Prep Guide: Excretory System

Key Concepts (Recall & Understanding)

Excretion: The removal of metabolic waste products from the body, such as urea, to maintain homeostasis.
Urinary System Organs: Kidneys, ureters, urinary bladder, urethra.
Kidney Structure: Composed of an outer cortex and an inner medulla. The renal pelvis collects urine.
The Nephron: The functional unit of the kidney. Each nephron consists of a renal corpuscle (glomerulus + Bowman's capsule) and a renal tubule (PCT, Loop of Henle, DCT).
Urine Formation: A three-step process:
1. Ultrafiltration (in Bowman's Capsule): High pressure in the glomerulus forces water and small solutes from the blood into the Bowman's capsule, forming the glomerular filtrate. Blood cells and large proteins remain in the blood.
2. Selective Reabsorption (in Renal Tubule): Essential substances like glucose, amino acids, most water, and ions are reabsorbed from the filtrate back into the blood.
3. Tubular Secretion (in Renal Tubule): Waste products and excess ions (like H+ and K+) are actively transported from the blood into the filtrate.
Osmoregulation: The control of water and solute balance in the body, a key function of the kidneys, regulated by the hormone ADH (Antidiuretic Hormone).

Application Corner

A person is diagnosed with diabetes mellitus, a condition where blood sugar levels are very high. Why would you expect to find glucose in this person's urine?
- Answer: During ultrafiltration, glucose is freely filtered into the Bowman's capsule. Normally, all of this glucose is reabsorbed back into the blood in the proximal convoluted tubule (PCT). However, in a person with uncontrolled diabetes, the blood glucose level is so high that the amount of glucose filtered exceeds the reabsorptive capacity of the tubules. The transport proteins responsible for reabsorbing glucose become saturated, and the excess glucose remains in the filtrate and is excreted in the urine.
On a hot day, you drink very little water. What changes would you expect in the volume and concentration of the urine you produce? Which hormone is responsible for this change?
- Answer: You would produce a smaller volume of more concentrated (darker yellow) urine. The hormone responsible is Antidiuretic Hormone (ADH). When the body is dehydrated, the pituitary gland releases more ADH, which increases the permeability of the collecting ducts to water, causing more water to be reabsorbed back into the blood.

Analytical Thinking

Odd One Out: Glomerulus, Ureter, Bowman's Capsule, Loop of Henle.
- Odd One: Ureter.
- Category: The rest are all parts of a single nephron.
Compare and Contrast: What is the key difference between the fluid in the Bowman's capsule (glomerular filtrate) and the blood plasma in the glomerulus?
- Answer: The main difference is the absence of large proteins and blood cells in the glomerular filtrate. The filtration process in the glomerulus is selective based on size, so while water, ions, glucose, and urea pass through, larger components like plasma proteins (e.g., albumin) and red and white blood cells are retained in the blood.

Kidney failure, or end-stage renal disease, occurs when the kidneys can no longer adequately filter waste products from the blood. This leads to a buildup of toxic substances like urea and a disruption of the body's water and electrolyte balance, which can be fatal. For patients with kidney failure, a common treatment is hemodialysis. The patient's blood is passed through a machine called a dialyzer or "artificial kidney." Inside the dialyzer, the blood flows next to a special fluid called dialysate, separated by a semi-permeable membrane. Waste products from the blood diffuse into the dialysate, while essential substances are retained in the blood. This case study illustrates the life-sustaining function of the kidneys and how technology can replicate the processes of filtration and diffusion to save lives when these organs fail.

Assertion-Reason Practice

Assertion (A): Ultrafiltration occurs in the glomerulus. Reason (R): The efferent arteriole (leading out of the glomerulus) is narrower than the afferent arteriole (leading into it), creating high pressure.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A). The difference in the diameter of the afferent and efferent arterioles is the primary reason for the high hydrostatic pressure within the glomerulus, which is the driving force for ultrafiltration.

HOTS (Higher-Order Thinking Skills) Question

Diuretic drugs are often prescribed to people with high blood pressure. These drugs work by inhibiting the reabsorption of sodium ions (Na+) in the renal tubules. How does inhibiting sodium reabsorption lead to a decrease in blood pressure?

Answer: The reabsorption of water in the kidneys is closely linked to the reabsorption of sodium ions through osmosis ("water follows salt"). By inhibiting the reabsorption of Na+, the diuretic drug causes more Na+ to remain in the filtrate. This, in turn, reduces the osmotic gradient for water to move back into the blood. As a result, more water is retained in the tubules and excreted in the urine. This increased urine output reduces the overall volume of fluid in the blood vessels. A lower blood volume leads to lower blood pressure.

Specimen Prep - Excretory System