Anatomy and Physiology of the Urinary System
TITLE
Anatomy and Physiology of the Urinary System
Detailed Lecture Notes
INTRODUCTION
The urinary system is one of the most important systems of the human body. Its main function is to remove waste products from the blood, maintain water and electrolyte balance, regulate blood pressure, and help in the production of red blood cells.
The system includes two kidneys, two ureters, one urinary bladder, and one urethra.
SECTION ONE
ANATOMY OF THE URINARY SYSTEM
Kidneys
The kidneys are two bean shaped organs located on either side of the vertebral column, in the upper abdominal cavity. The right kidney is slightly lower than the left due to the presence of the liver. Each kidney is about eleven to twelve centimeters long and weighs about one hundred and fifty grams.
External Structure
Each kidney has two surfaces called anterior and posterior, two poles called upper and lower, and two borders called medial and lateral.
The lateral border is convex and the medial border is concave. The medial border contains the hilum which allows the entry and exit of the renal artery, renal vein, lymphatics, and ureter.
The kidney is covered by three layers.
Fibrous capsule
Perirenal fat
Renal fascia
Internal Structure
The inside of the kidney has two main regions.
Cortex is the outer part containing renal corpuscles and convoluted tubules.
Medulla is the inner part containing renal pyramids.
The apex of each pyramid forms a renal papilla which opens into minor calyces. Several minor calyces join to form major calyces. Major calyces open into the renal pelvis which continues as the ureter.
Nephron
The nephron is the functional and structural unit of the kidney. Each kidney contains about one million nephrons.
Parts of the nephron
Renal corpuscle which includes Bowmans capsule and glomerulus.
Tubular portion which includes proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.
Ureters
The ureters are two muscular tubes about twenty five to thirty centimeters long. They carry urine from the renal pelvis to the urinary bladder.
They have three layers.
Mucosa
Muscular layer
Fibrous layer
They enter the bladder at an oblique angle which helps prevent backflow of urine.
Urinary Bladder
The urinary bladder is a hollow muscular organ that stores urine before it is expelled. It is located in the pelvic cavity.
The bladder has three layers.
Mucosa made up of transitional epithelium
Muscular coat called detrusor muscle
Outer serous or fibrous layer
The bladder has a special triangular area called the trigone that is important because it remains smooth even when the bladder is empty.
Urethra
The urethra is a tube that carries urine from the bladder to the outside of the body.
In males it is about twenty centimeters long and has three parts called prostatic, membranous, and spongy.
In females it is about four centimeters long. Because it is short, females are more prone to urinary infections.
SECTION TWO
PHYSIOLOGY OF THE URINARY SYSTEM
Formation of Urine
Urine formation occurs in three main steps.
Glomerular Filtration
Blood enters the glomerulus and high pressure forces water and small solutes out of the blood into Bowmans capsule.
This filtered fluid is called filtrate. Large proteins and blood cells do not pass through.
Tubular Reabsorption
Most of the filtrate is reabsorbed back into the blood.
Essential substances such as glucose, amino acids, water, and ions are reabsorbed mainly in the proximal convoluted tubule.
Tubular Secretion
Additional waste products such as hydrogen ions, potassium ions, and drugs are secreted from the blood into the tubules.
This helps maintain acid base balance and remove toxins.
Functions of the Kidneys
Regulation of water balance
The kidneys adjust the amount of water reabsorbed by the tubules to maintain body fluid balance.
Regulation of electrolytes
The kidneys control levels of sodium, potassium, calcium, and chloride.
Regulation of acid base balance
The kidneys secrete hydrogen ions and reabsorb bicarbonate to keep blood pH in a normal range.
Production of hormones
The kidneys produce erythropoietin which stimulates red blood cell formation.
They also produce renin which helps regulate blood pressure.
The kidneys also activate vitamin D for calcium absorption.
Excretion of waste products
Metabolic wastes such as urea, uric acid, creatinine, and drugs are removed from the body.
Micturition or Urination
Micturition is the process of emptying the bladder.
When the bladder fills with urine, stretch receptors send signals to the brain.
The detrusor muscle contracts and the internal sphincter relaxes.
Urine passes through the urethra and the external sphincter can be consciously controlled.
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