The Excretory System

By: Haden Edmonson

• • • • • • The function is to rid the body of waste products from cellular metabolism The lungs in the respiratory system excrete some waste products, such as carbon dioxide and water. The skin is another excretory organ that rids the body of wastes through the sweat glands. The liver and intestines excrete bile pigments that result from the destruction of hemoglobin.

The urinary system maintains homeostasis by regulating the amount of water that is excreted in the urine. Other aspects of its function include regulating the concentrations of various electrolytes in the body fluids and maintaining normal pH of the blood. The urinary system controls red blood cell production by secreting the hormone erythropoietin. It also maintains normal blood pressure by secreting the enzyme renin.

Main Organs

• • • •

The urinary system consists of… kidneys ureters urinary bladder urethra

Kidneys

• • The kidneys are the organs that filter the blood, remove the wastes, and excrete the wastes in the urine. They perform the functions of the urinary system. In the adult, each kidney is approximately 3 cm thick, 6 cm wide, and 12 cm long. It is roughly bean-shaped with an indentation, called the hilum, on the medial side. The hilum leads to a large cavity, called the renal sinus, within the kidney. The ureter and renal vein leave the kidney, and the renal artery enters the kidney at the hilum.

Where is it?

• The paired kidneys are located between the twelfth thoracic and third lumbar vertebrae, one on each side of the vertebral column. The right kidney usually is slightly lower than the left because the liver displaces it downward.

• The kidneys, protected by the lower ribs, lie in shallow depressions against the posterior abdominal wall and behind the parietal peritoneum. This means they are retroperitoneal (behind the peritoneum).

• Each kidney is held in place by connective tissue, called renal fascia, and is surrounded by a thick layer of adipose tissue, called perirenal fat, which helps to protect it.

The Outer Region of the Kidney

• • The outer, reddish region, next to the capsule, is the renal cortex. This surrounds a darker reddish-brown region called the renal medulla. The renal medulla consists of a series of renal pyramids, which appear striated because they contain straight tubular structures and blood vessels. The wide bases of the pyramids are adjacent to the cortex and the pointed ends, called renal papillae, are directed toward the center of the kidney. Portions of the renal cortex extend into the spaces between adjacent pyramids to form renal columns. The cortex and medulla make up the parenchyma, or functional tissue, of the kidney.

The Central Region of the Kidney

• • • The central region of the kidney contains the renal pelvis, which is located in the renal sinus, and is continuous with the ureter. The renal pelvis is a large cavity that collects the urine as it is produced. The periphery of the renal pelvis is interrupted by cuplike projections called calyces. A minor calyx surrounds the renal papillae of each pyramid and collects urine from that pyramid. Several minor calyces converge to form a major calyx. From the major calyces, the urine flows into the renal pelvis; and from there, it flows into the ureter.

Nephrons

Each kidney contains over a million functional units, called nephrons. The nephron is the functional unit of the kidney that is responsible for the actual purification and filtration of the blood. A nephron has two parts: (1) A renal corpuscle (2) A renal tubule

Ureters

Each ureter is a small tube, about 25 cm long, that carries urine from the renal pelvis to the urinary bladder. The wall of the ureter consists of 3 layers. (The outer layer) The fibrous coat is a supporting layer of fibrous connective tissue. (The middle layer) The muscular coat consists of the inner circular and outer longitudinal smooth muscle. The main function of this layer is peristalsis: to propel the urine. (The inner layer) The mucosa is transitional epithelium that is continuous with the lining of the renal pelvis and the urinary bladder. This layer secretes mucus, which coats and protects the surface of the cells.

Urinary bladder

• • • • The urinary bladder is a temporary storage reservoir for urine.

It is located in the pelvic cavity. The size and shape of the urinary bladder varies with the amount of urine it contains and with the pressure it receives from surrounding organs.

It has 3 layers and a trigone area.

1

st

layer

• • • The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is continuous with that in the ureters. When the bladder is empty, the mucosa has numerous folds called rugae.

The rugae and transitional epithelium allow the bladder to expand as it fills.

2

nd

layer

• • The second layer in the walls is the submucosa, which supports the mucous membrane. It is composed of connective tissue with elastic fibers.

