Cracking the AP Biology Exam

11 Animal Structure and Function

V. THE EXCRETORY SYSTEM

As you already know, all organisms must get rid of wastes. In this chapter, we’ll focus primarily on how organisms get rid of nitrogenous wastes (products containing nitrogen) and regulate water. When cells break down proteins, one of the byproducts is ammonia (NH₃), a substance that is toxic to the body. Consequently, organisms had to develop ways of converting ammonia to a less poisonous substance. Some animals convert ammonia to uric acid, while others convert ammonia to urea. Some examples of excretory organs among invertebrates are nephridia(found in earthworms) and Malpighian tubules (found in arthropods).

THE HUMAN EXCRETORY SYSTEM

In humans, the major organ that regulates excretion is the kidney. Each kidney is made up of a million tiny structures called nephrons. Nephrons are the functional units of the kidney. A nephron consists of several regions: the Bowman’s capsule, the proximal convoluted tubule, theloop of Henle, the distal convoluted tubule, and the collecting duct. The renal cortex, or outer-most section, contains the Bowman’s capsule and the proximal convoluted tubules. The renal medulla, the inner section, contains the loop of Henle and the distal convoluted tubules.

Here’s an illustration of a nephron:

How does a nephron work? Let’s trace the flow of blood in a nephron. Blood enters the nephron at the Bowman’s capsule. A blood vessel called the renal artery leads to the kidney and branches into arterioles, then tiny capillaries. A ball of capillaries that “sits” within a Bowman’s capsule is called a glomerulus. Blood is filtered as it passes through the glomerulus and the plasma is forced out of the capillaries into the Bowman’s capsule. This plasma is now called a filtrate.

The filtrate travels along the entire nephron. From the Bowman’s capsule, the filtrate passes through the proximal convoluted tubule, then the loop of Henle, then the distal convoluted tubule, and finally the collecting duct. As it travels along the tube, the filtrate is modified to formurine.

What happens next? The concentrated urine moves from the collecting ducts into the ureters, then into the bladder, and finally out through the urethra.

How Urine Is Made

Urine is produced in the nephron by three processes: filtration, reabsorption, and secretion.

Filtration—The blood is filtered as it passes through the glomerulus to the Bowman’s capsule. Small substances, such as ions, water, glucose, urea, and amino acids, easily pass through the capillary walls. Large substances, such as proteins and blood cells, are too big to pass through.
Reabsorption—As the filtrate moves through the proximal convoluted tubule, some materials are reabsorbed. The small solutes, such as water, nutrients, and salts, leave the proximal convoluted tubule and are reabsorbed by a network of capillaries, the peritubular capillaries, that surround the tubules. The material remaining in the tubule is urine.
Secretion—As the filtrate moves through the convoluted tubules, some substances, such as H⁺, potassium, and ammonium ions, are secreted from the surrounding capillaries into the tubule.

Hormones of the Kidney

Two hormones regulate the concentration of water and salt in the kidneys: vasopressin (also known as antidiuretic hormone) and aldosterone. Antidiuretic hormone (ADH) allows water to be reabsorbed from the collecting duct. If your fluid intake is low (or if you’re dehydrated), ADH helps your body retain water by concentrating the urine. If, however, your fluid intake is high, ADH levels will be low, and the body won’t reabsorb most of the water. Your urine will contain lots of water and therefore be dilute. For now, just remember that ADH controls the volume of urine. The other hormone, aldosterone, is responsible for regulating sodium reabsorption at the distal convoluted tubule.

Skin

The skin is also an excretory organ that gets rid of excess water and salts from the body. Believe it or not, your skin is the largest organ in the body! It contains 2.5 million sweat glands that secrete water and ions through pores. The skin’s primary function is to regulate body temperature.

The skin has three layers: the epidermis, dermis, and subcutaneous tissue (or hyperdermis). Sweat glands are found in the dermis layer along with blood vessels, nerves, and sebaceous—or oil—glands.

The epidermis is covered by a layer of dead cells called the stratum corneum. These cells form a barrier against invading microorganisms. The bottom layer of skin, the subcutaneous tissue, is mostly fats.

To summarize, in humans, two organs control fluid balance and dispose of metabolic wastes:

The kidney—gets rid of nitrogenous wastes and reabsorbs water and salt.
The skin—gets rid of excess salt and water.

EXCRETORY SYSTEM QUIZ

Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the answer that is best in each case. Answers can be found here.

1. The illustrations below depict the nephron of the kidney in the kangaroo rat and the human.

Compared to the human kidney, the kangaroo rat’s kidney is capable of producing more concentrated urine because its kidney

(A) allows the glomerulus to filter the blood more rapidly

(B) allows more filtrate to be reabsorbed in the proximal convoluted tubule

(D) produces uric acid instead of urea

(E) causes urine to pass through the right atrium and ventricle

Directions: Each group of questions consists of five lettered headings followed by a list of numbered phrases or sentences. For each numbered phrase or sentence, select the one heading that is the most closely related to it and fill in the corresponding oval on the answer sheet. Each heading may be used once, more than once, or not at all in each group.

Questions 2–5 refer to the following excretory organs

(A) Ureter

(B) Urinary Bladder

(D) Kidney

(E) ADH

2. Structure that expands as it stores urine

3. Structure through which urine leaves the bladder

4. Structure that carries the urine from the kidney to the bladder

5. Promotes water retention by the kidneys

6. All of the following would most likely filter through the glomerulus into the Bowman’s capsule EXCEPT

(A) monosaccharides

(B) proteins

(D) salts

(E) ions