the renal system consists of the kidneys the connecting arteries and veins and the urinary tract the kidneys are paired bean-shaped organs lying outside the peritoneal cavity in the back of the upper abdomen one on each side of the vertebral column the kidneys function as the body's main excretory organs eliminating the body's metabolic waste products by filtering the blood substances that are unneeded or are present in excess are filtered out of the blood and form into urine moving via the ureter into the bladder before being expelled through the urethra the kidneys also selectively reabsorb those substances that are needed to maintain the normal composition of the blood by adjusting blood composition the kidneys are able to maintain blood volume and pressure ensure the proper balance of sodium chloride potassium calcium hydrogen phosphate and pH and eliminate products of metabolism such as urea uric acid and creatinine the medial border of the kidney is indented by a deep fissure called the Hylas where the blood vessels nerves and ureters connect to the kidney the kidney is composed of up to 18 lobes each lobe is composed of nephrons nephrons are the functional units of the kidney and each kidney contains more than 1 million nephrons each nephron consists of a glomerulus and a system of tubules the glomerulus is a unique high-pressure mass of capillaries that filters the blood the glomerulus is encased in a thin double walled capsule called Bowman's capsule the space inside the capsule and surrounding the glomerulus is called Bowman's space plasma like fluid is filtered from the capillary blood into Bowman's space through the glomerular filtration membrane the glomerular filtration membrane consists of three layers of capillary wall the endothelium the basement membrane and the epithelium this membrane allows some particles from the blood to pass through but not all the fluid that is filtered from the capillary blood into the Bowman's space is called filtrate and forms the primary urine the filtrate then diffuses across Bowman's space and into the tubules system of the nephron in the tubules some substances are added to the filtrate as part of the urine formation and some substances are reabsorbed out of the filtrate and back into the blood the nephron to Buhl is divided into four segments the filtrate passes through each of these segments before reaching the ureter a highly coiled segment called the proximal convoluted tubules which drains Bowman's capsule and we're almost complete absorption of nutritionally important substances takes place a thin looped structure called the loop of Henle which reabsorbs water and ions from the urine and plays a role in controlling the concentration of urine a distal coiled portion called the distal convoluted tubules which regulates potassium sodium and pH and where further dilution of the urine takes place and the collecting tubules which joins with several tubules to collect the filtrate and where final sodium regulation takes place each kidney is supplied with blood by a single renal artery that arises on its respective side of the aorta before dividing into five segmental arteries that enter the Hylas within the kidney each segmental artery branches into several lobular arteries the lobular arteries further subdivide to form inter lobular arteries which branch off into afferent arterioles blood flows into the glomeruli through the afferent arterioles blood flows out of the glomerulus through the efferent arteriole the afferent and efferent arterioles regulate glomerular capillary pressure by selectively dilating or constricting the kidneys venous blood now filtered flows from the glomerulus via the efferent arterioles into the peritubular capillary network a low-pressure reabsorb t'v system surrounding all portions of the tubules this arrangement permits rapid movement of solutes and water between the fluid in the tubular lumen and the blood in the capillaries the peritubular capillaries rejoined to form the venous channels by which blood leaves the kidneys and empties into the inferior vena cava urine formation involves the filtration of the blood by the glomerulus to form an ultra filtrate of urine the tubular reabsorption of electrolytes and nutrients needed to maintain the constancy of the internal environment and the secretion of waste materials filtration occurs as blood flows into the glomerulus from its afferent arteriole and plasma moves through the glomerular capillaries into Bowman's space from Bowman's space the glomerular filtrate moves into the tubular segments of the nephron here through tubular reabsorption electrolytes and nutrients move from the filtrate back into the bloodstream here also through tubular secretion substances move from the peritubular capillaries into the urine filtrate the filtrate concentrates in the collecting tubules then finds its way to the renal pelvis where it is directed to the ureter the bladder and the urethra for elimination the kidneys perform an excretory function by filtering the blood and then selectively reabsorbing those materials that are needed to maintain a stable internal environment the nephron is the functional unit of the kidney and is composed of a glomerulus which filters the blood and a tubular component where necessary substances are reabsorbed into the bloodstream and unneeded materials are secreted into the tubular filtrate for elimination in urine