How Your Kidneys Work

The kidneys are located on either side of the spinal column in the posterior abdominal wall and they are very important for our health. Adrenal glands sit on top of the kidney. The medullary pyramid and outer cortex compose the renal lobe. Branches of the renal artery supply each renal lobe. The smallest branches of the renal artery called afferent arterioles, supply the excretory components of the kidney called nephrons. Each nephron consists of a renal corpuscle and a renal tubule. The first step of urine production, called glomerular filtration, occurs in the renal corpuscle. It consists of a network of capillaries called the glomerulus and Bowman's capsule. The diameter of the afferent arteriole is larger than the efferent arteriole. This size difference produces high pressure within the glomerulus. The pressure forces the smallest substances such as water, nitrogenous waste, amino acids, glucose, sodium ions, and calcium ions out of the blood into Bowman's capsule through tiny spaces in the membrane separating them. Other substances such as plasma proteins and red blood cells, are too large to pass through the membrane, so they remain in the blood. During the second step of urine production called tubular reabsorption, the fluid passes through the renal tubule which consists of the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct. As the fluid passes through the renal tubule, it reabsorbs useful substances into the peritubular capillaries, specifically glucose, amino acids, and some sodium and calcium ions, are reabsorbed by active transport. The peritubular capillary reabsorbs water through osmosis. The final step of urine production is tubular secretion, which occurs primarily in the distal convoluted tubules. These tubules extract larger nitrogenous wastes, excess hydrogen ions, and excess potassium ions from the peritubular capillaries and secrete them into the tubular fluid. From here, these wastes will continue through the urinary system until they pass out of the body in urine.