BIOL 310 - Exam 4 Review Notes

Lecture 13: Respiratory Physiology

Introduction to Respiratory System

• Purpose: Obtain O2 for use by body cells and eliminate CO2 produced by the cells.
• Respiration involves two processes:
  • External respiration: Exchange of O2 and CO2 between the external environment and tissue cells.
  • Cellular respiration: Intracellular metabolic processes within mitochondria; uses O2 to derive energy (ATP) from nutrient molecules, producing CO2 as a byproduct.

Ventilation and Airflow

• Ventilation: Exchange of air between the atmosphere and alveoli.
• Air moves from a region of high pressure to an area of low pressure.
• $F = frac{ΔP}{R}$, where:
  • F = Flow
  • ΔP = Pressure difference
  • R = Resistance
• Flow (F) is directly proportional to the pressure difference (ΔP) and inversely proportional to resistance (R).
• Relevant pressures:
  • Alveolar/intrapulmonary pressure (Palv)
  • Atmospheric pressure (Patm)
• $F = frac{(P<em>{alve} – P</em>{atm})}{R}$
  • Palv < Patm: Air flows into the lungs.
  • Palv > Patm: Air flows out of the lungs.
• Altering the volume of the lungs changes Palv.
• Boyle’s law: $P = frac{1}{V}$ (Pressure is inversely proportional to Volume).
• Respiratory muscle activity changes the volume of the thoracic cavity, leading to changes in lung volumes.
• Pressure change leads to airflow until pressures are equal.

Inspiration

• During inspiration:
  • Muscles of the chest wall contract, causing the chest wall to expand.
  • The diaphragm contracts downward, further enlarging the thoracic cavity.
  • As the volume of the thoracic cavity increases:
    • Pip (intrapleural pressure) decreases.
    • Palv (alveolar pressure) becomes more negative compared to Patm; Palv < Patm.
    • Ptp (transpulmonary pressure) becomes more positive, and the lungs expand; $Ptp = Palv - Pip$.
  • Air flows inward (inspiration).

Lung Compliance

• Ability of lungs to stretch; inverse of stiffness.
• Change in lung volume for a change in Ptp.
  • Decreased compliance (increased lung stiffness):
    • Less increase in lung volume for any given increase in Ptp.
    • More work is required to inflate lungs.
    • Examples: pulmonary fibrosis, pneumonia.
  • Increased compliance:
    • Easier to expand lungs at any given change in Ptp.
    • Decreased elastic recoil.
    • Example: emphysema.

Lung Compliance and Surfactant

• Two major determinants of lung compliance:
  1. Stretchability of elastin fibers in lung tissues (elastic recoil).
     • Thickening of lung tissues decreases lung compliance.
  2. Surface tension within alveoli.
     • Greater alveolar surface tension = less compliant lungs.
• Type II alveolar cells secrete surfactant:
  • A mixture of phospholipids and proteins.
  • Lowers surface tension by interspersing between water molecules in the lining of alveoli.
  • Increases lung compliance, reducing the work of inflating lungs.
  • Reduces the lungs’ tendency to recoil.
  • Production in fetal lung occurs in late gestation; stimulated by an increase in cortisol (glucocorticoid) secretion.

Clinical Application

• Surfactant deficiency leads to Newborn Respiratory Distress Syndrome:
  • A leading cause of death in premature infants.
  • Surfactant-synthesizing cells may be too immature to function adequately.
  • Low lung compliance causes infants to make strenuous inspiratory efforts to overcome high surface tension, leading to complete exhaustion, inability to breathe, lung collapse, and death.

Respiratory Performance and Volume Relationships

• Pulmonary volumes:
  • Tidal volume (VT): Volume of air moved into or out of the lungs in a breath.
  • Inspiratory reserve volume (IRV): Additional amount of air that can be inhaled.
  • Expiratory reserve volume (ERV): Additional amount of air capable of being exhaled.
  • Residual volume: Volume of air in the lungs after maximal exhalation.
  • Minimal volume: Volume of air in a collapsed lung.

