8.2 Review of Anatomy & Physiology of the Urinary System (77/84) -- Health Alterations

8.2 Review of Anatomy & Physiology of the Urinary System Anatomy The urinary system, also referred to as the renal system or urinary tract, consists of the kidneys, ureters, bladder, and urethra. The kidneys form urine, and the ureters carry the urine away from kidneys to the urinary bladder, where it is stored until it is expelled during urination. The urethra carries the urine from the urinary bladder to outside of the body during urination.[1] See Figure 8.1[2] for an illustration of the male urinary system. Kidneys The kidney is an organ that filters waste from the blood and produces urine. Kidneys lie on either side of the spine behind the abdominal cavity, well-protected by muscle, fat, and ribs. Because they lie posterior to the peritoneum membrane, their location is described as retroperitoneal. They are roughly the size of your fist. The male kidney is typically a bit larger than the female kidney. The kidneys are well-vascularized, receiving about 25 percent of the blood pumped out of the heart with each heartbeat. Figure 8.2[3] displays the location of the kidneys. The outer region of the kidney is called the renal cortex, and the inner region is called the medulla. See Figure 8.3[4] for an illustration of the internal structures of the kidney. The renal hilum is the entry and exit site for vessels, nerves, and ureters. The renal arteries (colored red) come directly from the descending aorta, and the renal veins (colored blue) return filtered blood to the inferior vena cava. Arterioles and Nephrons The renal artery branches down into small arteries called afferent arterioles that bring blood to the nephron. Afferent arterioles service about 1.3 million nephrons in each kidney. See Figure 8.4[5] for an illustration of blood flow from the renal artery to the afferent arterioles to a nephron. The nephron is the functional unit of the kidney and filters the blood, removes wastes, and balances fluid and electrolyte levels. The renal corpuscle consists of a cluster of high-pressure capillaries called the glomerulus, surrounded by a glomerular capsule. An afferent arteriole leads into the renal corpuscle, and an efferent arteriole leaves the renal corpuscle. See Figure 8.5[6] for an illustration of blood flow through the nephron. The capillaries in the glomerulus initially filter the blood based on particle size. After blood is filtered by the glomerulus, it continues to pass through the renal tubule. Very thin and a little over an inch long, the renal tubule has three segments that the filtrate flows through called the proximal convoluted tubule, the loop of the nephron (referred to as the “loop of Henle”), and the distal convoluted tubule. Different portions of the renal tubule have different permeabilities for solutes and water, and efferent arterioles recover most of the water and electrolytes and return them back into the circulation. The remaining wastes pass as urine from the nephrons into collecting ducts and eventually into the uret