•

3

rd

layer

The next layer is the muscularis, which is composed of smooth muscle.

• The smooth muscle fibers are interwoven in all directions and, collectively, these are called the detrusor muscle. • Contraction of this muscle expels urine from the bladder, like a balloon.

Trigone

• • • There is a triangular area, called the trigone, formed by three openings in the floor of the urinary bladder.

Two of the openings are from the ureters and form the base of the trigone. Small flaps of mucosa cover these openings and act as valves that allow urine to enter the bladder but prevent it from backing up from the bladder into the ureters.

The third opening, at the apex of the trigone, is the opening into the urethra.

Urethra

• • • The final passageway for the flow of urine is the urethra, a thin-walled tube that conveys urine from the floor of the urinary bladder to the outside. The mucosal lining of the urethra is transitional epithelium. The wall also contains smooth muscle fibers and is supported by connective tissue.

The opening to the outside is called the external urethral orifice.

Urethra Sphincters

• • The internal urethral sphincter surrounds the beginning of the urethra, where it leaves the urinary bladder. This sphincter is smooth (involuntary) muscle. Another sphincter, the external urethral sphincter, is skeletal (voluntary) muscle and encircles the urethra where it goes through the pelvic floor. These two sphincters control the flow of urine through the urethra.

• • • • • • • •

Review Key Points

The urinary system rids the body of waste materials, regulates fluid volume, maintains electrolyte concentrations in body fluids, controls blood pH, secretes erythropoietin, and renin.

The components of the urinary system are the kidneys, ureters, urinary bladder, and urethra.

The central region of the kidney is the renal pelvis, which collects the urine as it is produced.

The functional unit of the kidney is a nephron, which consists of a renal corpuscle and a renal tubule.

The ureters transport urine from the kidney to the urinary bladder.

The urinary bladder is a temporary storage reservoir for urine.

The urethra is the final passageway for the flow of urine.

The flow of urine through the urethra is controlled by an

involuntary internal

urethral sphincter and

voluntary external

urethral sphincter.

True/False Practice Questions

1 of 3

• The urinary system controls red blood cell production by secreting the enzyme renin.

True or False

False

• The urinary system controls red blood cell production by secreting the hormone erythropoietin.

2 of 3

• The kidneys are the organs that filter the blood, remove the wastes, and excrete the wastes in the urine.

True or False

True

3 of 3

• The internal and external urethral sphincters control the flow of urine through the urethra.

True or False

True

Fun facts

• In a healthy adult, almost 440 gallons of blood is passed through the kidneys on a daily basis, thereby resulting in formation of almost one to two liters of urine. However, this amount varies with the amount of water (or fluid) intake and the amount of sweat produced by the body.

• Though we keep describing kidneys as bean-shaped organs, the beans were named after the organ and not vice versa!

Diseases

ACUTE KIDNEY FAILURE

Caused by nephritis, shock, injury, bleeding, sudden heart failure or poisoning

CHRONIC RENAL FAILURE

- gradual loss of function of nephrons

CYSTITIS

•Inflammation of the mucous membrane lining of the urinary bladder •Most common cause – E. Coli

KIDNEY STONES

•Made of crystals of calcium phosphate and uric acid •Gradually they get larger until they block ureters •Symptoms: nausea and vomiting, pain, frequency, chills, and fever •Diagnosis: by symptoms, ultrasound, or x ray

Extra video:

• http://www.neok12.com/Urinary-System.htm

Picture locations:

• • • • http://english lss.com/E4MS/Urinary/Media/urinary system_color.gif

http://buzzle.com/images/diagrams/urinary system-labeled-diagram.jpg

http://d1.yimg.com/sr/img/4/bab94a59-a7cf 30b6-a0d4-eaa02316e800 http://ts4.mm.bing.net/th?id=H.46828855887

15195&pid=15.1

Sources:

• • • http://www.buzzle.com/articles/urinary system-facts.html

http://mhs.mcsnc.org/UserFiles/Servers/Serv er_2082346/File/medscience/Medical%20Scie nces%20I/O%20Urinary%20System%20%20M.

pdf http://training.seer.cancer.gov/anatomy/urina ry/