Urine Formation – MCAT Biology | MedSchoolCoach

Urine Formation

MCAT Biology - Chapter 7 - Section 7.2 - Organ Systems - Excretory System
Play Video

Sample MCAT Question - Urine Formation

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.

Get 1-on-1 MCAT Tutoring From a Specialist

With MCAT tutoring from MedSchoolCoach, we are committed to help you prepare, excel, and optimize your ideal score on the MCAT exam.


For each student we work with, we learn about their learning style, content knowledge, and goals. We match them with the most suitable tutor and conduct online sessions that make them feel as if they are in the classroom. Each session is recorded, plus with access to whiteboard notes. We focus on high-yield topics if you’re pressed for time. If you have more time or high-score goals, we meticulously cover the entire MCAT syllabus.

Urine Formation Intro

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.

Renal Blood Circulation

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.

Urine = Filtration - Reabsorption + Secretion

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.

Urine formation steps for the MCAT, including filtration from the glomerulus, reabsorption into the blood, and secretion into the renal tubule
Figure 1. Process of Urine Formation

Explore More MCAT Masterclass Chapters

Take a closer look at our entire MCAT Masterclass or explore our Biochemistry lessons below.

Interview - Job interview

One-on-One Tutoring

Are you ready to take your MCAT performance to a whole new level? Work with our 99th-percentile MCAT tutors to boost your score by 12 points or more!

See if MCAT Tutoring can help me

Talk to our enrollment team about MCAT Tutoring

Medical College Admission Test - MCAT Physics

MCAT Go Audio Course

Engaging audio learning to take your MCAT learning on the go, any time, any where. You'll be on the way to a higher MCAT score no matter where you are. Listen to over 200+ lessons.

Stock photography - Image

MCAT Practice Exams

Practice makes perfect! Our mock exams coupled with thorough explanations and in-depth analytics help students understand exactly where they stand.

MCAT Prep App Mobile

MCAT Prep App

Access hundreds of MCAT videos to help you study and raise your exam score. Augment your learning with expert-created flashcards and a question banks.