. 1 to 1.5 litres, excreted as urine. The difference in volume and concentration is due to selective reabsorption of water and electrolytes from the filtrate by the renal tubules and tubular secretion of unwanted substances such as urea and drugs in to the urine. Selective reabsorption In different parts of the renal tubule proximal convoluted tubule, loop of Henle, Distal convoluted tubule and collecting duct different mechanisms are used for reabsorption of water and electrolytes eg. Sodium, Potassium, Chloride. The general purpose of this process is to reabsorb into the blood those filtrate constituents needed by the body to maintain fluid and electrolyte balance and the ph of the blood.

7 Secretion Substances which are not required (eg. Urea, excess H + ) and foreign materials e.g. drugs including penicillin aspirin, may not be cleared from the blood by filtration because of the short time it remains in the glomerulus. Such substances are cleared by secretion into the convoluted tubules and excreted from the body in the urine. Tubular secretion of hydrogen (H + ) ions is important in maintaining the acid- base balance. Mechanisms of tubular reabsorption and secretion - simple and facilitated diffusion - active transport - carried out at carrier sites in the epithelial membrane using chemical energy to transport substances against their concentration gradients. - Water is reabsorbed by osmosis Proximal tubule (PT) Nearly 60-70% of filtered solutes reabsorbed and an equal percentage of water reabsorbed. The fluid in the proximal tubule remains isotonic. Sodium reabsorption in PT More than 60% of the filtered sodium is actively re absorbed by: - Co- transported with glucose, amino acids, phosphate and other organic acids. - Counter transported with H + Na + that is taken into the cell is actively pumped into the interstitium by the Na + / K + ATPase in the baso lateral membrane.

8 Glucose reabsorption in PT Glucose is completely reabsorbed in the proximal tubule until the transport maximum for glucose (TmG) is reached by secondary active transport. Energy is provided by the Na +/K+ ATPase in the basal membrane. The maximum capacity for reabsorption of a substance is the transport maximum, or the renal threshold for that substance. If the blood glucose level rises above the transport maximum of about 9mmol/ (160mg/100ml) glucose appears in the urine because all the carrier sites are occupied and the mechanism for active transfer out of the tubules is overloaded. Water reabsorption in PT Water is absorbed passively due to the osmotic gradient created by active transport of solutes % of water is reabsorbed in PT. The fluid in the entire proximal tubule remains isotonic. The loop of Henle (LOH) The loop of Henle has a descending limb and an ascending limb. The thin descending limb of the loop of Henle is permeable to water - -water reabsorption occurs. The thick part of the ascending limb is impermeable to water but Na+, K+ and Cl- are reabsorbed by secondary active transport. Due to the active transport of Na+, Cl- and K+, the tonicity in the medullary region of the kidney is high. This action of the Loop of Henle to maintain a gradient of increasing osmolality down the medullary pyramids is known as a counter-current system. The counter-current system of the loop of Henle helps to increase water reabsorption and concentrate the urine.

9 Collecting duct (CD) In the upper part of the CD, Na+ is reabsorbed while K+ or H+ is secreted. This is regulated by Aldosterone hormone. Aldesterone secreted by the adrenal cortex, increases the reabsorption of sodium and excretion of potassium and H+. Water reabsorption in the CD is regulated by ADH (Vasopressin antidiuretic hormone) - Antidiuretic hormone from the posterior lobe of the pituitary gland increases the permeability of the collecting duct to water by inserting aquaporin water channels Regulation of the functions of the urinary system Renal regulation of water balance Maintenance of water balance in the body is an important function of the kidney. Kidney is the only organ which can regulate the water excretion to balance the water intake. With a GFR of 125 ml/min about 180L of fluid is filtered at the glomerulus per day. The normal urine out put varies from 1 to 1.5L per day - nearly 99% of the filtered fluid is reabsorbed. Main hormone regulating water reabsorption is antidiuretic hormone (ADH). ADH is a peptide hormone synthesized in the hypothalamus and is secreted by the posterior pituitary. It acts on the collecting duct and increases the permeability to water by increasing the number of aquaporin 2 channels. ADH secretion is stimulated by, increased osmolality detected by osmoreceptors in the hypothalamus

10 decreased circulating blood volume detected by volume receptors decreased arterial pressure detected by baroreceptors in the cardiovascular system ADH secretion is inhibited by, decreased in osmolality of plasma increased in the ECF volume Therefore when ECF volume is decreased in situations such as severe blood loss or dehydration there is an increase in plasma osmolality resulting in increased ADH secretion by the posterior pituitary. This causes an increase in water reabsorption in the collecting ducts of the nephrons. Figure Negative feedback regulation of ADH secretion Figure Negative feedback regulation of ADH secretion

