Detailed Notes on the Digestive and Urinary Systems

Digestive System

• The digestive system is primarily responsible for the digestion of food.
• This process begins with ingestion, followed by mechanical processing, which includes the secretion of enzymes like amylase.
• Subsequently, digestion and absorption occur, primarily in the small intestine, followed by excretion of waste products.

Digestive Tract (Gastrointestinal Tract)

• The digestive tract, also known as the gastrointestinal (GI) tract or alimentary canal, is a muscular tube extending from the mouth to the anus.
• It comprises the pharynx, esophagus, stomach, and small and large intestines.

Organs of the Digestive System

• Oral Cavity (Mouth): Involved in ingestion and mechanical processing.
• Pharynx: Facilitates muscular propulsion of materials into the esophagus.
• Esophagus: Transports materials from the mouth to the stomach using peristalsis.
• Stomach: Responsible for the chemical breakdown of materials via acids and enzymes.
• Small Intestine: Facilitates enzymatic digestion and absorption of water, organic substrates, vitamins, and ions.
• Large Intestine: Involved in the reabsorption of water. The rate of peristalsis affects the consistency of feces; slow peristalsis leads to constipation due to increased water reabsorption, while rapid peristalsis results in diarrhea.

Accessory Organs of the Digestive System

• Teeth: Perform mechanical processing through chewing or mastication.
• Tongue: Assists in the mechanical processing of food and swallowing.
• Salivary Glands: Secrete lubricating fluid containing enzymes like amylase to break down carbohydrates.
• Liver: Secretes bile, which is stored in the gallbladder.
  • The liver also stores nutrients, converting excess sugar into glycogen.
• Pancreas: Secretes buffers and digestive enzymes, as well as endocrine hormones.
  • Alpha cells secrete glucagon, and beta cells secrete insulin.

Oral Cavity Function

• Sensory analysis of materials before swallowing.
• Mechanical processing via the teeth, tongue, and palatal surfaces.
• Lubrication through saliva production, which mixes with mucus.
• Limited digestion due to salivary gland secretions containing enzymes for carbohydrate breakdown.
• The oral cavity is defined by the teeth, tongue, hard palate, and soft palate, essential for ingestion, where teeth and tongue work with salivary glands to break down food into smaller masses.

Salivary Glands

• Saliva, secreted by salivary glands, moistens food and initiates chemical digestion.
• Saliva is about 99% water and cleanses the mouth, dissolves food chemicals for taste, and contains amylase to break down carbohydrates.
• There are three pairs of salivary glands: parotid, sublingual, and submandibular.
• The tongue forms a bolus of food to be swallowed, moving it down the pharynx and into the esophagus.

Peristalsis and Digestion Time

• Peristaltic waves move nutrients and waste through the intestine.
• Swallowing takes approximately 2-3 seconds.
• Digestion in the stomach lasts for 2-4 hours.
• Nutrient absorption occurs over 3-5 hours.
• Elimination takes 10 hours or more and should occur daily.

Stomach Structure and Function

• The stomach is the widest part of the digestive tube, a J-shaped muscular sac where food is stored, churned, and mixed with gastric juices.
• Gastric juices include digestive enzymes and hydrochloric acid, breaking down food and killing microbes.
• Hydrochloric acid can kill harmful microbes, but some bacteria, like Helicobacter pylori, are acid-resistant and can cause peptic ulcers.

Accessory Organs: Liver, Gallbladder, and Pancreas

• The salivary glands, liver, gallbladder, and pancreas aid in ingestion, digestion, and absorption by secreting or storing substances that pass through ducts into the alimentary canal.
• Liver: Produces bile, a yellowish-brown fluid that emulsifies fats in the duodenum.
  • Bile flows from the liver into the hepatic ducts, then to the common hepatic duct, and into the small intestine, or it is stored in the gallbladder.
• Gallbladder: Stores and concentrates bile, which is used to emulsify fats.
  • Excessive fat intake can lead to gallstone formation or pancreas problems.
• Pancreas: Secretes pancreatic enzymes, water, buffers, and electrolytes.
  • Pancreatic juice drains through the main pancreatic duct into the common bile duct and into the small intestine, aiding in the digestion of various products.

Absorption and Elimination

• The small intestine, which contains villi, increases the surface area for absorption.
• After digestion, food passes from the small intestine through the ileocecal valve into the large intestine.
• The large intestine absorbs remaining nutrients and water as peristaltic waves move the chime through the ascending and transverse colon.
• Dehydration compacts the chime into feces, which are stored in the descending and sigmoid colons before elimination.
• Feces are expelled through the rectum and anus via defecation, which involves the contraction of rectal muscles and relaxation of anal sphincters.
• The defecation reflex is primarily involuntary, controlled by the autonomic nervous system, but can be voluntarily controlled by the somatic nervous system.

Summary of Digestive Processes

• Mouth: Mastication (chewing) occurs; saliva containing amylase breaks down starch.
• Esophagus: Swallowing occurs; delivers chewed food to the stomach.
• Stomach: Upper muscles relax to allow food entry; lower muscles mix food with digestive juices.
  • Stomach acid starts the digestion process, breaking down proteins.
• Small Intestine: Peristalsis occurs; digestive juices break down starch, protein, and carbohydrates, but fats require bile.
• Liver: Produces bile acids to emulsify fats.
• Pancreas: Secretes pancreatic juice to aid in the digestion of fats, proteins, starch and carbohydrate.

