Alila Academy

The kidneys filter blood plasma, removing metabolic wastes, toxins, and excrete them in urine. During this process, they also maintain constant volume and composition of the blood, or homeostasis. 

Blood enters the kidney via the renal artery, which divides into smaller arteries, then arterioles. The arterioles get into contact with functional units of the kidney called the nephrons. This is where blood filtration and urine formation take place. The filtered blood then passes through a series of veins and exits the kidney via the renal vein. The urine is collected in collecting ducts and leaves the kidney via the ureters. 

Each kidney contains over a million nephrons. A nephron consists of 2 major parts: a glomerular capsule, or Bowman’s capsule; and a long renal tubule. Renal tubules of several nephrons connect to a common collecting duct. 

There are 3 steps in the formation of urine: glomerular filtration; tubular reabsorption and secretion; and water conservation. 

1. Glomerular filtration:

Blood enters the Bowman’s capsule via the afferent arteriole, passes through a ball of capillaries called the glomerulus, then leaves via the efferent arteriole. 

The afferent arteriole is significantly larger than the efferent arteriole, creating a blood flow with a large inlet and small outlet. As a result, the blood hydrostatic pressure in these capillaries is much higher than usual. 

Hydrostatic and osmotic pressures drive water and solutes from blood plasma, through a filtration membrane, into the capsular space of the nephron. 

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