MCAT Biology : Other Excretory Physiology

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Other Excretory Physiology

The process of excretion officially begins after the collecting duct since the collecting duct is the last point of reabsorption. 

Urine passes through all of the following structures except the __________.

Possible Answers:

vasa recta

ureters 

renal pelvis 

urethra 

Correct answer:

vasa recta

Explanation:

The path of urine excretion from the body, after filtrate passes through the collecting ducts of the nephrons, begins in the renal pelvis. The renal pelvis collects the concentrated filtrate and transfers it to the bladder via the ureters. During urination, the bladder contracts, and urine exits the body through the urethra.

Vasa recta are capillaries found near nephrons within the kidneys. 

Example Question #2 : Other Excretory Physiology

The interaction between blood pressure and kidney function in humans requires coordination by the renin-angiotensin-aldosterone system (RAAS). This system involves the dynamic interplay of the kidneys, lungs, and blood vessels to carefully regulate sodium and water balance.

A normal human kidney has cells adjacent to the glomerulus called juxtaglomerular cells. These cells sense sodium content in urine of the distal convoluted tubule, releasing renin in response to a low level. Renin is an enzyme that converts angiotensinogen to angiotensin I (AI). AI is converted to angiotensin II (AII) by angiotensin converting enzyme (ACE) in the lung.

AII stimulates aldosterone secretion in the zona glomerulosa of the adrenal gland. Aldosterone then acts to upregulate the sodium-potassium pump on the basolateral side of distal tubule epithelial cells to increase sodium reabsorption from the urine, as well as increasing potassium excretion.

After exiting the nephron, urine is transported to the urinary bladder by which structure?

Possible Answers:

Ureter

Urethra

Collecting duct

Loop of Henle

Distal tubule

Correct answer:

Ureter

Explanation:

The ureters extend from the kidneys to the urinary bladder, where urine is stored until it can be excreted from the body.

The loop of Henle, distal tubule, and collecting duct are all considered parts of the nephron and are involved in filtration and concentration of urine. After passing through the collecting duct, urine enters the ureters for transport to the bladder. From the bladder, urine enters the urethra before expulsion from the body.

Example Question #3 : Other Excretory Physiology

A certain individual is born with a mutation that resulted in the lack of a bladder. How will this affect the individual?

Possible Answers:

Urination will remain voluntary, but its frequency will increase

Urination will become involuntary and its frequency will decrease

Urination will become involuntary and its frequency will increase

Urination will remain voluntary, but its frequency will decrease

Correct answer:

Urination will become involuntary and its frequency will increase

Explanation:

The urinary bladder is an important structure in humans that stores urine until urination. The walls of the bladder contain both smooth and somatic muscle. The smooth muscle serves to detect the degree of stretch of pressure in the bladder, stimulating the sensation of a full bladder and the need for urination. The somatic muscle allows for actual contraction of the bladder, making urination a voluntary process.

Without a bladder, the urine from the kidneys would continuously flow to the ureter and, subsequently, to the urethra where it would be excreted. Due to this continuous flow, urine would not be stored, and the person will have to urinate constantly. This means that without a urinary bladder, urination will become involuntary and the individual will urinate constantly (urination frequency will increase dramatically).

Example Question #4 : Other Excretory Physiology

Listed below are four structures that participate in urine storage and elimination.

1. Ureter

2. Urethra

3. Renal pelvis

4. Bladder

Which of the following is the correct order of urine flow after it exits the collecting duct?

Possible Answers:

3, 1, 4, 2

4, 2, 3, 1

2, 3, 1, 4

2, 1, 3, 4

Correct answer:

3, 1, 4, 2

Explanation:

After exiting the collecting duct, urine enters the renal pelvis. The renal pelvis is part of the ureter and is found inside the medulla of the kidney. Numerous collecting ducts will pool into the renal pelvis, allowing urine to accumulate. After it exits the kidney (via the renal pelvis), urine enters the ureter where it is pushed towards the urinary bladder. In the bladder, urine is stored until it reaches a maximum threshold pressure and volume. Once this maximum is reached, the individual feels the urge to urinate and the process of urination begins. During urination, the urine is propelled via smooth muscle contraction from the urinary bladder to the urethra, and excreted from the body.

Example Question #5 : Other Excretory Physiology

The basement membrane that separates the glomerular capillary from Bowman's capsule is negatively charged. What is the purpose of this?

Possible Answers:

To decrease osmotic pressure within Bowman's capsule

To generate an electrical gradient for increased ion flow

To increase hydrostatic pressure within the glomerular capillary

To attract positvely charged substances

To prevent proteins from entering the filtrate

Correct answer:

To prevent proteins from entering the filtrate

Explanation:

The basement membrane is negatively charged in order to repel other negatively charged substances, specifically proteins. This is important because it ensures that proteins don't enter the filtrate, which is eventually excreted as urine. Old or malfunctioning proteins can be broken down, and their amino acids can often be recycled. Excretion of protein molecules would be extremely unfavorable.

Example Question #6 : Other Excretory Physiology

Kidney stones are crystals that most commonly form from calcium and uric acid. They are typically found in the ureters, but they can form in the calyces, the collecting ducts, or even in the nephrons themselves. Suppose an obstructive kidney stone is found in a nephron. Which of the following would occur?

Possible Answers:

Increased glomerular filtration rate 

Decreased membrane fluidity 

Decreased glomerular filtration rate 

Increased clearance rate

Increased membrane fluidity

Correct answer:

Decreased glomerular filtration rate 

Explanation:

An obstructive kidney stone in a nephron would impact bulk forces and filtration. Specifically, it would affect the hydrostatic pressure in Bowman's capsule. As a result, the glomerular filtration rate would decrease due to increased hydrostatic pressure in the capsule opposing filtration. Consequenty, fluid would be prevented from being filtered out of the kidney.

Example Question #7 : Other Excretory Physiology

Which of the following correctly depicts the order of liquid waste elimination?

Possible Answers:

Micturition, calyces, urinary bladder, papillary duct

Papillary duct, calyces, urinary bladder, micturition

Papillary duct, calyces, micturition, urinary bladder

Papillary duct, urinary bladder, calyces, micturition

Correct answer:

Papillary duct, calyces, urinary bladder, micturition

Explanation:

The papillary duct forms from collecting ducts in the kidney, where it travels to the calyces in the renal pelvis and then out of the kidney (via the ureter) to the bladder. Here, it is stored until micturition (urination).

Example Question #8 : Other Excretory Physiology

Which of the following is not true of the function and/or structure of the rectum?

Possible Answers:

The rectum contains stretch receptors in walls that signal the urge to defecate

The rectum precedes the anal canal and acts as temporary storage for feces

The rectum forces waste into the anal canal by use of internal and external sphincter muscles

The rectum makes up part of the lower gastrointestinal tract and shortens as waste is forced into the anal canal

Correct answer:

The rectum forces waste into the anal canal by use of internal and external sphincter muscles

Explanation:

While internal and external sphincter muscles are used to expel waste, these are located in the anal canal, not the rectum. The rectum precedes the anal canal and serves as storage; it signals the nervous system when it is full and defecation is needed.

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