Urinary system

      The urinary system is the main excretory system and consists of the following structures :-

       * Two kidneys, which secrete urine.

       * Two ureters that convey the urine from the kidneys to the urinary bladder

       * The urinary bladder, which collects and stores urine.

      * The urethra, through which urine leaves the body.

      The urinary system plays a vital part in maintaining homeostasis of water and electrolytes within the body. The kidneys produce urine , which contains metabolic waste products, including the nitrogenous compounds urea and uric acid, excess ions and sometimes,excreted drugs. The main functions of the kidneys are :-

        • Formation of urine, maintaining water, electrolyte and acid-base balance.

        • Excretion of waste products.

        • Production and secretion of erythropoietin, the hormone that stimulates formation of red blood cells (erythropoiesis)

       • Production and secretion of renin, an important enzyme in the long-term control of blood pressure.

      Urine is stored in the bladder and excreted by the process of micturition.

                    

                        Kidneys

     The kidneys lie on the posterior abdominal wall, one on each side of the vertebral column, behind the peritoneum and below the diaphragm. They extend  from the level of the 12th thoracic vertebra to the 3rd lumbar vertebra, receiving some protection from the lower ribcage. The right kidney is usually slightly lower than the left, probably because of the considerable space occupied by the liver.

       Kidneys are bean- shaped organs, about 11 cm long , 6 cm wide and 3 cm thick ; they weigh around 150 g. They are embedded in and held in position by , a mass of fat . A sheath of fibrous  connective tissue , the renal fascia , encloses the kidney and the renal fat .

* Organs associated with the kidneys :-

       The kidneys lie on either side of the vertebral column and so each is associated with different structures.

    • Right kidney-

       ° Superiorly - The right adrenal gland.

       ° Anteriorly - The right lobe of the liver , the duodenum and the hepatic flexure of the colon .

       ° Posteriorly - The diaphragm and muscles of the posterior abdominal wall.

    • Left kidney-

       ° Superiorly- The left adrenal gland.

       ° Anteriorly- The spleen, stomach , pancreas, jejunum and splenic flexure of the colon .

       ° Posteriorly- The diaphragm, and muscles of the posterior abdominal wall .

* Gross Structure of kidney :-

        Three different areas of tissue are clearly distinguishable to the naked eye when viewing a longitudinal section of the kidney.

 They are -

       • An outer fibrous capsule, surrounding the kidney.

       • The cortex, a reddish- brown layer of tissue immediately below the capsule and surrounding the renal pyramids.

       • The medulla, the innermost layer, consisting of pale, conical - shaped, striated structure, the renal pyramids. Each pyramid has a pointed end called the papilla.

        The hilum is the concave medial border of the kidney, where the renal blood and lymph vessels, the ureter and nerves enter.

       Urine formed within the kidney passes through a renal papilla into the drainage system that begins at a minor calyx(see the picture). Several minor calyces merge into a major calyx, and two or three major calyces combine, forming the renal pelvis, a hollow funnel- shaped structure that narrows when it leaves the kidney as the ureter. The walls of the calyces and renal pelvis are lined with transitional epithelium and contain smooth muscle. Peristalsis, intrinsic constraction of smooth muscle, propels urine through the calyces, renal pelvis and ureters to the bladder.

* Microscopic structure of kidney :-

       The kidney contains 1-2 million functional units, the nephrons , and a much smaller number of collecting ducts. The collecting ducts transport urine through the pyramids to the calyces, giving the pyramids their striped appearance (see image in Gross structure ). The collecting ducts are supported by connective tissue,containing blood vessels, nerves and lymph vessels .

   • The nephron :- 

         The nephron is essentially a tubule that is closed at one end and opens into a collecting duct at the other end. The closed or blind end forms the cup-shaped glomerular(Bowman's) capsule, which almost completely encloses a coiled tuft of capillaries, the glomerulus.

       Continuing from the glomerular capsule , the remainder of the nephron is about 3 cm long and described in three parts -

    -> The proximal convoluted tubule

    -> The medullary loop (loop of Henle)

    -> The distal convoluted tubule , leading into a collecting duct .

    The collecting ducts unite, forming larger ducts that empty into the minor calyces.

    The kidneys receive about 20 % of the cardiac output. After entering the kidney at the hilum , the renal artery divides into smaller arteries and arterioles. In the cortex an arteriole, the afferent arteriole, enters each glomerular capsule and then subdivides into a cluster of tiny arterial capillaries, forming the glomerulus. Between this capillary loops are connective tissue phagocytic mesangial cells, which are part of the mononuclear phagocyte defence system. The blood vessel leading away from the glomerulus is the efferent arteriole. The afferent arteriole has a larger diameter than the efferent arteriole, which increases pressure inside the glomerulus and drives filtration across the glomerular capillary walls.