Effects of Various Conditions on Alveolar Gas Pressures

Gas Transport

• Blood plasma cannot transport enough O2 or CO2 to meet physiological needs; hence, red blood cells (RBCs) are used.
• RBCs transport O2 to and CO2 from peripheral tissues.
• Remove O2 and CO2 from plasma, allowing gases to continue to diffuse into blood.
• $O_2$ transport:
  • O2 binds to iron atoms on heme groups in hemoglobin (Hb) molecules in a reversible reaction.
  • Forms oxyhemoglobin (HbO2).

Hb and pH

• Bohr effect = effects of CO2 due to tissue metabolism.
• CO2 diffuses into RBCs.
• Enzyme carbonic anhydrase catalyzes reaction with H2O, producing carbonic acid (H2CO3).
• H2CO3 dissociates into H+ and bicarbonate ion (HCO3 –).
• H+ diffuses out of RBC, lowering pH.
• The rate of carbonic acid formation depends on the amount of CO2 in the solution.
  • PCO2 increases:
    • Rate of carbonic acid formation accelerates.
    • More H+ diffuses out of RBCs, decreasing plasma pH.
    • More O2 is released; the curve shifts to the right.
  • PCO2 decreases:
    • H+ diffuses into RBCs, increasing plasma pH.
    • Less O2 is released; the curve shifts to the left.

Hb and 2,3-DPG

• RBCs generate ATP by glycolysis, forming lactate and 2,3-di-/bis-phosphoglycerate (2,3-DPG/BPG).
• During hypoxia:
  • 2,3-DPG directly affects O2 binding and release.
  • More 2,3-DPG, more oxygen is released; the curve shifts to the right.
  • Increase in 2,3-DPG due to:
    • High blood pH
    • Thyroid hormones, GH, epinephrine, and androgens.

Oxygen Release

• Left shift = Less O2 released
• Right shift = More O2 released
• CADET, face right and release:
  • $↑CO_2$
  • Acidity ($↓pH$)
  • $↑2,3- DPG$
  • Exercise
  • $↑Temperature$

$CO_2$ Transport

• Generated as a byproduct of cellular respiration.
• CO2 in the bloodstream is carried three ways:
  1. 70% transported as bicarbonate ions (HCO3 –):
     • Most CO2 entering the bloodstream diffuses into RBCs.
     • CO2 is converted to carbonic acid (H2CO3), which dissociates into H+ and bicarbonate (HCO3 –).
     • H+ binds to hemoglobin in RBCs.
     • In tissue cells, bicarbonate ions move into plasma in exchange for Cl– ions (chloride shift).
  2. 23% bound to protein portions of Hb molecules in RBCs, forming carbaminohemoglobin.
  3. 7% transported as dissolved gas molecules in plasma.

Local Control of Respiration

• When peripheral tissues become more active, PO2 decreases and PCO2 increases.
• Decreased PO2 levels in tissues cause vasodilation, increasing blood flow.
• More O2 is delivered, and more CO2 is carried away.
• Ventilation-to-perfusion ratio (V/Q ratio):
  • Matches lung perfusion (blood flow to alveoli) with alveolar ventilation (airflow).
  • If blood flow > airflow to alveoli: ↓O2 in alveoli → vasoconstriction of pulmonary arteriole; decreases blood flow to match smaller airflow; blood is shifted to alveoli with high PO 2.
  • If airflow > blood flow to alveoli: ↑O2 in alveoli → vasodilation of pulmonary arteriole; increases blood flow to match large airflow; promoting blood flow to ventilated alveoli.

Local Control of Respiration (Bronchioles)

• Smooth muscles in bronchiole walls are sensitive to PCO2 levels, controlling bronchoconstriction and bronchodilation.
  • ↑ PCO2 → bronchodilation (smooth muscle relaxation) due to little airflow to alveolus.
  • ↓ PCO2 → bronchoconstriction (smooth muscle contraction) due to too much air to alveolus.
• Local adjustments improve the efficiency of gas transport:
  • Directs blood flow to alveoli with low CO2 levels.
  • Increases airflow to alveoli with high CO2 levels.

