Table of Contents
URINARY SYSTEM
INTRODUCTION. The urinary (renal) system is comprised of organs that produce urine and excrete it from the body. It is a major system in the maintenance of homeostasis (constancy of the internal environment).
Components. The urinary system consists of two kidneys, which produce urine; two ureters, which carry urine to a single urinary bladder for temporary storage; and the urethra, which passes urine outside of the body through the external urethral orifice .
1. Removal of organic wastes. The kidneys excrete urea, uric acid, creatinine, and the breakdown products of hemoglobin and hormones.
2. Regulation of the concentrations of important ions. The kidneys excrete sodium, potassium, calcium, magnesium, sulfate, and phosphate ions.
3. Regulation of the acid-base balance of the body. The kidneys control the elimination of hydrogen (H+), bicarbonate (HCO3-), and ammonium (NH4+) ions and produce an acid or alkaline urine, depending on the body's requirements.
4. Regulation of RBC production. The kidneys release erythropoietin, which regulates the production of RBCs in the bone marrow.
5. Regulation of blood pressure. The kidneys regulate the fluid volume of the body and thus blood pressure. They also produce the enzyme renin, which is an important component of the renin-angiotensin-aldosterone mechanism. This regulates blood pressure and water retention.
6. Limited control of blood glucose and blood amino acid concentration. Excrete excess amounts of glucose and amino acids from the blood.
7. Elimination of toxic substances. The kidneys eliminate pollutants, food additives, drugs, or other chemicals foreign to the body.
Appearance. The kidneys are bean-shaped, dark red organs, approximately 5 in. long and 1 in. thick. Each kidney weighs 125 to 175 g in men and 115 to155 g in women.
2. Location The kidneys are located high on the posterior abdominal wall. They are retroperitoneal;that is, behind the peritoneum.
3. Connective tissue coverings. Each kidney is surrounded by three layers of connective tissue.
The renal fascia is the outermost covering. It anchors the kidney to surrounding structures and maintains the position of the organ.
1. The hilus (hilum) is a depression or "dimple" on the medial surface of the kidney.
2. The renal sinus is a fat filled space near the hilus. It serves as an attachment site for the ureter, the renal artery and vein, nerves, and Iymphatic vessels.
3. The renal pelvis is the expanded proximal end of the ureter. It is formed by two to four major calyces, which extend into the kidney. Each major calyx branches can form (about 12) minor calyces.
4. The renal parenchyma is tissue that surrounds the renal sinus. It has two parts; an inner medulla and an outer cortex.
a. Renal pyramids, triangular shaped masses of tissue, are found in the medulla. The tip each pyramid, the papilla, extends into a minor calyx and is covered by the openings of urinary collecting ducts.
b. The cortex consists of the tubules and blood vessels of nephrons; the structural and functional units of the kidneys. Renal columns, extensions of cortical tissue between each pyramid, consists of collecting tubules that drain into collecting ducts.
Kidney lobes subdivide the kidney. Each lobe consists of a renal pyramid, the adjacent renal columns, and the overlying cortical tissue.
Each kidney contains 1 to 4 million nephrons, which are urine- forming units. Each nephron has a vascular (capillary) component and a tubular component.
The glomerulus is a tuft of capillaries, which is surrounded by a double walled, epithelial capsule called Bowman's capsule.
Together, the glomerulus and Bowman's capsule form a renal corpuscle.
The visceral layer of Bowman's capsule is the internal layer of epithelium. The visceral layer cells are modified into podocytes(foot-like cells"), which are specialized epithelial cells that surround the glomerular capillaries.
Each podocyte adheres to the outer surface of a glomerular capillary by several long primary processes that bear secondary processes called foot processes or pedicels ("little feet").
The pedicels interdigitate with similar processes from adjacent podocytes. The narrow spaces between interdigitating pedicels are filtration slits (slit pores) approximately 25 nm wide. Each slit is covered by a thin membrane, which allows the passage of some molecules and restricts the passage of others.
(3) The glomerular filtration barrier is the tissue barrier that separates the blood in the glomerular capillaries from the space in Bowman's capsule. It consists of the capillary endothelium, the basement membrane (basal lamina) of the capillary, and the filtration slits.
b. The parietal layer of Bowman's capsule forms the periphery of the renal corpuscle.
(1)At the vascular pole of the renal corpuscle, the afferent arteriole enters the glomerulus and the efferent arteriole exits the glomerulus.
(2) At the urinary pole of the renal corpuscle, the glomerular filtrate drains into the proximal convoluted tubule.
2. The proximal convoluted tubule (PCT) is approximately 15 mm in length and highly coiled. On the surface that faces the lumen of the PCT, the cuboidal epithelial cells have abundant microvilli (brush border), which increase the luminal surface area.
3. Loop of Henle. The proximal convoluted tubule leads into the descending limb of the loop of Henle, which dips down into the medulla, forms a sharp hairpin turn (the loop), and returns as the thick ascending limb of the loop of Henle.
4. The distal convoluted tubule (DCT) is also highly coiled, about 5 mm in length, and forms the last segment of the nephron.
a. Along itís path, the distal tubule establishes contact with the wall of the afferent arteriole. The portion of the tubule that contacts the arteriole contains modified cells called the macula densa. The macula densa functions as a chemoreceptor and is stimulated by a decrease in sodium ions.
b. The wall of the afferent arteriole next to the macula densa contains modified smooth muscle cells called juxtaglomerular cells. The juxtaglomerular cells are stimulated by a decrease in blood pressure to produce renin.
c. Together, the macula densa, the juxtaglomerular cells, and the mesangial cells between them form the juxtaglomerular apparatus (GA), which is important in blood pressure regulation.
Collecting tubules and ducts. As each collecting tubule descends in the cortex, it drains an increasing number of distal convoluted tubules. The collecting tubules join and form larger, straight collecting ducts.
The collecting ducts form larger tubes, which drain the urine into minor calyces, which empty into the kidney pelvis via the major calyces.
From the kidney pelvis the urine is funneled into the ureter, which leads to the urinary bladder.
F. Blood supply
3. An efferent arteriole leaves each glomerulus and gives rise to another capillary network, the peritubular capillaries, which surrounds the proximal and distal tubules to nourish them and carry away reabsorbed materials.
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The loops of the vasa recta form hairpin turns that run alongside the loops of Henle. They permit an interchange of materials between the loops of Henle and the capillaries and play a role in the concentration of urine.
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G. THE URETERS, URINARY BLADDER, AND URETHRA
2.The urinary bladder is a hollow, muscular organ that functions as a storage container for urine.
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ii. Structure. The urinary bladder is supported in the pelvic cavity by folds of peritoneum and condensations of fascia.
a. The wall of the bladder consists of four layers.
3. The urethra conveys urine from the urinary bladder to the exterior of the body.
1. In males, the urethra carries both seminal fluid and urine but not at the same time. The male urethra is 20 cm long and passes through the prostate gland and the penis.
2. In females, the urethra is short (3.75 cm). It opens to the outside by the external urethral orifice located in the vestibule between the clitoris and the vaginal opening.
Define glomerulus.
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Author: Anil Rao
Email: raoak@mscd.edu
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