Urinary System Notes

The urinary system plays a crucial role in maintaining homeostasis within the body. It has three major functions:

• Excretion: This is the process of removing organic waste products from body fluids. Waste products include urea, creatinine, and uric acid, which are byproducts of metabolism and must be eliminated to prevent toxicity.

• Elimination: This involves the discharge of waste products into the external environment, primarily through urine formation and expulsion from the body.

• Homeostatic regulation of the blood: The urinary system regulates blood volume, blood pressure, electrolyte balance, and acid-base balance by adjusting the reabsorption of various substances in the kidneys. This regulation helps maintain the stability of internal conditions despite external changes.

Anatomical Structures

• The structures associated with the storage and transport of urine in the urinary system are largely visible to the naked eye, consisting primarily of the kidneys, ureters, bladder, and urethra.

• The urethra serves as the channel that transports urine from the bladder to the outside of the body, and its length and structure vary between males and females.

• The bladder is a muscular sac that functions as a reservoir for urine, expanding and contracting as it fills and empties, respectively.

Urethra

• The urethra shows significant anatomical differences based on gender and plays a vital role in the urinary system. It commences at the inferior and central part of the bladder's base and extends posteriorly and inferiorly to the pubic symphysis.

• In terms of histology, the proximal urethra is lined with transitional epithelium, ideal for accommodating fluctuating volumes of urine, while the terminal portion consists of non-keratinized stratified squamous epithelium for protection against abrasions.

Female Urethra

• The external urethral orifice in females is located inferior to the clitoris and superior and anterior to the vaginal opening, which is a unique positioning that can affect urinary tract health.

• Its length is approximately 4 centimeters, significantly shorter than in males, which influences the likelihood of urinary tract infections (UTIs).

Male Urethra

• The male urethra is approximately 20 centimeters long and uniquely integrates both urinary and reproductive systems as it traverses through the prostate gland just inferior to the bladder.

• It exits at the tip of the penis, serving as a passageway for both urine and semen, necessitating coordinated muscular contractions for excretion during urination and ejaculation.

• Due to the comparatively shorter female urethra positioned closer to the rectum, women have a higher predisposition to urinary tract infections (UTIs).

Urinary Bladder

• The flow of urine through the urethra is regulated by two sphincters: the internal urinary sphincter (controlled involuntarily by the autonomic nervous system, comprised of smooth muscle) and the external urethral sphincter (under voluntary skeletal muscle control).

• The urinary bladder collects and stores urine produced by the kidneys through the ureters. In females, the bladder is anatomically positioned anterior to the uterus, posterior to the pubic bone, and anterior to the rectum, while in males, it is situated superior to the prostate gland.

• The bladder functions as a retroperitoneal organ that expands to accommodate urine volume. Its walls consist of crisscrossing muscle bands known as the detrusor muscle, which is internally lined with transitional epithelial tissue, allowing for stretch and contraction.

Micturition

• Micturition (urination or voiding) is a regulated process resulting from the stimulation of stretch receptors in the bladder wall that trigger a spinal reflex leading to urination.

• Parasympathetic neural outflow causes contraction of the detrusor muscle while the involuntary internal urethral sphincter relaxes, facilitating urine release.

• The external urinary sphincter, controlled voluntarily, allows for conscious control over urination. Damage to either muscle can lead to urinary incontinence, affecting quality of life.

Ureters

• The ureters are paired tubes that originate at the renal pelvis of each kidney, stretching retroperitoneally along the spine before sweeping laterally and hugging the pelvic cavity walls.

• They turn medially as they approach the bladder, piercing the bladder wall obliquely to ensure a one-way valve mechanism that prevents backflow of urine.

• Urine is propelled through the ureters via peristalsis, a rhythmic contraction of smooth muscles, ensuring continuous flow. Each ureter measures approximately 30 centimeters in length, and its inner lining comprises transitional epithelium that allows for expansion and contraction during urine transport.

Kidneys

• The kidneys are retroperitoneally located on either side of the spine, cushioned and protected by surrounding muscle, fat, and rib structures. They are roughly fist-sized and fundamentally contribute to the body's filtration system, receiving about 25% of the cardiac output at rest to perform their functions efficiently.

• The left kidney is typically positioned at the T12 to L3 vertebrae level, while the right kidney is lower due to displacement from the liver. Each kidney is crowned with an adrenal gland on its superior aspect.

• Each kidney comprises an outer renal cortex and an inner renal medulla, with renal columns extending from the cortex through the medulla to separate the renal pyramids.

• The renal hilum is a crucial area where blood vessels, nerves, lymphatics, and the ureter enter and exit, whereas the renal pelvis is formed from the merging of minor and major calyces within the kidney.