      The efferent arteriole divides into a second peritubular(meaning 'around tubules') capillary network, which wraps around the remainder of the tubule, allowing exchange between the fluid in the tubule and the bloodstream. This supplies tubular tissues with oxygen and nutrients and removes waste products, Venous blood drained from this capillary bed eventually leaves the kidney in the renal vein, which empties into the inferior vena cava.

      The walls of the glomerulus and the glomerular capsule consist of a single layer of flattened epithelial cells. The glomerular walks are very permeable, to facilitate filtration. The walls of the reminder of the nephron and the collecting duct are formed by s single layer of simple squamous epithelium.

      Renal blood vessels are supplied by both sympathetic and parasympathetic nerves. The presence of both divisions of the autonomic nervous system controls renal blood vessels diameter and renal blood flow independently of autoregulation .

  * Functions of kidney :- 

      • Formation of urine -

            The kidneys form urine , which passes to the bladder for storage prior to excretion . The composition of urine reflects the exchange of substances between the nephron and the blood in the renal capillaries. Waste products of protein metabolism are excreted , water and electrolyte levels are controlled , and pH (acid-base balance) is maintained by excretion of hydrogen ions . 

     There are three processes involved in the formation of urine :-

  -> Filtration -

           Filtration takes place through the semipermeable walls of the glomerulus and glomerular capsule. Water and other small molecules readily pass through, although some are re-absorbed later. The fluid filtered from the bloodstream into the glomerular capsule is now called filtrate, and its composition will be adjusted as it passes through the other parts of the renal tubule. Blood cells, plasma proteins and other large molecules are too large to filter through and therefore remain in the capillaries. The filtrate is very similar in composition to plasma, with the important exceptions of plasma proteins and blood cells .

          Filtration takes place because there is a defference between the blood pressure in the glomerulus and the pressure of the filtrate in the glomerular capsule. The efferent arteriole is narrower than the afferent arteriole , and so a capillary hydrostatic pressure of about 7.3 kPa(55 mmHg) builds up in the glomerulus. This pressure is opposed by the osmotic pressure of the blood, provided mainly by plasma proteins, about 4 kPa(30mmHg) , and by filtrate hydrostatic pressure of about 2 kPa(15mmHg) in the glomerular capsule.

      The net filtration pressure is , therefore :-

            7.3 - (4+2) = 1.3 KPa or

            55 - (30+15) = 10 mmHg

   The volume of filtrate formed by both kidneys each minute is called the glomerular filtration rate (GFR). In a healthy adult the GFR is about 125 mL/min, i.e.180 litres of filtrate are formed each day by the two kidneys. Nearly all of the filtrate is later re- absorbed from the kidney tubules, with less than 1% , i.e. 1 - 1.5litres, excreted as urine. The differences in volume and concentration are due to selective re-absorption of some filtrate constituents and tubular secretion of others.

   • Autoregulation -

           Renal blood flow , and therefore glomerular filtration, is protected by a mechanism called Autoregulation, whereby renal blood flow is maintained at a constant pressure across a wide range of systolic blood pressure ( from around 80 - 200 mmHg ). Because glomerular filtration is entirely dependent on blood pressure, which fluctuates significantly depending on activity, emotional state and so on, renal blood flow must be locally controlled . In this way, for example, in strenuous exercise when blood pressure rises, the kidney can independently reduce blood flow through the glomeruli in order to reduce water loss in the urine. Autoregulation operates independently of nervous control: i.e. if the nerve supply to the renal blood vessels is interrupted, autoregulation continues to operate. It is therefore a property inherent in renal blood vessels; it may be stimulated by changes in blood pressure in the renal arteries or by fluctuating levels of certain metabolites, e.g. prostaglandins.

     In sever shock, when the systolic blood pressure falls below 80 mmHg, autoregulation fails and renal blood flow and the hydrostatic pressure decrease, impairing filtration within the glomeruli. Damage to the kidneys caused by such impairment may or may not be reversible.

    -> Selective re-absorption -

        Most reabsorption from the filtrate back into the blood takes place in the proximal convoluted tubule, whose epithelial lining possesses microvilli to increase surface area for absorption. Many substances are re- absorbed here, including water, electrolytes and organic nutrients such as glucose and amino acids. Some re-absorption is passive but some substances, e.g. glucose, are actively transported. Only 60-70% of filtrate reaches the medullary loop. Much of this, especially water,sodium, and chloride, is re-absorbed in the loop, so that only 15-20% of the original filtrate reaches the distal convoluted tubule, and the composition of the filtrate is now very different. More electrolytes are re- absorbed here, especially sodium, so the filtrate entering the collecting ducts is actually quite dilute. The main function of the collecting ducts is to re- absorb as much water as the body needs.