11 Regulation of electrolyte balance Plasma Na+ and K+ levels are maintained by the kidney by regulating the amount reabsorbed in the renal tubules by the action of Aldosterone hormone which is secreted by the adrenal gland. Action of Aldosterone - Aldosterone acts on the renal tubular cells to increase Na + reabsorption in the collecting duct (upper part) in association with the secretion of H + or K +. - Aldosterone increases epithelial sodium channels (ENaC) on the luminal membrane and the Na + /K + ATPase pumps on the basolateral membrane of cells of CD renal tubules Net effect is increase in the reabsorption of sodium together with water, and increased secretion of potassium and H+ in to the urine. Regulation of aldosterone secretion by Renin angiotensin aldosterone (RAA) pathway Renin is an enzyme, secreted by the juxtaglomerular (JG) cells in the kidney. JG cells are in the afferent arteriole adjacent to the glomerulus. Renin is an important hormone in the regulation of ECF volume and blood pressure. Renin converts the plasma protein angiotensinogen to angiotensin 1. Angiotensin 1 is converted to angiotensin II by action of angiotensin converting enzyme (ACE) in the lungs. Angiotensin II acts on the adrenal gland and increases secretion of Aldosterone hormone. Angiotensin II is also has a vasoconstrictor action and increases the blood pressure by increasing the peripheral resistance due to vasoconstriction of arterioles. Factors increasing Renin secretion

12 1. Reduced blood pressure (hypotension) - when pressure at the level of the afferent arteriole reduces, it increases renin secretion. 2. Reduced extracellular fluid volume (hypovolaemia) 3. Increased sympathetic discharge in renal nerves The feedback regulation of aldosterone hormone by RAA mechanism

13 Regulation of acid / base balance by the Kidney Acidification of urine important to maintain H + balance in the body H ions are secreted in the proximal tubules, distal tubule and the collecting duct In the proximal tubule, H + is secreted by secondary active transport. It is a counter transport with Na + Carbonic anhydrase (CA) catalyses the reaction of forming H 2 CO 3. CO2 + H2O H2CO3 H+ + HCO3- This enzyme is present in the brush border of the proximal tubular cells. For each H ion secreted, one sodium ion & one HCO 3 - enter the interstitial fluid There is a maximal H ion gradient across which the transport mechanism can secrete H ions to the lumen - corresponds to a urine ph of the limiting ph. This concentration would be reached rapidly if there was nothing to tie up with the H ions that are secreted to the tubular lumen. This action is produced by the urinary buffers.

14 Micturition Micturition is the passing of urine by emptying the badder Bladder has a smooth muscle layer in the wall known as the detrusor muscle. Detrusor muscle has a parasympathetic nerve supply which causes it to contract. Around the opening of the urethra there is an internal urethral sphincter consisting of smooth muscle and is therefore involuntary. External urethral sphincter is made of the surrounding skeletal muscle and is under voluntary conscious control. Micturition reflex - When urine volume reaches 200 to 400 ml it causes stretching of the detrusor muscle of the bladder This is the stimulus for the micturition reflex. This is seen in infants where micturition cannot be controlled voluntarily. - the afferent sensory impulses travel to the sacral spinal cord (S2,3,4 segments) and the motor impulses return along parasympathetic nerves to the detrusor muscle, causing contraction of the bladder wall. At the same time, the internal urethral sphincter relaxes and urine is passed. - In adults micturition can be inhibited for a period of time by conscious effort and by voluntary contraction of the external urethral sphincter. However, if the bladder continues to fill and be stretched, voluntary control is eventually no longer possible.

15 Micturiton reflex where there is no conscious inhibition of relex Control of micturition by conscious effort

16 Self Assessment Questions 1. List the parts of the nephron and outline which parts are involved in filtration of plasma, reabsorption and secretion of substances. 2. List the main functions of the Kidneys 3. Define glomerular filtration rate and explain the forces involved in filtration. 4. Explain how water is reabsorbed in the different parts of the renal tubule. 5. Explain the function of the proximal tubule in reabsorption of Sodium and Glucose. 6. Explain the action of the loop of Henle in the concentration of urine. 7. Explain how the body water balance is regulated by the Kidneys. 8. Explain how the plasma Sodium and Potassium balance is regulated by the Kidneys. 9. Explain how the acid/base balance is maintained by the Kidneys. 10. Outline the micturition reflex mechanism and compare the micturition mechanism in infants and adults. References Ross and Wilson s Physiology - Ganong Review of Physiology