CH 16 Urinary System

Chapter 16: The Urinary System and Excretion

16.1 Introduction to the Urinary System

Excretion: The physiological process of removing metabolic wastes and toxins from the body to maintain homeostasis and overall health.Kidneys: The primary organs of the urinary system, the kidneys filter blood and produce urine through a complex process involving multiple layers of tissue and the nephron units within them. They are responsible for regulating the body's internal environment, including fluid and electrolyte balance, and play a critical role in the filtration and removal of waste products.

16.2 Functions of the Urinary System

Overview of Functions

The urinary system serves several vital functions:

• Excretion of Metabolic Wastes:

  • Urea: A nitrogenous waste product formed from the metabolism of amino acids; it is synthesized in the liver from ammonia, a toxic by-product. Urea is efficiently removed from the bloodstream through renal filtration.

  • Creatinine: A waste product generated from the normal breakdown of creatine phosphate in muscles, it is produced at a fairly constant rate and provides a valuable measure of kidney function.

  • Uric Acid: Produced from the breakdown of purines found in nucleic acids; while necessary in small amounts, excess uric acid can crystallize and lead to conditions like gout, which is characterized by joint inflammation.

  • Urochrome: The pigment that gives urine its characteristic yellow color, derived from the breakdown of hemoglobin.

• Preservation of Water-Salt Balance:

  • The kidneys play a crucial role in regulating the body’s hydration status by adjusting the concentration of sodium (NaCl) in the blood, which directly influences osmotic pressure and blood volume.

  • They also maintain the balance of other ions, including potassium, bicarbonate, and calcium, essential for numerous cellular functions and metabolic processes.

• Maintenance of Blood Pressure:

  • Through the regulation of blood volume and vascular resistance, the kidneys help maintain normal blood pressure levels. They produce renin, an enzyme crucial for the activation of angiotensin, which constricts blood vessels and promotes further blood volume increase through the action of aldosterone.

• Maintenance of Acid-Base Balance:

  • By excreting hydrogen ions and reabsorbing bicarbonate, the kidneys play a critical role in regulating blood pH and maintaining acid-base homeostasis, crucial for normal cellular function.

• Secretion of Hormones:

  • Renin: Secreted by the kidneys in response to decreased blood flow; it activates the renin-angiotensin-aldosterone system to regulate blood pressure.

  • Erythropoietin (EPO): Released when oxygen levels in blood are low, stimulates increased production of red blood cells in bone marrow.

  • Activation of Vitamin D: The kidneys convert inactive vitamin D into its active form, promoting calcium and phosphate absorption in the intestines.

• Kidney Filtering Capacity:

  • Each kidney filters approximately 180 liters of blood daily, with less than 1% excreted as urine, highlighting the efficiency of kidney function in waste removal and substance reabsorption.

16.3 Organs of the Urinary System

Structure and Location

• Kidneys:

  • Bean-shaped, reddish-brown organs located retroperitoneally in the lumbar region, supported by renal fascia and surrounded by adipose tissue for protection. Each kidney contains one million or more nephrons, which are the functional units that perform the filtering and excretion.

• Ureters:

  • Muscular tubes that convey urine from the kidneys to the bladder; they utilize peristaltic contractions to propel urine effectively toward the urinary bladder.

• Urinary Bladder:

  • A hollow, muscular organ that stores urine temporarily before it is expelled from the body; it is composed of smooth muscle (detrusor muscle) and contains sphincters that control the release of urine.

• Urethra:

  • The tube leading from the bladder to the external opening; it differs in length between genders, being longer in males, which influences the mechanism of urination.

16.4 Urination

Micturition

The process of urination (micturition) is coordinated by a complex interaction between the nervous system and the urinary system. Stretch receptors in the bladder signal fullness, resulting in involuntary contractions of the detrusor muscle and voluntary relaxation of the external sphincters for urine expulsion.

16.5 Anatomy of the Kidney

The kidneys are divided into three primary regions:

• Renal Cortex: The outer layer, which extends into the medulla with renal columns. It contains the renal corpuscles and convoluted tubules of nephrons.

• Renal Medulla: The inner portion that contains renal pyramids, where urine is collected before being funneled into the renal pelvis.