Urine Flow

• Urine is produced in both the renal cortex and medulla, ultimately emptying into a minor calyx. Two or three minor calyces converge to create a major calyx, from which urine flows into the renal pelvis.

• The renal pelvis leads the urine into the ureter, which it transports to the urinary bladder via peristaltic waves, a vital function for maintaining homeostasis.

Blood Flow in the Kidney

• The renal artery branches into segmental arteries, which further divide into interlobar arteries that traverse the renal columns to access the cortex. This branching continues into arcuate arteries and cortical radiate arteries, eventually leading to afferent arterioles.

• Each afferent arteriole feeds into a glomerulus, a tuft of capillaries that generates high pressure to facilitate filtration. The nephron, the kidney's functional unit, consists of a continuous tubule, with its proximal end encasing the glomerulus, termed the Bowman's capsule.

• The combination of the glomerulus and Bowman's capsule constitutes the renal corpuscle, while the efferent arterioles exiting this structure create a capillary network around the nephron, known as the peritubular capillaries.

Nephron

• The nephron serves as the kidney's essential functional unit, playing a fundamental role in blood purification, waste excretion, and fluid and electrolyte balance. Each kidney contains approximately one million nephrons.

• Structurally, a nephron consists of four major parts:

  • The renal corpuscle

  • The proximal convoluted tubule (PCT)

  • The loop of Henle (nephron loop)

  • The distal convoluted tubule (DCT)

• Renal corpuscles and proximal convoluted tubules primarily reside in the cortex, while loop of Henle lengths vary; some dip into the renal medulla significantly.

• The distal convoluted tubule also remains within the renal cortex, highlighting the nephron's compact and efficient structure.

Nephron Structure

• Proximal Convoluted Tubule: Positioned closest to the renal corpuscle, it is convoluted to maximize surface area for absorption.

• Loop of Henle: This section includes descending and ascending limbs, crucial for the reabsorption of water and solutes.

• Distal Convoluted Tubule: Located furthest from the renal corpuscle, it is convoluted to facilitate controlled solute exchange.

Collecting Ducts

• The collecting ducts carry urine through the renal medulla, culminating at the tips of renal pyramids in the renal papillae.

• Although part of the nephron's drainage system, they are technically distinct as multiple nephrons feed into a single collecting duct, thus contributing to final urine concentration and composition.

Renal Corpuscle Details

• The renal corpuscle encompasses the glomerulus, a cluster of high-pressure capillaries, enveloped by the Bowman's capsule, which captures and directs filtrate toward the proximal convoluted tubule.

• The parietal layer of the Bowman's capsule consists of simple squamous epithelium, while the inner layer contains podocytes—specialized cells with finger-like extensions (pedicels) that wrap around glomerular capillaries, forming filtration slits crucial for effective filtration.

Glomerular Capillary

• The endothelial cells of glomerular capillaries have fenestrations that allow substances smaller than 8 nanometers to pass freely, maintaining blood cell integrity while facilitating filtration.

• The combinations of the glomerulus's fenestrated endothelium, the basement membrane, and the podocytes form the renal corpuscle's filtration membrane, a dynamic barrier to blood contaminants.

• During filtration, approximately 10-20% of plasma is pushed into the Bowman's capsule, ultimately producing filtrate for further processing in the nephron.

Proximal Convoluted Tubule (PCT)

• The PCT comprises simple cuboidal epithelial cells with microvilli that enhance surface area for maximizing solute transport.

• These cells actively reabsorb vital solutes such as sodium, chloride, and potassium, which helps create an osmotic gradient for water reabsorption (about 75% of the total filtrate traverses through here).

Loop of Henle

• The Loop of Henle is structured with parallel descending and ascending segments, each specialized for different permeabilities:

  • The descending limb promotes water recovery, whereas the ascending limb actively reabsorbs sodium without permitting water to follow.

• By the time filtrate reaches the distal convoluted tubule, roughly 90% of the original water has been reabsorbed.

Collecting Ducts (Water Reabsorption)

• Within the collecting ducts, specific channels called aquaporins support selective water recovery.

• While collecting ducts are functionally linked to nephrons, they serve as the final passageway for adjusting and concentrating urine before it is excreted.

Reabsorption & Secretion

• Reabsorption refers to the recovery of essential materials back into the bloodstream, ensuring vital nutrients and electrolytes are preserved and not lost in urine.

• Secretion is the process where waste products are removed from surrounding blood and added back into the tubular fluid for eventual exclusion from the body.

Process Summary

• The sequence of processes includes:

  • Filtration (in the renal corpuscle) leads to filtrate formation.

  • Reabsorption (from the tubules back to blood).

  • Secretion (into the tubules for elimination).

  • Elimination (removal from the body as urine).