Effects of Local Changes in O2

Chemoreceptors

• Respiratory centers are strongly influenced by chemoreceptor input from:
  • Glossopharyngeal nerve (CN IX): Stimulated by changes in blood pH or PCO2 at carotid bodies.
• Vagus nerve (CN X): Stimulated by changes in blood pH or PCO2 at aortic bodies.
• Chemoreceptor stimulation leads to an increased depth and rate of respiration due to:
  • Decreased PO2 (hypoxia).
• Increased PCO2 (respiratory acidosis).
• Any condition altering the pH of blood because chemoreceptors monitoring CO2 levels are also sensitive to pH.
• An increase in lactic acid levels after exercise causes a decrease in pH that stimulates respiratory activity (metabolic acidosis).

Chemoreceptor Response to Changes in PCO2

• Hypercapnia:
  • An increase in arterial PCO2 caused by low RR (hypoventilation).
  • Chemoreceptors stimulate respiratory centers to increase the rate and depth of respiration.
• Hypocapnia:
  • Abnormally low PCO2 caused by high RR (hyperventilation).
• Chemoreceptor activity decreases, and RR falls.

Control of Respiration

• Respiratory rhythmicity centers: In the medulla oblongata, set the pace (rate and rhythm) of respiration.
• Each center is divided into:
  1. Dorsal respiratory group (DRG):
     • Inspiratory center.
     • Functions in quiet and forced breathing.
     • In quiet breathing: Brief activity in DRG stimulates inspiratory muscles; DRG neurons become inactive, allowing passive exhalation.
  2. Ventral respiratory group (VRG):
     • Inspiratory and expiratory center.
     • Functions only in forced breathing.
     • Increased activity in DRG stimulates VRG, activating accessory inspiratory muscles; expiratory center neurons stimulate active exhalation.

Lecture 14: Digestive System

Histology of GIT

• Mucosa:
  • Inner lining; the superficial layer exposed at the lumen of the digestive tract.
• Submucosa:
  • Contains submucosal plexus (Meissner’s plexus).
  • Contains sensory neurons, parasympathetic ganglionic neurons, and sympathetic postganglionic fibers.
  • Innervates mucosa and submucosa.
• Muscular layer:
  • Dominated by smooth muscle cells arranged in inner circular and outer longitudinal layers (like smooth muscle cells in muscularis mucosa).
  • Essential role in mechanical digestion and in moving materials along GIT.
  • Movements coordinated by the enteric nervous system (ENS).
  • Innervated primarily by the parasympathetic nervous system.
  • Myenteric plexus (Auerbach’s plexus):
    • Between circular and longitudinal muscle layers.
    • A network of parasympathetic ganglia, sensory neurons, interneurons, and sympathetic postganglionic fibers.
• Serosa:
  • Formed by visceral peritoneum.

Movement of Food Along GIT

• Peristalsis:
  • Waves of muscular contractions by the muscular layer that move a bolus along the length of GIT.
    1. Circular muscles contract behind bolus, while circular muscles ahead of bolus relax.
    2. Longitudinal muscles ahead of bolus contract, shortening adjacent segments.
    3. The wave of contraction in circular muscles forces bolus forward.
• Segmentation:
  • Cycles of contraction that churn and fragment bolus, mixing contents with intestinal secretions.
  • Does not follow a set pattern and does not push materials in any one direction.

Salivary Glands

• Three pairs secrete into the oral cavity:
  1. Parotid salivary glands:
     • Inferior to the zygomatic arch, anterior to the ear between the masseter muscle.
     • Produce serous secretion with salivary amylase (breaks down starches).
     • Drained by the parotid duct, which empties into the vestibule at the second molar.
  2. Sublingual salivary glands:
     • Covered by mucous membrane of the floor of the mouth.
     • Produce mucous secretion that acts as a buffer and lubricant.
     • Sublingual ducts open along the base of the tongue.
  3. Submandibular salivary glands:
     • In the floor of the mouth (lie along inner surfaces of the mandible within the mandibular groove).
     • Secrete buffers, glycoproteins (mucins), and salivary amylase, producing the majority of saliva.
     • Submandibular ducts open on each side of the frenulum of the tongue immediately posterior to teeth.

