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BIOC33H3 (127)
Lecture

Urinary System

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Department
Biological Sciences
Course
BIOC33H3
Professor
Stephen Reid
Semester
Fall

Description
Chapter 45:Urinary System STRUCTURES AND FUNCTIONS  The urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra.  The bladder provides storage, and the ureters and urethra are the drainage channels for the urine after it is formed by the kidneys. Kidneys  The primary functions of the kidneys are (1) to regulate the volume and composition of extracellular fluid (ECF), and (2) to excrete waste products from the body.  The kidneys function to control blood pressure, produce erythropoietin, activate vitamin D, and regulate acid-base balance.  The outer layer of the kidney is termed the cortex, and the inner layer is called the medulla.  The nephron is the functional unit of the kidney. Each kidney contains 800,000 to 1.2 million nephrons.  A nephron is composed of a glomerulus, Bowman’s capsule, and a tubular system. The tubular system consists of the proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and a collecting tubule.  The kidneys receive 20% to 25% of cardiac output.  The primary function of the kidneys is to filter the blood and maintain the body’s internal homeostasis.  Urine formation is the result of a multistep process of filtration, reabsorption, secretion, and excretion of water, electrolytes, and metabolic waste products. Glomerular Function  Blood is filtered in the glomerulus.  The hydrostatic pressure of the blood within the glomerular capillaries causes a portion of blood to be filtered across the semipermeable membrane into Bowman’s capsule.  The ultrafiltrate is similar in composition to blood except that it lacks blood cells, platelets, and large plasma proteins.  The amount of blood filtered by the glomeruli in a given time is termed the glomerular filtration rate (GFR). The normal GFR is about 125 ml/min. Tubular Function  The functions of the tubules and collecting ducts include reabsorption and secretion. Reabsorption is the passage of a substance from the lumen of the tubules through the tubule cells and into the capillaries. Tubular secretion is the passage of a substance from the capillaries through the tubular cells into the lumen of the tubule. o The loop of Henle is important in conserving water and thus concentrating the filtrate. In the loop of Henle, reabsorption continues. o Two important functions of the distal convoluted tubules are final regulation of water balance and acid-base balance.  Antidiuretic hormone (ADH) is required for water reabsorption in the kidney.  Aldosterone acts on the distal tubule to cause reabsorption of sodium ions + + + (Na ) and water. In exchange for Na , potassium ions (K ) are excreted. o Acid-base regulation involves reabsorbing and conserving most of the bicarbonate (HCO )3and secreting excess H . + o Atrial natriuretic peptide (ANP) acts on the kidneys to increase sodium excretion. o Parathyroid hormone (PTH) acts on renal tubules to increase reabsorption of calcium. Other Functions of the Kidney  The kidneys produce erythropoietin in response to hypoxia and decreased renal blood flow. Erythropoietin stimulates the production of red blood cells (RBCs) in the bone marrow.  Vitamin D is activated in kidneys. Vitamin D is important for calcium balance and bone health.  Renin, which is produced and secreted by juxtaglomerular cells, is important in the regulation of blood pressure.  Prostaglandin (PG) synthesis (primarily PGE a2d PGI ) o2curs in the kidney, primarily in the medulla. Th+se PGs have a vasodilating action, thus increasing renal blood flow and promoting Na excretion. Ureters  The ureters are tubes that carry urine from the renal pelvis to the bladder.  Circular and longitudinal smooth muscle fibers, arranged in a meshlike outer layer, contract to promote the peristaltic one-way flow of urine. Bladder  The urinary bladder is a distensible organ positioned behind the symphysis pubis and anterior to the vagina and rectum.  Its primary functions are to serve as a reservoir for urine and to help the body eliminate waste products.  Normal adult urine output is approximately 1500 ml/day, which varies with food and fluid intake.  On the average, 200 to 250 ml of urine in the bladder causes moderate distention and the urge to urinate. Urethra  The urethra is a small muscular tube that leads from the bladder neck to the external meatus.  The primary function of the urethra is to serve as a conduit for urine from the bladder neck to ou
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