Vasa recta

The vasa recta are modified peritubular capillaries. As with the peritubular capillaries, the vasa recta arise from efferent arterioles. However, these vessels are associated only with the juxtamedullary nephrons and are found only in the medullary region of the kidney. The vasa recta pass straight through to the inner region of the medulla, form a hairpin loop, and return straight toward the cortex. This structure allows these vessels to lie parallel to the Loop of Henle and collecting ducts.

The vasa recta perform several important functions, including:

• Providing nourishment to tubules of the medullary region of the kidneys

• Returning NaCl and water reabsorbed from the Loop of Henle and collecting ducts back to the general circulation

• Maintaining the vertical osmotic gradient within the interstitial fluid of the medulla

Blood entering the vasa recta has an osmolarity of about 300 mOsm/ l. As the vessels travel through the increasingly concentrated medulla, the osmolarity of the blood within them equilibrates with that of the surrounding interstitial fluid. In other words, the blood also becomes increasingly concentrated. Water leaves the vasa recta down its concentration gradient and NaCl enters the vasa recta down its concentration gradient. Therefore, at the innermost region of the medulla, the osmolarity of the blood is 1200 mOsm/l. If the process were to be interrupted at this point, all the NaCl that had initially created the vertical gradient would eventually be washed away, or removed from the medulla, by the blood flowing through it. However, like the Loop of Henle, the vasa recta form a hairpin loop and travel back toward the cortex through an increasingly dilute interstitial fluid. Once again, the osmolarity of the blood within them equilibrates with that of the surrounding interstitial fluid. In other words, the blood now becomes increasingly dilute. Water enters the vasa recta down its concentration gradient and NaCl leaves the vasa recta down its concentration gradient. Consequently, when this blood has reached the cortex, its osmolarity has returned to 300 mOsm/l. Therefore, the blood leaving the vasa recta has an osmolarity similar to that of the blood that entered it. What does change is the volume of blood that leaves the vasa recta. Once again, the excess NaCl and water reabsorbed from the tubules within the medulla have been picked up by these vessels and returned to general circulation. It is important to note that this process has been performed without disrupting the vertical medullary gradient.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment