What is a Kidney?

What is a Kidney?

The kidneys are two small organs located towards the back of the abdomen, close to the base of the rib cage. There is one at each side, the left kidney being slightly higher than the right. Their function is to remove waste products from the blood and so regulate salts, minerals and fluid levels in the body.

The kidneys are oval structures, red/brown in color with a tube called the ureter that runs down from each kidney to the base of the bladder. The renal aorta, branching from the aorta from the heart, brings oxygenated blood to them and the renal vein takes deoxygenated blood away. There are two main parts to the structure of the inside of the kidney, the inner known as the medulla and the outer known as the cortex.  The renal artery carrying oxygenated blood to the kidney, branches through the medulla and then divides in to many arterioles and capillaries inside the cortex. Here, the tiny capillaries branch further and form coils, with each coil surrounded by structures called the Bowman’s capsule. The Bowman’s capsule is a cup like structure that leads to small tubules that pass back through the cortex of the kidney into the medulla, and finally join the ureter that exits the kidney and leads down to the bladder.

The method by which the kidney filters waste products, is determined by the blood pressure in the coiled capillaries being high after going through so many twists and turns. Additionally, the renal aorta from which they branch being close to the heart, heightens the pressure. This pressure causes fluid to filter out through the capillary walls and enter through the porous walls of the Bowman’s capsule where it collects. The filtered fluid contains water, glucose, amino acids, salts and nitrogenous waste. The larger particles in the blood such as larger proteins and the red blood cells themselves are too big to pass through this filtering process.

As the filtered fluid passes down the tubules from the Bowman’s capsule and back through the medulla of the kidney, all amino acids, glucose, some of the salts and much of the water are absorbed back into a network of capillaries surrounding the tubule. This re-absorption stops the loss of useful substances that have been filtered from being wasted. The remaining fluid, now called urine, contains only the waste products such as salts, excess water and other waste substances. The urine passes down towards the ureter where more water is re-absorbed. If the blood is too dilute then less is absorbed but if the blood is concentrated then more is absorbed. Contractions in the ureter carry the urine down from the kidney to the bladder.  When the kidneys become dysfunctional, dialysis becomes necessary. In dialysis, the blood passes through a semi-permeable membrane in the dialysis machine that allows waste products to be filtered from the blood. Because waste products build up constantly, it is usually necessary to have dialysis three times a week, or nightly depending on the type of dialysis used.