     Active transport takes place at carrier sites in the epithelial membrane, using chemical energy to transport substances against their concentration gradients.

    Some ions e.g. sodium and chloride, can be absorbed by both active and passive mechanisms, depending on the site in the nephron.

    Some constituents in the glomerular filtrate (e.g. glucose, amino acids ) do not normally appear in urine because they are completely re- absorbed, unless blood levels are excessive.

     Re-absorption of nitrogenous waste products, such as urea, uric acid and creatinine, is very limited.

   The kidneys maximum capacity for re- absorption of a substance is the transport maximum, or renal threshold. For example, the normal blood glucose level is 3.5-8 mmol/L(63-144mg/100mL); if this rises above the transport maximum of about 9 mmol/L (160 mg/100ml ), glucose appears in the urine . This occurs because all the carrier sites are occupied and the mechanism for active transport out of the tubules is overloaded. Other substances re- absorbed by active transport include sodium , calcium, potassium , phosphate and chloride.

      The transport maximum, or renal threshold, of some substances varies according to body need at a particular time, and in some cases re- absorption is regulated by hormones .

   • Hormones that influence selective re-absorption :-

     Several hormones play such a role, each being regulated by a negative feedback mechanism.

     Parathyroid hormone - This is secreted by the parathyroid glands and, together with calcitonin from the thyroid gland, regulates the reabsorption of calcium and phosphate from the distal collecting tubules, so that normal blood levels are maintained. Parathyroid hormone increases the blood calcium level and calcitonin lowers it . 

     Antidiuretic hormone - Antidiuretic hormone (ADH) is secreted by the posterior pituitary. It increases the permeability of the distal convoluted tubules and collecting ducts, increasing water reabsorption.  Osmoreceptors in the hypothalamus monitor the water content of the blood, and ADH secretion is adjusted accordingly.

   Aldosterone - This secreted by the adrenal cortex, this hormones increases the reabsorption of sodium and water, and the excretion of potassium.

   Atrial natriuretic peptide - Atrial natriuretic peptide (ANP) is a hormone secreted by the atrial of the heart in response to stretching of the atrial wall when blood volume is increased. It decreases reabsorption of sodium and water from the proximal convoluted tubules and collecting ducts.

 ->  Tubular Secretion -

        Filtration occurs as blood flows through the glomerulus. Substances not required and foreign materials, e.g. drugs including penicillin and aspirin, may not be entirely filtered out of the blood because of the short time they remain in the glomerulus or because the molecules are too large to pass through the filtration pores. Such substances are cleared by secretion from the peritubular capillaries into the filtrate within the convoluted tubules.  Tubular secretion of hydrogen ions is important in maintaining normal blood pH.

* Summary of urine formation :-

      The three processes involved - filtration, selective re-absorption and tubular secretion have been described and summarised.

* Composition of urine :- 

      Urine is clear and amber in colour due to the presence of the pigmented substance urobilin. This is generated following haemolysis when some of the urobilinogen produced in the kidneys is converted to urobilin. The specific gravity is between 1020 and 1030, and the pH is around 6 (normal range 4.5 - 8). A healthy adult passes from 1000 to 1500 mL per day. The volume of urine produced and the specific gravity vary, according to fluid intake and the amount of solute excreted. The constituents of urine are -

  ° Water 96%

  ° Urea 2%

   ° Uric acid

   ° Creatinine

   ° Ammonia

   ° Sodium

   ° Potassium

   ° Chlorides

   ° Phosphates

   ° Sulphates

   ° Oxalates ,

 except water and uric acid all are together 2%.

   

                                Ureters

      

   Figure- Ureters and their relationship to kidneys and bladder.   

       

      The ureters carry urine from the kidneys to the urinary bladder. They are hollow muscular tubes about 25-30 cm long with a diameter of approximately 3 mm. The ureter is continuous with the funnel-shaped renal pelvis. It travels downwards through the abdominal cavity,behind the peritoneum in front of the psoas muscle into the pelvic cavity, and passes obliquely through the posterior wall of the bladder. This arrangement means that, as urine accumulates and the pressure in the bladder rises, the ureters are compressed and the openings into the bladder are occluded. This prevents back flow (reflux) of urine into the ureters (towards the kidneys) as the bladder fills and also during micturition , when pressure increases as the muscular bladder wall contracts.

   * Structure of Ureters :-

    The walls of the ureters consists of three layers of tissue , they are :-

        • An outer covering of fibrous connective tissue, continuous with the fibrous capsule of the kidney.

       • A middle muscular layer consisting of interlacing smooth muscle fibres that form a functional unit round the ureter and an additional outer longitudinal layer in the lower third.

      • An inner layer, the mucosa, composed of transitional epithelium.

