The kidneys are important organs for maintaining blood water potential and excreting waste products from the blood. In this article, we will look at the structure of the kidneys and how substances are filtered out and reabsorbed into the blood using ultrafiltration and selective reabsorption.
Kidney structure
The kidneys have an outer layer called the cortex, and an inner layer called the medulla. They have a good blood supply – oxygenated blood is delivered via the renal artery, and deoxygenated blood is removed via the renal vein. About a million tiny structures called nephrons span across the cortex and medulla. Nephrons are tiny long tubules surrounded by bundles of capillaries. Different areas of the nephron have different names:
- Glomerulus – the bundle of capillaries inside the Bowman’s capsule.
- Bowman’s capsule – a hollow cup-like structure containing the glomerulus, forming the start of the tubule. Located in the cortex.
- Proximal convoluted tubule (PCT) – a folded part of the tubule between the Bowman’s capsule and loop of Henle. Located in the cortex.
- Loop of Henle – part of the tubule which dips down into the medulla. More about this in the next article.
- Distal convoluted tubule (DCT) – another folded part of the tubule after the loop of Henle. Located in the cortex.
- Collecting duct – the final part of the tubule which eventually joins to the ureter (the tube which carries urine to the bladder).
Ultrafiltration
Ultrafiltration takes place at the glomerulus and Bowman’s capsule. The afferent arteriole entering the glomerulus has a wider lumen than the efferent arteriole leaving it. This means that the blood is under high hydrostatic pressure in the glomerulus as it tries to force its way through. This high hydrostatic pressure forces water and small molecules into the Bowman’s capsule. There are three layers which the molecules must pass through:
- The capillary endothelium.
- A basement membrane.
- The Bowman’s capsule epithelium.
There are gaps between the cells on each side of the basement membrane, but the basement membrane acts as a filter to only allow small molecules such as water, urea, glucose and amino acids to pass into the tubule. Ions can also pass through. Large molecules such as large proteins and blood cells are not able to pass through so remain in the blood. Any substances that do pass through form the glomerular filtrate which flows to the PCT.
Selective reabsorption
When the glomerular filtrate enters the proximal convoluted tubule (PCT), it contains water and the small molecules and ions that were able to leave the blood during ultrafiltration e.g. glucose, amino acids and urea. The body wants to keep some of those substances but get rid of others, so selective reabsorption allows only the useful substances to be transported back into the blood.
The epithelium of the PCT is adapted to enable this absorption to occur – it has microvilli to increase surface area for absorption, and contains the correct carrier and channel proteins, for example the carrier proteins that reabsorb glucose. Glucose is reabsorbed by co-transport with sodium ions, very similar to glucose absorption in the ileum. The vast majority of the urea will remain in the filtrate to be excreted in urine. Some water is reabsorbed the blood by osmosis down a water potential gradient. Water absorption continues further down the nephron tubule. In the next article we will look at how the kidneys are involved in the control of blood water potential, including a closer look at how the loop of Henle is important for water reabsorption.
Summary
- Nephrons span across the cortex and medulla of the kidneys.
- Ultrafiltration occurs at the glomerulus and Bowman’s capsule. High hydrostatic pressure forces water and small molecules from the blood into the glomerular filtrate.
- Selective reabsorption occurs in the proximal convoluted tubule. Useful substances such as glucose are reabsorbed back into the blood.
- Water is reabsorbed by osmosis in the proximal convoluted tubule.
