Technische Universität München

The Entrepreneurial University

The kidney is an organ which modulates the volume and the composition of body fluid by filtration of the blood and selective reabsorption or secretion functions. The kidneys excrete water, salts and metabolites. As such they have a major role in controlling the water and electrolyte balance within the body and regulating the acid-base balance of blood. The kidney secretes hormones such as
  • renin (to activate the renin-angiotensin-aldosterone system),
  • calcitriol (a metabolite of vitamin D3) and
  • erythropoietin (to stimulate the formation of red blood cells by the bone marrow).

The waste products leave the kidneys as urine and are conveyed through the ureter to the urinary bladder in the pelvis.

The kidneys are retroperitoneal organs and they are ovoid-shaped. They are approximately 10-11cm in length, 5cm in width, 2.5cm in thickness and weigh about 150g in the adult human. The left kidney is lightly higher located in the abdomen than the right, due to the presence of the liver on the right side. The kidneys are supplied with blood directly from the aorta via the renal arteries. Blood is returned to the inferior vena cava via the renal veins. On sectioning, the kidney presents a pale outer region (the cortex) and a darker inner region (the medulla). The cortex and the medulla are made up of nephrons.

Nephron is the functional unit of the kidney and there are about 1.3 million of nephrons in each kidney. The nephron is responsible for ultrafiltration of the blood and reabsorption or excretion of products in the subsequent filtrate. Each nephron is made up of:  
  • The glomerulus
    A filtering unit of the kidney that forms 125 ml/min of fluid filtrate out of the blood.
  • The proximal convoluted tubule
    It regulates and controls the absorption of glucose, sodium, and other substances.
  • The loop of Henle
    This region is responsible for concentration and dilution of urine by utilizing a counter-current multiplying mechanism. It is water impermeable but can pump sodium out which in turn regulates the osmolarity of the surrounding tissues.
  • The distal convoluted tubule
    This region is responsible for absorbing water back into the body. Almost 99% of the water is reabsorbed leaving the concentrated urine to flow into the collecting duct and then into the renal pelvis.