Gastrointestinal Tract (GIT)

• Pharynx:
  • Common passageway for food, liquids, and air.
  • Regions: nasopharynx, oropharynx, laryngopharynx.
  • Food passes through the oropharynx and laryngopharynx to the esophagus.
• Esophagus:
  • A hollow muscular tube that conveys solid food and liquids to the stomach.
  • Begins posterior to the cricoid cartilage and enters the abdominopelvic cavity through the esophageal hiatus (opening in the diaphragm).

Swallowing

• Deglutition:
  • Initiated voluntarily but proceeds automatically.
  • Divided into three phases:
    1. Buccal (oral) phase:
       • Voluntary phase where bolus is forced into the oropharynx.
    2. Pharyngeal phase:
       • An involuntary phase that stimulates tactile receptors to send signals to the swallowing center in the medulla oblongata.
       • Results in the movement of food to the esophagus.
    3. Esophageal phase:
       • An involuntary phase.
       • The upper esophageal sphincter contracts to move the bolus deeper into the esophagus.
       • Peristaltic contractions then move bolus into the stomach in <8s.

Stomach - Histology

• Gastric glands:
  • In the fundus and body of the stomach.
  • Two types of secretory cells that secrete gastric juice:
    • Parietal cells secrete:
      • Intrinsic factor: Helps absorb vitamin B12 across the intestinal lining (essential for normal erythropoiesis).
      • HCl (hydrochloric acid): Keeps stomach contents in a highly acidic environment.
    • Chief cells:
      • Most abundant near the base of the gastric gland.
      • Secrete pepsinogen (inactive proenzyme) converted by HCl in the gastric lumen to pepsin, a protein-digesting enzyme.

Stomach - Hormones

• Enteroendocrine cells scattered among mucus-secreting cells produce hormones:
  • G cells produce gastrin:
    • Stimulates secretion of gastric acid by parietal cells.
    • Stimulates contraction of the gastric wall to mix and stir gastric contents (gastric motility).
    • Also located in the duodenum of the small intestine.
    • Its secretion is stimulated by the entry of food into the stomach and distension of the stomach.
• D cells release somatostatin:
  • Inhibits the release of gastrin.

Stomach - Phases

• Control of gastric secretion involves three phases:
  1. Cephalic phase
  2. Gastric phase
  3. Intestinal phase

Cephalic Phase

• Begins when you see, smell, taste, or think of food and prepares the stomach to receive food.
• Efferent pathways for these reflexes are mediated by parasympathetics.
• The vagus nerve innervates the submucosal plexus of the stomach.
• Next, postganglionic parasympathetic fibers innervate mucous, chief, parietal, and G cells of the stomach.
• Affects secretory and contractile activity.

Gastric Phase

• Begins with the arrival of food in the stomach.
• Stimuli that initiate the gastric phase:
  • Distension of the stomach.
  • Increase the pH of gastric contents.
  • The presence of undigested material in the stomach, especially proteins and peptides.
• Local response:
  • Distension of the wall stimulates the release of histamine, which binds to receptors (H2) on parietal cells and stimulates acid production.
• Neural response:
  • Stimulation of stretch receptors and chemoreceptors triggers short reflexes in the submucosal and myenteric plexuses.
  • Activates the stomach’s secretory cells.
    • Stimulation of the myenteric plexus produces powerful contractions = mixing waves in the muscular layer.

Intestinal Phase

• Begins when chyme first enters the small intestine.
• Function: Control the rate of gastric emptying to ensure efficient secretion, digestion, and absorption in the small intestine.
• Chyme leaving the stomach decreases distension of the stomach, reducing stimulation of stretch receptors.
• The arrival of chyme in the duodenum causes:
  • Distension of the duodenum, leading to the contraction of the pyloric sphincter, prevents further discharge of chyme.
  • Triggers hormonal responses: the arrival of lipids and carbohydrates leads to cholecystokinin (CCK) and gastric inhibitory peptide (GIP) secretion, and a decrease in pH leads to secretin secretion.

