Excretion of waste products : blood enters the kidney through the renal artery and then passes through capillaries in the cortex (outer layer) of the kidneys. As the blood passes through capillaries in the cortex, substances are filtered out of the blood and into long tubules that around the capillaries. This process is called ultrafiltration. Useful substances, such as glucose and the right amount of water, are then reabsorbed back into the blood. This process is called selective reabsorption. The remaining unwanted substances pass along to the bladder and are excreted as urine.
What is ultrafiltration?
A filtration process that separates particles based on size using a semi-permeable membrane.
What are the nephrons?
The long tubules with the bundles of capillaries where the blood is filtered. There are around one million nephrons in each kidney.
The process of ultrafiltration:
blood from the renal artery enters smaller arterioles in the cortex of the kidney
each arteriole splits in a structure called a glomerulus (glomeruli = plural) - a bundle of capillaries looped inside a hollow ball called a Bowman's capsule - this is where ultrafiltration takes place.
The arteriole that takes blood into each glomerulus is called the afferent arteriole and the arteriole that takes the filtered blood away from the glomerulus is called the efferent arteriole.
The efferent arteriole is smaller in diameter than the afferent arteriole, so the blood in the glomerulus is under high pressure. The high pressure forces liquid and small molecules in the blood out of the capillary and into to Bowman's capsule.
The liquid and small molecules pass through three layers to get into the Bowman's capsule and enter the nephron tubules:
The capillary endothelium
a membrane called the basement membrane
the epithelium of the Bowman's capsule
Larger molecules like proteins and blood cells can't pass through so stay in the blood
The substances that enter the Bowman's capsule are known as the glomerular filtrate - it passes along the rest of the nephron and useful substances are reabsorbed along the way. Finally, the filtrate flows through the collecting duct and passes out of the kidney along the ureter
Selective reabsorption of useful substances takes place as the glomerular filtrate flows along the proximal convoluted tubule (PCT), through the loop of Henle, and along the distal convoluted tubule (DCT). Useful substances leave the tubules of the nephrons and enter the capillary network that's wrapped around them.
The epithelium of the wall of the PCT has microvilli to provide a large surface area for reabsorption of useful materials from the glomerular filtrate (in the tubules) into the blood (in the capillaries)
Useful solutes, like glucose, are reabsorbed along the PCT by active transport and facilitated diffusion.
Water enters the blood by osmosis because the water potential of the blood is lower than the water potential of the filtrate. Water is reabsorbed from the PCT, loop of Henle , DCT and the collecting duct. The filtrate that remains is urine, which passes along the ureter to the bladder
Urine is usually made up of water and dissolved salts, urea and other substances such as hormones and excess vitamins. Urine doesn't usually contain proteins or blood cells as they are too big to e filtered out of the blood. Glucose is actively reabsorbed back into the blood, so is not usually found in urine either.