18. Excretory System

The Excretory System is one of the most important topics in Biology for the ENEM test. It is responsible for removing metabolic waste and toxic substances from the body, maintaining internal balance, also known as homeostasis. This chapter of our e-book will cover this system in detail, helping you to fully understand its workings and importance.

The human excretory system is composed mainly of the kidneys, ureters, bladder and urethra. The kidneys are the main excretory organs and perform a number of vital functions, including filtering blood, reabsorbing useful substances, secreting unwanted substances, and regulating blood pressure, blood volume, and electrolyte balance.

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The functional unit of the kidney is the nephron, which is composed of a Bowman's capsule, a proximal tubule, a loop of Henle, and a distal tubule. Blood enters the kidney through the afferent arteriole and is filtered in Bowman's capsule. This filtration is a passive process that removes water, ions, and small molecules, but leaves behind blood cells and large proteins. The filtrate then passes through the proximal tubule, where most of the useful substances are reabsorbed back into the bloodstream.

In the loop of Henle, water and salts are reabsorbed to help maintain fluid and electrolyte balance. The remaining filtrate, now concentrated, passes into the distal tubule, where further reabsorption and secretion take place. Finally, the filtrate, now called urine, passes into the ureter, which carries it to the bladder for storage before excretion through the urethra.

It is important to note that the kidneys not only remove waste and toxic substances, but also play a crucial role in maintaining the body's internal balance. They regulate blood volume and pressure by adjusting the amount of water and salts reabsorbed or excreted. They also help regulate blood pH by excreting hydrogen ions and reabsorbing bicarbonate.

In addition, the kidneys produce hormones such as erythropoietin, which stimulates the production of red blood cells, and renin, which helps control blood pressure. They also convert vitamin D into its active form, which is essential for the absorption of calcium and phosphorus in the gut.

In summary, the excretory system plays a vital role in many aspects of human physiology. It not only removes waste and toxic substances from the body, but also helps maintain the body's internal balance and produces important hormones. This chapter of our e-book will help you to fully understand the functioning and importance of this system, preparing you for any questions about the excretory system that may appear on the ENEM test.

Understanding the excretory system is essential to understanding how the human body maintains homeostasis and protects itself against potentially harmful substances. We hope that this chapter of our Biology for ENEM e-book will provide you with the knowledge you need to answer questions on this topic with confidence.

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