Small Intestine - Motility

• After chyme arrives in the duodenum, weak peristaltic contractions move it slowly toward the jejunum.
• Contractions are myenteric reflexes that are not under CNS control.
• Parasympathetic stimulation accelerates local peristalsis and segmentation.
• The migrating motility complex sweeps the intestine clean between meals.
• Reflexes of the small intestine:
  • Gastroenteric reflex: Stimulates motility and secretion along entire small intestine.
  • Gastroileal reflex: Triggers the opening of the ileocecal valve, allowing materials to pass from the small intestine into the large intestine.
  • Enterogastric reflex: Causes constriction of the pyloric sphincter and inhibits gastric motility and secretion.

Pancreas

• Lies posterior to the stomach and extends from the duodenum toward the spleen.
• Functions:
  • Endocrine cells of pancreatic islets secrete insulin and glucagon into bloodstream.
  • Exocrine cells (acinar cells and epithelial cells) of the duct system secrete pancreatic enzymes.
• Pancreatic secretions are controlled by hormones from the duodenum:
  • Secretin: Triggers pancreatic secretion of a watery alkaline buffer solution (with HCO3 -) to neutralize the highly acidic chyme in the small intestine.
  • Cholecystokinin (CCK): Stimulates the production and secretion of pancreatic enzymes.

Liver - Physiology

• Metabolic Regulation:
  1. Composition of circulating blood
  2. Nutrient metabolism (carbohydrate, lipid, and amino acid)
  3. Waste product removal
  4. Nutrient storage
  5. Drug inactivation
• Hematological Regulation
  • The largest blood reservoir in the body and receives 25% of CO.
    1. Phagocytosis and antigen presentation
    2. Synthesis of plasma proteins
    3. Removal of circulating hormones
    4. Removal of antibodies
    5. Removal or storage of toxins
    6. Synthesis and secretion of bile

Liver - Functions of Bile

• Bile contains:
  • $HCO_3$: Neutralizes acid from the stomach
  • Cholesterol
  • Phospholipids
  • Bile pigments
  • Organic wastes
  • Bile salts: Solubilize dietary fat and break fat globules apart (emulsification); creates tiny emulsion droplets coated with bile salts and phospholipids, which increases the surface area exposing them to enzymatic attack; bile salts and pancreatic lipase digest emulsion droplets into micelles in the small intestine.

Gallbladder

• The gallbladder stores and concentrates bile.
• Releases bile into the cystic duct, which extends to join the common hepatic duct and forms the CBD, where it travels into the duodenum.
• Under the stimulation of the intestinal hormone cholecystokinin (CCK):
  • CCK is secreted when chyme arrives in the duodenum, causing the hepatopancreatic sphincter to relax and the gallbladder to contract, resulting in the ejection of bile into the duodenum.

Activities of Major Digestive Tract Hormones

Large Intestine

• Extends from the end of the ileum to the anus.
• Sections: cecum, colon, appendix, rectum.
• Functions:
  • Reabsorption of water: The primary absorptive process is the active transport of Na+ from the lumen to the ECF with accompanying osmotic absorption of water.
  • Compaction of intestinal contents into feces.
  • Absorption of vitamins produced by bacteria:
    1. Vitamin K (fat-soluble): Role in coagulation; required by the liver for synthesizing clotting factors, including prothrombin.
    2. Biotin (water-soluble): Important in glucose metabolism.
    3. Vitamin B5 (pantothenic acid) (water-soluble): Required in the manufacture of steroid hormones and some neurotransmitters.
  • Storage of fecal material prior to defecation.

Large Intestine - Movements

• Movements:
  • Gastroileal and gastroenteric reflexes move materials into the cecum while you eat.
  • Peristaltic waves move material along the length of the colon.
  • Segmentation movements (haustral churning) mix the contents of adjacent haustra.
  • Movement from the ascending and transverse colon through the rest of the large intestine results from powerful peristaltic contractions (mass movements).
  • The stimulus is the distension of the stomach and duodenum; relayed over intestinal nerve plexuses.
  • Distension of the rectal wall triggers the defecation reflex:
    • Two positive feedback loops, both loops are triggered by stretch receptors in the rectum, causing relaxation of internal and external anal sphincters.

