FUNCTIONAL ANATOMY AND PHYSIOLOGY

The kidneys play a central role in the excretion of many metabolic breakdown products, including ammonia, urea, and creatinine from protein, and uric acid from nucleic acids, drugs, and toxins. They also regulate fluid and electrolyte balance. This is achieved by making large volumes of an ultrafiltrate of plasma (120 mL/min,170 L/day) at the glomerulus, and selectively reabsorb ing components of this ultrafiltrate at points along the nephron. The rates of filtration and reabsorption are controlled by many hormonal and hemodynamic signals. The kidneys also regulated base homeostasis, calcium and phosphate homeostasis, vitamin D metabolism, and the production of red blood cells. They are important in regulating blood pressure. Renin is secreted from the juxtaglomerular apparatus in response to reduced afferent arteriolar pressure, stimulation of sympathetic nerves and changes in the sodium content of the fluid in the distal convoluted tubules the macula densa, and is the first step in the generation of angiotensin II and aldosterone release, which in turn regulate systemic vasoconstriction and extracellular volume.

 

Each kidney is approximately 11-14 cm in length in healthy adults; they are located retroperitoneally on either side of the aorta and inferior vena cava between the 12th thoracic and 3rd lumbar vertebra. The right kidney is usually a few centimeters lower because the liver lies above it. Both kidneys rise and descend several centimeters with respiration. and The kidneys have a rich blood supply and receive approximately20-25% of cardiac output through the renal arteries, which arise from the abdominal aorta. The renal arteries undergo various subdivisions within the kidney, eventually forming interlobular arteries that run through the renal cortex. These eventually give rise to afferent glomerular arterioles that supply individual nephrons, which are the functional units of the kidneys. The efferent arteriole, leading from the glomerulus, supplies the distal nephron and medulla in ‘portal circulation.

The Nephron

Healthy kidneys contain approximately1 a million individual nephrons. Each nephron consists of a glomerulus, which is responsible for ultrafiltration of blood, a proximal renal tubule, a loop of Henle, a distal renal tubule, and a collecting duct, which together are responsible for selective reabsorption of water and electrolytes that have been filtered at the glomerulus. Under normal circumstances, more than 99% of the 170 liters of glomerular filtrate that is produced each day is reabsorbed in the tubules. The remainder passes through the collecting ducts of multiple nephrons and drains into the renal pelvis and ureters

The glomerulus

The glomerulus comprises a tightly packed loop of capillaries supplied by an afferent arteriole and drained by an efferent arteriole. It is surrounded by a cup-shaped extension of the proximal tubule termed Bowman’s capsule, which is comprised of epithelial cells. Blood enters the glomerular basement membrane (GBM), which is formed by the fusion of the basement membranes of tubular epithelial and vascular endothelial cells. The glomerular capillary endothelial cells contain pores (fenestrae), through which circulating molecules can pass to reach the underlyingGBM. Glomerularepithelialcells (podocytes) have multiple long foot processes that interdigitate with those of the adjacent epithelial cells. As well as maintaining a selective barrier to filtration, podocytes are involving regulating the turnover of the GBM.