Medical brain notes 41

  Functions of the Urinary System The urinary system consists of organs involved in the  elimination of waste products  produced by the cells of the body. The respiratory system eliminates carbon dioxide—one waste product. All other organic waste products found in the extracellular fluid, such as bilirubin from hemoglobin breakdown, uric acid from nucleic acid in cells, creatinine from creatine phos-phate in muscle, urea and ammonia from amino acid metabolism, are taken care of by the urinary system. This system helps  conserve nutrients  by retaining them in the body and excreting only the unwanted products. In addition to these important functions, the urinary system  regulates blood osmolality, blood volume,  and  blood pressure  by altering the volume of water lost inthe urine. It  regulates the levels of sodium, potassium,chloride, calcium,  and  other ions  by altering the quan-tity excreted in the urine. By monitori...

  COMPONENTS OF THE URINARY SYSTEM The urinary system consists of two  kidneys  that produce urine, two  ureters  that convey the urine to the  urinary bladder,  where it is stored, and the  urethra,  which transports the urine out of the body (see Fig-ures 12.1and 12.2) THE KIDNEYS The paired kidneys (Figure 12.2) are bean-shaped, with the indentation (the  hilum ) facing medially. The renal artery, renal vein, lymphatics, and renal nerves  enter and leave at the hilum.   An adult kidney is about 11.25 cm (4.43 in) long, 5–7.5 cm (2–3 in) wide, and about 2.5 cm (1 in) thick. The kidneys are located on either side of the vertebral column between the T12 and L3 vertebrae. The right kidney is slightly lower than the left because of the presence of the liver. These organs are retroperitoneal  (i.e., they are located behind the peritoneum). The anterior aspect of the  right  kidney is related to the liver, the hepati...

  THE KIDNEYS The paired kidneys (Figure 12.2) are bean-shaped, with the indentation (the  hilum ) facing medially. The renal artery, renal vein, lymphatics, and renal nerves  enter and leave at the hilum. An adult kidney is about 11.25 cm (4.43 in) long, 5–7.5 cm (2–3 in) wide, and about 2.5 cm (1 in) thick.  The kidneys are located on either side of the vertebral column between the T12 and L3 vertebrae. The right kidney is slightly lower than the left because of the presence of the liver. These organs are  retroperitoneal  (i.e., they are located behind the peritoneum). The anterior aspect of the  right  kidney is related to the liver, the hepatic flexureof the colon, and the duodenum. The  left  kidney is covered by the stomach, pancreas, spleen, jejunum, and splenic flexure of the colon. The superior aspect of both the kidneys is covered by the adrenal glands. Posteriorly, the kidneys are related to the muscles of the back; the tendo...

  The Nephron Each kidney is made up of about 1.25 million tubu-lar, microscopic structures known as the  nephrons  (see Figure 12.4). The nephrons are the functional units of the kidney. These nephrons, if laid end to end, would extend about 145 kilometers. Study of the nephron structure is required to un-derstand the process of urine formation. Each nephron can be considered to be a long tube with one end closed. Imagine the closed end of the tube to be  dilated and indented to form a cup.   The long tube is the  renal tubule,  and the cup-shaped end is the  glomerular capsule,  or  Bowman’s capsule.  Nowimagine a network of capillaries nestled in the cup with both ends of the capillaries continuous with a blood vessel—one bringing blood to the cup and the other taking blood from the capillaries. This network of blood vessels is the  glomerulus  (plural, glomeruli). The blood vessel that brings blood  to  the c...

  Urine Formation The filtrate originally formed in the renal corpuscle undergoes many changes as it travels down the renal tubule. These changes are made in different segments  of the renal tubule, according to the internal envi-ronment (i.e., in accordance to the blood volume, blood pressure, plasma concentration, and level of such electrolytes as sodium, potassium, and calcium) The structure of the cells lining the different seg-ments of the tubule is modified according to the func-tions they perform (see Figure 12.8). In the renal corpuscle, substances from the blood are allowed to pass passively through the filtration membrane, based on their size. Therefore, solutes, such as glucose, amino acids, and fatty acids, are also filtered. To increase efficiency and avoid waste, useful substances such as these must be reabsorbed. In ad-dition, the large volume of water filtered from the plasma cannot be wasted. To give an idea of the water volume involved, the rate at which the ...

  THE COMPOSITION OF URINE Urine is composed of 93% to 97% water. Urine pH ranges from 4.5–8.0. Normally, about 1,200 mL (73.2 in 3 ) of urine is excreted each day. Urine is a sterile fluid. It only becomes contaminated with bacteria when it passes through the external genitalia. Some organic substances present include urea, creatinine, ammonia, uric acid, urobilin, and bilirubin. Urea is derived from the metabolism of amino acids by the liver and kidneys. Its content in the urine would, therefore, increase when protein breakdown is greater than protein buildup. Creatinine is derived from the breakdown of creatine phosphate in skeletal muscle. Its excretion is proportional to the skeletal muscle mass. Ammonia is derived largely from pro-tein breakdown. Uric acid is derived from breakdown of nucleic acid, present in large amounts in the nu-cleus of cells. Urobilin and bilirubin are breakdown products of hemoglobin and give urine its yellow color. Increased amounts are excreted when ...