66 Chapter Summary

From Blood to Urine:

Now that you’ve learned the main structures of the renal system, let’s walk through the journey of blood as it enters the kidney, gets filtered, and eventually leaves the body as urine.

Blood first enters the kidney through the renal artery, which branches into smaller arteries until it reaches the afferent arterioles. These arterioles deliver blood into the glomerulus, a capillary network housed within the Bowman’s capsule in the renal cortex. Here, filtration begins as water, ions, and small molecules are pushed out of the blood and into the nephron, forming a fluid known as filtrate.

From the glomerulus, the filtrate flows into the proximal convoluted tubule, still located in the cortex, where most reabsorption of water, glucose, and electrolytes occurs. It then enters the loop of Henle, which dips down into the medulla. The descending limb allows water to leave, while the ascending limb reabsorbs salts – creating a concentration gradient that helps the kidney concentrate urine.

Next, the filtrate moves into the distal convoluted tubule back in the cortex for further selective reabsorption and secretion. It then drains into the collecting duct, which travels down through the medulla. As the filtrate descends, more water can be reabsorbed depending on the body’s needs.

By the time it reaches the end of the collecting duct, the filtrate has become urine. From there, it passes into the renal pelvis, then down the ureter, and eventually to the bladder for excretion.

Despite their humble shape and simple appearance the kidneys are hyper-organized structures which transform blood to filtrate to urine, balancing several important functions for us to carry out our day-to-day.

Tracing the path from artery to nephron to urine highlights the kidneys’ essential role in blood filtration, fluid and electrolyte regulation, and waste elimination – allowing the body to maintain homeostasis despite constantly changing variables.

Figure 123 Summary Diagram of the renal system from the broad anatomy to a coronal cross section of the kidney to a microscopic representation of the nephron