• Renal Pelvis: A funnel-shaped structure that connects the kidneys to the ureters, serving as the collecting area for urine.

Nephrons

Nephrons are the functional units of the kidneys, numbering over a million per kidney; they consist of two types:

• Cortical Nephrons: Located primarily in the renal cortex, responsible for the majority of the kidney's regulatory processes.

• Juxtamedullary Nephrons: Extend deeper into the medulla, playing a critical role in concentrating urine and maintaining water balance.

16.6 Urine Formation

Steps in Urine Formation

1. Glomerular Filtration: Blood is filtered in the glomerulus; water and small solutes enter the glomerular capsule while larger items, such as proteins and blood cells, remain in the bloodstream.

   Filtrate Contents: It includes water, nitrogenous wastes, essential nutrients, and salts.

2. Tubular Reabsorption: This process reabsorbs water and vital nutrients back into the bloodstream and occurs mainly in the proximal convoluted tubule through both passive diffusion and active transport.

   Reabsorption Rate: This is influenced by the availability of transporters; conditions like diabetes can lead to glycosuria, where excess glucose exceeds the transport capacity.

3. Tubular Secretion: The kidneys actively transport select substances into the tubular fluid, such as H+ ions and certain drugs, for final excretion.

   Final Urine Product: The urine consists of substances that were filtered at the glomerulus and not reabsorbed, in addition to those that were secreted from the blood.

16.7 Regulatory Functions of the Kidneys

Fluid and Electrolyte Balance

• Average body water percentages indicate approximately 60% in males and 50% in females, influenced by body composition and age.

• Water intake must equal output, including losses via urine, sweat, and respiration.

• Electrolytes such as sodium, potassium, and bicarbonate are crucial for maintaining cellular functions, nerve conduction, and overall bodily health.

16.8 Hormonal Regulation

Key Hormones

• Antidiuretic Hormone (ADH): Synthesized in the hypothalamus and released from the posterior pituitary gland; it increases water reabsorption in the kidneys, conserving blood volume and pressure during dehydration.

• Aldosterone: Secreted by the adrenal cortex; it promotes sodium reabsorption in exchange for potassium or hydrogen ions, thus regulating blood pressure and electrolyte balance.

• Atrial Natriuretic Hormone (ANH): Released by the heart in response to increased blood volume, it decreases blood pressure by promoting sodium and water excretion.

16.9 Acid-Base Balance

• The normal blood pH is maintained between 7.35 and 7.45, crucial for enzyme function and metabolic processes.

• Buffers, respiratory adjustments, and kidney functions work together to maintain this balance. Kidneys are vital for excreting excess hydrogen ions and reabsorbing bicarbonate to regulate acid-base equilibrium.

16.10 Problems with Kidney Function

Common Issues

• Alterations in urine composition can indicate underlying health conditions, such as renal disease or dysfunction.

• Common diseases associated with kidney function include diabetes mellitus (leading to nephropathy), hypertension (causing vascular damage), and infections (e.g., urethritis, cystitis, and pyelonephritis).

• Renal Calculi (Kidney Stones): The formation of stones from crystallized minerals can obstruct urine flow and cause immense pain.

• Glomerular Damage: Conditions like glomerulonephritis can severely impact filtration capabilities, leading to waste accumulation in the bloodstream.

16.11 Treatment for Kidney Problems

Treatments

• Hemodialysis: A medical procedure in which a machine filters blood, removing excess substances and toxins when the kidneys are unable to do so effectively.

• Renal Transplantation: Involves the surgical implantation of a donated kidney; it poses risks of rejection by the recipient’s immune system and requires lifelong immunosuppressive therapy.

16.12 Aging Effects on the Urinary System

• Age-related changes include a decline in renal function, kidney shrinkage, and a decrease in nephron counts, impacting overall health and efficiency of waste removal.

• Older adults have an increased susceptibility to kidney stones and urinary tract infections, necessitating careful monitoring of kidney health.

16.13 Homeostasis

• The kidneys are the primary organs for excretion and maintaining homeostasis, regulating water-salt balance, blood pressure, and pH levels, and aiding the functioning of other systems in the body by ensuring a balanced internal environment.