     *Function of Ureters :-

       Peristalsis is an intrinsic property of the smooth muscle layer that propels urine along the ureter. Peristaltic waves occur several times per minute, increasing in frequency with the volume of urine produced , and sending little spurts of urine along the ureter towards the bladder.

   

                   Urinary Bladder

      The urinary bladder is a reservoir for urine. It lies in the pelvic cavity and its size and position vary, depending on the volume of urine it contains. When distended, the bladder rises into the abdominal cavity.

   * Structure of urinary bladder :- 

       The bladder is roughly pear-shaped, but becomes more balloon- shaped as it fills with urine. The posterior surface is the base. The bladder opens into the urethra at its lowest point, the neck.

       The peritoneum covers only the superior surface before it turns upwards as the parietal peritoneum, lining the anterior abdominal wall. Posteriorly, it surrounds the uterus in the female and the rectum in the male. The bladder wall is composed of three layers:-

     • The outer layer of loose connective tissue, containing blood and lymphatic vessels and nerves, covered on the upper surface by the peritoneum.

     • The middle layer, consisting of interlacing smooth muscle fibres and elastic tissue loosely arranged in three layers. This is called the detrusor muscle, and when it contracts, it empties the bladder.

     • The inner mucosa, composed of transitional epithelium that readily permits distension of the bladder as it fills.

     When the bladder is empty the inner lining is arranged in folds , or rugae, which gradually disappear as it fills. The bladder is distensible, but as it fills, awareness of the need to pass urine is felt. The total capacity is rarely more than about 600mL.

     The three orifices in the bladder wall form a triangle or trigone. The upper two orifices on the posterior wall are the openings of the ureters; the lower orifice is the opening into the urethra. The internal urethral sphincter, a thickening of the urethral smooth muscle layer in the upper part of the urethra, controls outflow of urine from the bladder . The sphincter is not under voluntary control.

     

                          Urethra

     The urethra is a canal extending from the neck of the bladder to the exterior, at the external urethral orifice. It is longer in the male than in the female.

      The male urethra is associated with both the urinary and the reproductive system.

       The female urethra is approximately 4 cm long and 6 mm in diameter. It runs downwards and forwards behind the symphysis pubis and opens at the external urethral orifice just in front of the vagina. The external urethral orifice is guarded by the external urethral sphincter, which is under voluntary control.

     The wall of the female urethra has two main layers : an outer muscle layer and an inner lining of mucosa , which is continuous with that of the bladder. The muscle layer has two parts : an inner layer of smooth muscle that is under autonomic nerve control, and an outer layer of striated ( voluntary ) muscle surrounding it . The striated muscle forms the external urethral sphincter and is under voluntary control. The mucosa is supported by loose fibroelastic connective tissue containing blood vessels and nerves. Proximally it consists of transitional epithelium, while distally it is composed of stratified epithelium.

           

                       Micturition

      In infants, accumulation of urine in the bladder activates stretch receptors in the bladder wall, generating sensory (afferent) impulses that are transmitted to the spinal cord, where a spinal reflex is initiated. This stimulates involuntary contraction of the detrusor muscle and relaxation of the internal urethral sphincter , and expels urine from the bladder : this is known as micturition, urination or voiding of urine.

      When bladder control is established, the micturition reflex is still stimulated but sensory impulses also pass upwards to the brain and there is awareness of the need to pass urine as the bladder fills(around 300-400 mL in adults).By learned and conscious effort, contraction of the external urethral sphincter and muscles of the pelvic floor can inhibit micturition until it is convenient to pass urine.

     Urination can be assisted by increasing the pressure within the pelvic cavity, achieved by lowering the diaphragm and contracting the abdominal muscles. Over-distension of the bladder is extremely painful, and when this occurs there is a tendency for involuntary relaxation of the external sphincter to occure, allowing a small amount of urine to escape, provided there is no mechanical obstruction. Incontinence is the involuntary loss of urine after bladder control has been established.

    * Effects of ageing on the urinary system:-

    The kidneys have a substantial functional reserve; the loss of one kidney does not cause problems in an otherwise healthy individual. The number of nephrons declines with age, glomerular filtration rate falls and the renal tubules function less efficiently; the kidneys become less able to concentrate urine.

      These changes mean that older adults become more sensitive to alterations in fluid balance, and problems associated with fluid overload or dehydration are more prevalent. Elimination of drugs also becomes less efficient with declining kidney function, which may lead to accumulation and toxicity.

      The ability to inhibit contraction of the detrusor muscle declines and may result in the urgent need to pass urine and urinary frequency. Nocturia becomes increasingly common in older adults. Incontinence is more prevalent in older adult, affecting 15% of women and 10% of men over 65, figures that double by 85 years of age. Enlargement of the prostate gland is common in older men and may cause retention of urine and problems with micturition.

                                *****