LAB 7 NOTES

Functions of the Urinary System

The urinary system plays a crucial role in maintaining homeostasis by:

• Regulating water balance.

• Maintaining acid-base balance in the blood.

• Controlling plasma ionic composition (electrolytes such as calcium, sodium, and potassium).

• Regulating blood pressure and blood volume.

• Stimulating blood cell production.

• Activating vitamin D3 (converting it to calcitriol, the active form).

Additionally, it eliminates waste products, including nitrogenous wastes, toxins, and drugs.

Major Structures of the Urinary System

The urinary system consists of the following key structures:

• Kidneys: Produce urine, performing the major functions of the urinary system.

• Ureters: Transport urine from the kidneys to the bladder.

• Bladder: Serves as a temporary storage reservoir for urine.

• Urethra: Excretes urine from the bladder out of the body.

Blood vessels enter and exit the kidney at the hilum. Specifically, renal arteries deliver blood to the kidneys for processing, while renal veins carry blood away from the kidneys.

Ureters

The ureters are paired tubes, with one running from each kidney to the bladder.

• They are continuous with the renal pelvis.

• They enter the posterior aspect of the bladder.

• They run behind the peritoneum.

• Peristalsis, aided by gravity, facilitates urine transport.

Bladder

The bladder is a smooth, collapsible, muscular sac that temporarily stores urine. A moderately full bladder is approximately 5 inches long and can hold about 500 mL of urine.

The trigone is a triangular region at the base of the bladder, characterized by three openings:

• Two from the ureters.

• One to the urethra.

In males, the prostate gland surrounds the neck of the bladder.

Urinary Bladder Wall

The urinary bladder wall comprises three layers of smooth muscle, collectively known as the detrusor muscle. The mucosa is made of transitional epithelium. The walls are thick and folded when the bladder is empty, allowing the bladder to expand significantly without increasing internal pressure.

Urethra

The urethra is a thin-walled tube that carries urine from the bladder to the outside of the body via peristalsis. The release of urine is controlled by two sphincters:

• Internal Urethral Sphincter: Involuntary and made of smooth muscle.

• External Urethral Sphincter: Voluntary and made of skeletal muscle.

Female vs. Male Urethra

There are notable differences between the female and male urethra:

• Length:

  • Female: 3-4 cm

  • Male: 20 cm

• Location:

  • Female: Next to the wall of the vagina.

  • Male: Through the prostate and penis.

• Function:

  • Female: Carries only urine.

  • Male: Carries urine and serves as a passageway for sperm cells.

Location of the Kidneys

The kidneys are located against the dorsal wall, at the level of the T12 to L4 vertebrae. The right kidney is slightly lower than the left due to the position of the liver.

Regions and Features of the Kidneys

The kidneys exhibit distinct regions and features:

• Renal Cortex: The outer region.

• Renal Medulla: Located inside the cortex.

• Renal Pelvis: The inner collecting tube.

• Renal Hilum: A medial indentation where structures enter or exit the kidney (ureters, renal blood vessels, and nerves).

An adrenal gland sits atop each kidney.

Coverings of the Kidneys

The kidneys are protected by several coverings:

• Renal/Fibrous Capsule: Surrounds each kidney.

• Renal Fascia: The outermost capsule that helps hold the kidney in place against the muscles of the trunk wall. It also divides the fat surrounding the kidney into two layers:

  1. Perirenal fat: Located inside the renal fascia, between the capsule of the kidney, cushions against blows, and helps attach it to the body wall.

  2. Pararenal fat: Located outside the renal fascia, and aids in maintaining the kidney's proper position.

Nephrotosis, also known as renal ptosis, occurs when the kidney's position changes and drops in the body cavity, potentially due to the loss of perirenal fat.

Structure of the Kidney

The internal structure of the kidney includes:

• Renal (Medullary) Pyramids: Triangular regions of tissue in the medulla.

• Renal Columns: Extensions of cortex-like material inward that separate the pyramids.

• Calyces: Cup-shaped structures that funnel urine toward the renal pelvis.

Blood Supply and Blood Flow Through the Kidneys

Approximately 25% of the total body's blood volume passes through the kidneys each minute. The renal artery branches from the abdominal aorta and provides arterial blood to the kidney. The renal vein drains blood from the kidneys into the inferior vena cava.

