The Excretory System & Kidneys – MCAT Biology | MedSchoolCoach - Page 2

The Excretory System & Kidneys

MCAT Biology - Chapter 7 - Section 7.1 - Organ Systems - Excretory System
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Sample MCAT Question - The Excretory System & Kidneys

Which of the following is a major function of the kidneys?

a) Pumps blood through the entire body

b) Removes old red blood cells from circulation

c) Maintains blood volume and composition

d) Breaks down insulin and other hormones

C is correct. Maintains blood volume & composition. The kidney can act in various capacities performing several different vital biological functions. One of the functions of the kidney is to maintain the volume and composition of blood. For example, if the blood volume in the body is too low, as in dehydration, the kidneys will decrease urine output to help regulate and maintain a healthy blood volume. Likewise, if the blood volume is too high, the kidneys will increase the volume of urine. Answer choices A and B are incorrect and describe the heart. Answer choice D is incorrect because while kidney cells may break down some hormones, this is not a major function.

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Introduction to the Excretory System & Kidneys

In this MCAT post, we’ll be introducing you to the excretory system, one of the major organ systems of the human body. We’ll discuss both the anatomy of the excretory system as well as its function in the body. Then we’ll zero in on the kidneys specifically, as these are the most important organs in the excretory system. We’ll discuss the unique anatomy of the kidneys, as well as the various roles they play in maintaining the health of the organism.

Anatomy and Function of the Excretory System

The excretory system is a biological system that removes waste from the body and helps to maintain internal homeostasis. It is a complex system that consists of many structures, as shown in Figure 1. The kidneys are an essential component of this system that help to filter blood and remove waste from the body in the form of urine. The urine formed by the kidneys is carried to the bladder by long tube-like structures called ureters. The bladder serves as a storage unit for urine. When the bladder is full, the urethra carries the urine from the bladder to the outside of the body. This process is known as micturition. Also, it is important to note that there are two urinary sphincters: the internal urinary sphincter and the external urinary sphincter. The internal urinary sphincter is made of smooth muscle and is under involuntary control. The external urinary sphincter, however, is made of skeletal muscle and is under voluntary control.

Anatomy of the excretory system for the MCAT, showing how the kidney connects to the ureter, bladder, and urethra
Figure 1. Anatomy of the Excretory System

Anatomy of the Kidneys

The kidneys are the central organs of the excretory system. In order to better understand the excretory system, it is therefore important to understand the anatomy of the kidneys and how they connect to the blood supply.


The kidneys receive blood from the renal artery and blood that passes through the kidney is then returned to the body through the renal vein (Figure 2). Moreover, the outer layer of the kidneys is called the renal cortex and the inner part of the kidney is called the renal medulla. The renal cortex is the part of the kidney where filtration of substances such as water, glucose, and amino acids occurs. Also, the hormone erythropoietin, which is involved in red blood cell production is produced in the renal cortex. The renal medulla contains several structures involved in the regulation of salt and water in the body.

Kidney anatomy for the MCAT, showing how the kidney connects to the blood supply and the renal cortex and medulla
Figure 2. Anatomy of the Kidney

Functions of the Kidney

In addition to kidney anatomy, the MCAT exam will also test your knowledge of kidney function.


The first function, as discussed above, is an excretory function. The kidney eliminates waste from the blood by producing urine. In particular, the kidney removes soluble nitrogenous wastes, salts, as well as excess water from the blood. One consequence of renal disease is the inability to eliminate waste products from the body, which can cause many problems. For example, high potassium can lead to abnormal heart rhythms, and high levels of urea can lead to altered mental status.


A second important function of the kidney is a regulatory function. As the kidneys are producing urine by eliminating wastes and salts, they are also helping to maintain the volume and composition of blood. For example, if the body’s blood volume is low, then the kidneys will decrease urine output to help increase the blood volume until it is back to normal. Likewise, if someone becomes over-hydrated, by drinking too much water, then the blood volume will increase. In this way, the kidneys will increase urine output to decrease blood volume.


The third important function of the kidneys is an endocrine function. As mentioned above, the kidney secretes the hormone erythropoietin that acts on the blood marrow to stimulate the production of red blood cells. It also produces the enzyme renin, which is involved in the renin-angiotensin-aldosterone system that helps to regulate the body’s blood pressure. Furthermore, the kidneys also help to convert vitamin D produced from the skin into its active form, called calcitriol. Calcitriol helps to increase blood calcium by stimulating calcium reabsorption in the intestines. Calcium is important for bone mineralization. For this reason, an important consequence of long-term, or chronic, kidney disease is the inability to mineralize bone properly, leading to excess fractures or bone pain.

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