Lecture 15: Renal Physiology

Functions of the Kidneys

• Maintain H2O balance.
  • Maintain proper osmolarity of body fluids, primarily through regulating H2O balance.
  • Regulate the quantity and concentration of most ECF ions.
  • Maintain proper plasma volume.
  • Help maintain proper acid-base balance.
  • Removal of metabolic waste products from blood and their excretion in urine.
  • Removal of foreign chemicals from blood and their excretion in the urine.
  • Production of hormones/enzymes:
    • Erythropoietin: controls erythrocyte production
    • Renin: Enzyme that controls the formation of angiotensin, which influences blood pressure and sodium balance.
    • Conversion of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol) which increases calcium absorption by intestines.

Juxtaglomerular Complex (JGC)

• Helps regulate blood pressure and filtrate formation.
• Consists of:
  • Macula densa:
    • Epithelial cells in the wall of the DCT.
    • Located on the tubule side next to the afferent arteriole.
    • Function as chemoreceptors or baroreceptors Detect changes in NaCl concentration of fluid in DCT.
  • Juxtaglomerular cells:
    • Smooth muscle cells in the wall of the afferent arteriole near the entrance to the renal corpuscle.
    • Function as baroreceptors.
    • Contract when stimulated by stretch or sympathetic stimulation.
    • Secrete renin.
  • Extraglomerular mesangial cells:
    • Located between afferent and efferent arterioles, outside the glomerulus.
    • Provide feedback control.

Renal Physiology

1. Glomerular filtration
   • Moves substances from the glomerulus into Bowman's capsule.
2. Tubular secretion
   • Transports substances from peritubular capillaries into renal tubules.
3. Tubular reabsorption
   • Moves substances from renal tubules into peritubular capillaries.

• Basic renal processes and urine excretion:
  • Amount excreted = amount filtered + amount secreted – amount reabsorbed.

Glomerular Filtration

• Filtration is driven by glomerular capillary hydrostatic (blood) pressure.
  • Force of water and all low molecular weight substances through a filtration membrane.
• Reabsorption is driven by osmotic force due to plasma proteins.
  • Blood colloid osmotic pressure: Pulls water back into glomerulus.
• Blood cells (hematuria) or protein (proteinuria) in urine indicate potential problems with the glomerular filtration barrier.

Glomerular Filtration Pressures

• Glomerular hydrostatic pressure (GHP)
  • BP in glomerulus which pushes water and solutes out of blood (glomerulus) into filtrate
• Blood leaving glomerulus flows into efferent arteriole
  • Efferent arteriole diameter is smaller than that of afferent arteriole
• Capsular hydrostatic pressure (CsHP)
  • Pressure in Bowman’s capsule: Results from resistance of filtrate already in the nephron and collecting system
  • Opposes glomerular hydrostatic pressure and favors reabsorption i.e. pushes water and solutes out of filtrate into blood
• Net hydrostatic pressure (NHP)
  • Difference between glomerular hydrostatic pressure and capsular hydrostatic pressure
  • $GHP – CsHP = NHP$

Glomerular Filtration Pressures(Cont.)

• Blood colloid osmotic pressure
  • Osmotic pressure resulting from suspended proteins in blood which opposes filtration i.e. pulls fluid back into glomerulus
• Net filtration pressure (NFP)
  • Difference between net hydrostatic pressure and blood colloid osmotic pressure in glomerulus
  • $NHP – BCOP = NFP$

Autoregulation of GFR

• Maintains adequate GFR despite changes in systemic BP and blood flow.
• Involves changing luminal diameters of:
  • Afferent arterioles
  • Efferent arterioles
• Ex: ↓BP = ↓GFR = less stretch of smooth muscle in arteriole
• Smooth muscle cells relax = vasodilation of afferent arteriole AND constriction of efferent arteriole More blood into glomerulus Increases GFR back to normal
• Note the changes in GFR due to changes in arteriole diameter.

Hormonal Regulation of GFR

• Renin-angiotensin-aldosterone system (RAAS)
  • Renin converts inactive angiotensinogen to inactive angiotensin I, Angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE) primarily in capillaries of lungs
• Effects of Angiotension II
  • Vasoconstriction
  • ↑ Aldosterone secretion, which increases Na+ reabsorption and K+ secretion in DCT and CD
  • ↑ production of ADH
• Overall effect
  • Increases in systemic BP and blood volume, and restoration of normal GFR.