Urinary System

• The urinary system excretes waste products and regulates blood volume, acidity, salinity, concentration, and chemical composition.
• Kidneys filter the blood, removing metabolic waste products.
• The renin-angiotensin-aldosterone system controls blood pressure through water reabsorption in the kidneys.
• The urinary system includes the kidneys, ureters, bladder, and urethra.

Components of the Urinary System

• Kidneys: Filter blood, producing urine.
• Ureters: Transport urine from the kidneys to the bladder.
• Bladder: Stores urine until it is released.
  • The autonomic nervous system controls bladder relaxation or constriction.
• Urethra: Conducts urine from the bladder to the outside of the body.

Kidney Function and Waste Removal

• The kidneys filter blood to remove waste products, such as creatinine (a metabolic waste product from muscles).
• High levels of creatinine in the blood indicate kidney malfunction.
• Blood urea nitrogen (BUN) levels also indicate kidney function; high levels suggest kidney problems.

Bladder and Urination

• The bladder is a muscular organ that stores urine.
• Stretch receptors initiate the desire to urinate.
• The urethra conducts urine from the bladder to the outside.

Urinary Tract Infections (UTIs)

• UTIs are often caused by Escherichia coli (E. coli).
• Females are prone to UTIs due to the anatomical proximity of the urethra, vagina, and rectum.
• E. coli, a normal flora of the intestine, can cause infection if it enters the urethra.
• Untreated UTIs can lead to kidney infections (pyelonephritis).
• Antibiotic resistance is a concern in UTI treatment.

Kidney Structures

• Nephrons filter blood in the renal pyramids.
• The kidneys reabsorb vital substances and remove unwanted ones, returning filtered blood to the body.
• 200 quarts of blood are filtered every 24 hours.
• Key structures include:
  • Renal capsule (thin covering).
  • Renal papilla (apex of renal pyramid).
  • Renal pelvis (funnel-shaped tube).
  • Major and minor calyxes.
  • Renal cortex and medulla.
  • Renal column.
  • Renal pyramid.
  • Renal artery and vein.
  • Renal hilus.

Nephrons and Urine Formation

• Each kidney contains over 1 million nephrons, located in the cortex and renal pyramids.
• Nephrons regulate water and substance concentrations in the body by filtering blood, reabsorbing needed substances, and excreting waste.
• Urine—95% water and 5% waste products—may contain urea, creatinine, ammonia, uric acid, dissolved salts, and ions.

Urination Process

• Urine travels from the kidneys down the ureters to the urinary bladder.
• The bladder stretches and expands, triggering micturition (urination), controlled by nervous impulses.
• The internal urethral sphincter is involuntary, while the external urethral sphincter is voluntary.

Types of Nephrons

• Cortical Nephrons: 85% of all nephrons, located primarily in the cortex; responsible for most regulatory functions.
• Juxtamedullary Nephrons: Have long nephron loops extending deep into the medulla; essential for concentrating urine.

Renal Corpuscle and Tubule

• Renal Corpuscle:
  • Blood pressure forces water and solutes out of the glomerular capillaries in a process called filtration.
  • Filtrate is collected in the surrounding capsular space.
• Renal Tubule:
  • A tubular passageway that receives filtrate and modifies to create urine.
  • PCT (proximal convoluted tubule) reabsorbs nutrients from the filtrate
  • Loop of Henle establishes osmotic gradient for water reabsorption
  • DCT (distal convoluted tubule) adjusts tubular fluid composition by reabsorption and secretion

Steps in Urine Formation

• Filtration: Blood pressure forces water and solutes across the membrane of glomerular capillaries into the capsular space.
• Reabsorption: Transport of water and solutes from tubular fluid across the tubular epithelium into peritubular fluid.
• Secretion: Transport of solutes from peritubular fluid across the tubular epithelium into tubular fluid.

Functions of Nephron Components

• Proximal Convoluted Tubule (PCT): Reabsorption of water, ions, and organic nutrients.
• Nephron Loop: Descending thin limb reabsorbs water; thick limb reabsorbs sodium and chloride ions.
• Distal Convoluted Tubule (DCT): Secretion of ions, acids, drugs, toxins, and variable reabsorption of water, sodium, and calcium ions.
• Collecting Duct: Variable reabsorption of water and secretion or reabsorption of sodium, potassium, hydrogen, and bicarbonate ions.

PCT Specific Function

• Active reabsorption of glucose and ions like sodium, potassium, calcium, magnesium, phosphate, and bicarbonate
• Passive reabsorption of urea, chloride lipid-soluble materials, and water
• Secretion of hydrogen or ammoniumions, as well as creatinine, drugs, and toxins

Nephron Loop Specific Function

• Reabsorption of water, sodium, and cloride ions

DCT and Collecting System Specific Function

• DCT:
  • Reabsorption of sodium and chloride ions, along with calcium and water, dependent on the body's needs.
  • Secretion of hydrogen, ammonium, creatinine, and toxins.
• Collecting Systems:
  • Absorption or secretion of sodium, bicarbonate, hydrogen and potasium, dependent on the body's needs.