The kidney is one of the most important organs in the human body for maintaining homeostasis. Although a person can live a relatively normal life with only one kidney instead of the usual two, having no kidney or having kidney failure will cause a person to die in just a few days. This article will explain the anatomy and functions of the kidneys, including a basic explanation of the process by which blood is filtered and urine is produced.
Function of the kidneys
The reason kidneys are so vital for survival is because they are responsible for removing a large portion of toxic waste products that naturally accumulate in the bloodstream as a result of protein digestion and muscle exertion. When proteins in the food we eat is metabolized by the liver, it creates the toxic byproduct ammonia. The liver is able to convert a large portion of the ammonia into the less toxic urea and uric acid, but the liver is not able to remove these byproducts from the body. Another toxic byproduct is created when muscles break creatine down into creatinine. The kidney’s job is to filter these toxic substances out of the bloodstream and putting them into urine so that they can be removed from the body through urination. Approximately 30% of your blood is filtered by the kidneys as it circulates through the body.
Anatomy of the kidneys
Humans normally have two kidneys, a left kidney and right kidney, which are located near the back of the abdominal cavity below the liver. Because the liver is larger on the right side of the body, the right kidney is slightly lower than the left kidney. Each kidney is bean-shaped (hence, kidney beans) with the indented side facing inwards.
The kidneys are primarily composed of delicate soft tissue which is protected by a rigid outer layer called the renal cortex. The indentation on the inner edge of the kidney is where blood vessels attach, the renal artery bringing blood from the heart to the kidneys and the renal vein carrying blood from the kidneys back to the heart. This is also where the ureters attach to the kidney, whose job it is to drain urine produced in the kidney into the urinary bladder for elimination.
The functional unit of the kidney is a very small, complex structure called a nephron. Each kidney contains approximately 1 million of these structures which receive small amounts of blood from the renal artery and filter out toxic solutes like creatinine and uric acid while allowing non-waste solutes, like sugars, electrolytes and some urea, to be reabsorbed into the bloodstream. The main components of the nephron are the Bowman’s capsule, glomerulus, proximal tube, loop of Henle, distal tube and the collecting duct.
How do kidneys filter blood to make urine?
Blood enters the glomerulus from blood vessels branching off from the renal artery. As it does, the plasma and the substances dissolved in it are separated from the blood cells, which remain concentrated in capillaries surrounding the proximal tube. Cells in this tube reabsorb valuable non-waste solutes, like sugars and amino acids, and deposit them back into the blood stream. Osmotic pressure causes water to move across the membrane from the filtered solution and into the concentrated blood. The solution then flows down into the loop of Henle which is surrounded by cells in the renal medulla which are very hypertonic.
Osmotic pressure again forces water out of the filtered solution and into these cells, which in turn return the water to the concentrated blood. As the solution moves up to exit the loop of Henle, ions (such as electrolytes) are reabsorbed. Finally, the filtered solution enters the distal tube, where more water is reabsorbed into the blood and excess potassium and hydrogen ions are excreted into the solution as waste. This final solution is urine, and it accumulates in the collecting ducts before draining into the renal pelvis and finally the bladder via the ureters.