Hormonal Regulation of GFR via RAAS(Cont.)

• Three stimuli cause JGC to release renin:
  1. ↓BP at glomerulus due to:
     • ↓blood volume
     • ↓systemic pressures
     • Blockage in renal artery
  2. Stimulation of juxtaglomerular cells by sympathetic innervation
  3. ↓ osmotic concentration of tubular fluid at macula densa due to ↓NaCl in tubular fluid at DCT.

Regulation of GFR

• Autonomic regulation of GFR
  • Sympathetic activation leads to Vasoconstriction of afferent arterioles and a Decreased GFR leading to decreased filtrate production.
  • Sympathetic activation can override local regulatory mechanisms to stabilize GFR During exercise or stressful conditions/emergency.
• Juxtaglomerular cells release renin angiotensin II production and contraction of mesangial cells
  • Mesangial cell contraction causes decreased surface area of the glomerulus, resulting in the decreased GFR
    Body conserves water and shunts blood away from kidneys to perfuse tissue and organs with higher needs

Tubular Reabsorption of Na+

• Most abundant cation in filtrate
• Active process occurring in all tubular segments except descending limb of nephron loop
• Basolateral Na+–K+ pump (primary active transport) actively transports Na+ from tubular cell into interstitial fluid within lateral space
  * Establishes concentration gradient for passive movement of Na+ from lumen into tubular cell and from lateral space into peritubular capillary
• Reabsorption of substances occurs via cotransport with Na+
  * Cotransported substance moves uphill and Na+ moves downhill into cell via a secondary active cotransporter Glucose and amino acids Glucose via facilitated diffusion transports glucose from tubular cell into interstitial fluid

Tubular Secretion

• Substances transferred from peritubular capillaries into tubular lumen H+ secretion.
• Secreted by PCT, DCT, and CD Regulates acid-base balance.
• When body fluids are too acidic, H+ secretion increases.
• K+ secretion
  • Secreted by DCT and CD Coupled to Na reabsorption by basolateral Na+-K+ pump
  • Resulting high intracellular K+ concentration favors net movement of K+ from cells into tubular lumen via passive K+ leak channels.
• Controlled by:
  • Aldosterone, H+ secretion
  • ↑ secretion of K+ or H+ is accompanied by ↓ secretion of other Importance of regulating plasma K+ concentration Changes in concentration changes resting membrane potential and reduces cardiac muscle excitability cardiac arrhythmia.

Buffering of H+ in Body

• Respiratory system and kidneys work together to balance H+ concentration

  • Respiratory response is rapid (minutes) and Renal response is slow (hours-days)

• Kidneys eliminate or replenish H+ from body by altering plasma HCO3 - concentration
  $↓$ plasma $HCO_3$ concentration (alkalosis) kidneys’ homeostatic response to excrete large quantities of HCO3 $↑$ plasma HCO3 concentration toward normal

  • ↑ plasma concentration (acidosis) kidney tubular cells produce new and secrete into plasma or reabsorbing filtered $HCO_3$ concentration toward normal

Vertical Osmotic Gradient

• Concentration of interstitial fluid (osmolarity) progressively increases from cortex to medulla
  • Results in osmotic gradient between tubular lumen and interstitial fluid Used by CD to concentrate tubular fluid so that a more concentrated urine is excreted Established by countercurrent multiplication in nephron loop
    • Countercurrent = flow in limbs of nephron loop moves in opposite direction. Multiplication

Concentrating effect multiplied due to countercurrent flow Ascending limb Permeable to NaCl hypertonic medullary interstitial fluid Impermeable to water Descending limb Responds by diffusion of $H_20$ into interstitial fluid Impermeable to NaCI

Vasopressin/ADH

For H2O absorption to occur across a segment of tubule, 2 criteria must be met:

1. Osmotic gradient must exist across tubule
2. Tubular segment must be permeable to H2O DCT and CD are impermeable to H2O except in the presence of vasopressin

• Binds to V2 receptors on DCT and CD cAMP activation promotes insertion of aquaporins H2O reabsorbed H2O passively leaves cells down osmotic gradient to enter interstitial fluid

Dilute Urine

• H2O deficit:
  • ↑blood osm