During the process of urine formation, where does the filtration of blood take place?
a) Afferent arterioles
b) Peritubular capillaries
c) Glomerular capillaries
d) Vasa recta
C is correct. Glomerular capillaries. There are many steps to the formation of urine that take place at various locations in the urinary tract system. One crucial step is the filtration of water and solutes from the blood in the circulatory system into the kidney tubules. Approximately 1% of this filtrated material becomes urine. This process takes place at the glomerular capillaries of the kidney glomerulus. From these capillaries, filtered material is passed into the kidney tubules.
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Urine is one of the body’s waste products. The formation of urine is a complex process that takes place in the kidneys. By filtering unwanted substances from the blood, the kidneys create urine that is excreted from the body.
Urine is composed of water and solutes filtered from the bloodstream. This means that in order to understand how urine is formed, we first need to understand how the blood circulates through the kidneys.
The main artery that brings blood to the kidneys is called the renal artery. There are two renal arteries (one for each kidney), and they usually arise from the interior aspect of the abdominal aorta. As the renal artery passes through the kidney, it branches many times to form afferent arterioles. Each afferent arteriole will deliver blood to a glomerulus. Within each glomerulus, the afferent arterioles will form glomerular capillaries. The glomerular capillaries are the site of nutrient and waste exchange from the systemic circulation into the renal tubules.
The fluid that passes through the glomerular capillaries into the renal tubules is forced out by the high hydrostatic pressure within the glomerular capillaries. This process is called filtration and is the starting point of urine formation. The blood that does not pass through the glomerular capillaries continues along the efferent arteriole and into the peritubular capillaries. Recall the meaning of the terms afferent and efferent. Afferent means toward and efferent means away. In this way, the afferent arteriole brings blood to the glomerulus, and the efferent arteriole takes blood away from the glomerulus.
The peritubular capillaries are the capillaries that are interwoven with the renal tubule, and one portion of these capillaries, called the vasa recta, is nearby the loop of Henle. The loop of Henle is an essential renal structure involved in concentrating the urine. At this point, the peritubular capillaries will combine to form renal venules, which will then combine to form the renal vein. The renal vein carries blood away from the kidney and into the inferior vena cava.
Now that we understand how the blood flows through the kidneys, we can better understand how the kidney filters unwanted elements from the blood to form urine.
The first step of urine formation is filtration. This is the process by which water and solutes are filtered from the glomerular capillaries into a part of the renal tubule called Bowman’s capsule. This process involves the movement of water and solutes, not all of which will end up being excreted in the urine. Some solutes like glucose and amino acids are essential for bodily function, and so these, as well as some water, usually move from the kidney tubules back into the blood circulation via the vasa recta portion of the peritubular capillaries. This process is called reabsorption and is the second step of urine formation.
Lastly, in the third step of urine formation, known as secretion, certain substances will pass from the peritubular capillaries back into a portion of the renal tubule. This process is basically the reverse of reabsorption: instead of substances being reabsorbed back into the blood, substances are being removed from the blood. Substances removed during secretion include urea, creatinine, and certain electrolytes.
Figure 1 shows a diagram of the process of urine formation, as well as where each step occurs in the blood circulation. As you can see, the urine exiting via the collecting duct is the product of the three steps of urine formation. It is the result of what was filtered from the blood during filtration, minus what was reabsorbed by the blood during reabsorption, plus what was secreted by the blood back into the renal tubule. Thus, what is excreted as urine can be thought of as filtration minus reabsorption plus secretion.
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