The Nephron

Nephrons are the structural and functional units of the kidney responsible for forming urine. They are associated with two capillary beds: the glomerulus and peritubular capillaries.

Each nephron comprises two major structures:

• Renal Corpuscle

• Renal Tubule: Extends from the glomerular capsule and ends at the collecting duct. It consists of:

  • Glomerular (Bowman's) capsule

  • Proximal convoluted tubule (PCT)

  • Nephron loop (Loop of Henle)

  • Distal convoluted tubule (DCT)

Nephron Anatomy: The Renal Corpuscle

The renal corpuscle is the site of filtration, where a portion of the blood passes from the glomerular capillaries into the glomerular capsule. The glomerulus is a knot of capillaries supplied by the afferent arteriole and drained by the efferent arteriole. The high pressure in the glomerulus forces fluid and small solutes out of the blood and into the glomerular capsule. The glomerulus is covered by podocytes of the visceral layer of the glomerular capsule.

The glomerular capsule encloses the glomerulus and marks the beginning of the renal tubule. It features a visceral layer of podocytes with foot processes that form part of the filtration membrane and a parietal layer, which is an outer impermeable wall.

Nephron Anatomy: Peritubular Capillary Beds

Peritubular capillaries arise from the efferent arteriole of the glomerulus. They are normal, low-pressure capillaries adapted for absorption rather than filtration. These capillaries cling closely to the renal tubule to reabsorb (reclaim) substances from collecting tubes.

Nephron Anatomy: PCT, Nephron Loop, and DCT

The path of filtrate through the renal tubule is as follows: glomerular capsule -> proximal convoluted tubule -> nephron loop -> distal convoluted tubule. The renal tubule is the site of tubular reabsorption and tubular secretion.

Nephron Anatomy: Collecting Duct

Distal convoluted tubules empty into the collecting duct, which receives urine from many nephrons. The collecting duct runs through the medullary pyramids and delivers urine into the calyces and renal pelvis.

Types of Nephrons

There are two main types of nephrons:

• Cortical Nephrons: Located entirely in the cortex (approximately 85% of nephrons).

• Juxtamedullary Nephrons: Found at the boundary of the cortex and medulla and play an important role in concentrating urine.

Basic Renal Processes

The basic renal processes involved in urine formation are:

• Filtration: Movement from glomerulus to glomerular capsule.

• Reabsorption: Movement from renal tubule to peritubular capillaries.

• Secretion: Movement from peritubular capillaries to renal tubule.

• Excretion: Elimination from renal tubules out of the body.

Urine Formation

Urine formation involves three main steps:

1. Glomerular Filtration: Water and solutes smaller than proteins are forced through capillary walls and pores of the glomerular capsule into the renal tubule.

2. Tubular Reabsorption: Water, glucose, amino acids, and needed ions are transported out of the filtrate into the tubule cells and then enter the peritubular capillaries.

3. Tubular Secretion: H^+, K^+, creatinine, and drugs are removed from the peritubular blood and secreted by the tubule cells into the filtrate.

Filtration, Reabsorption, and Secretion

These three processes are essential for the formation of urine and the regulation of blood composition.

Characteristics of Urine

Typically, 1-1.8 liters of urine are produced in 24 hours. Filtrate contains everything that blood plasma does (except proteins), while urine is what remains after the filtrate has lost most of its water, nutrients, and necessary ions.

The yellow color of urine is due to the pigment urochrome (from the destruction of hemoglobin). The intensity of the color indicates the ratio of solutes to water in the urine. Urine is slightly aromatic and has a normal pH of about 6. The specific gravity of urine ranges from 1.001 to 1.030. Specific gravity is defined as the relative weight of a specific volume of liquid compared with an equal volume of distilled water - dissolved solutes in urine cause a given volume of urine to weigh more than the same volume of water.

Composition of Urine

Urine consists of water, urea, sodium, potassium, creatinine, uric acid, and various ions.

Abnormal Urine Constituents

Abnormal constituents in urine may include glucose, proteins, blood, pus, bile pigments, and casts. These components can indicate underlying health issues, such as diabetes mellitus (glucose), kidney disease (proteins, blood), or infection (pus).