Biology Chapter on Digestion and Absorption

Human Physiology Test #4 Review

• Nephron Structure:
  • Some nephrons extend deep into the medulla.
  • The cortex contains Bowman's capsules, proximal, and distal tubules.
  • The medulla contains loops of Henle and collecting ducts.
  • One nephron has two arterioles and two sets of capillaries forming a portal system (afferent and efferent arterioles, glomerulus, peritubular capillaries, and vasa recta).
  • The juxtaglomerular apparatus is also part of the nephron.

Glomerular Filtration Rate (GFR)

• Filtration Pressure:

  • GFR is determined by the balance of hydrostatic pressure (blood pressure) and colloid osmotic pressure, and capsule fluid pressure.
  • Filtration pressure depends on blood pressure and opposes colloid osmotic pressure and capsule fluid pressure, with the difference being the net filtration pressure.

• GFR Regulation:

  • GFR is relatively constant at a blood pressure of 80-180 mm Hg.
  • GFR is regulated primarily by altering blood flow through the renal arterioles.
  • Increased resistance in the afferent arteriole decreases GFR.
  • Increased resistance in the efferent arteriole increases GFR.
  • Decreased resistance in the afferent arteriole increases GFR, and decreases in the efferent arteriole decreases GFR.

Reabsorption

• Types of Reabsorption:

  • Reabsorption can be active or passive.
  • Transcellular transport involves substances crossing the apical and basolateral membranes of the tubule epithelial cells.
  • Paracellular transport involves substances passing through cell-cell junctions between adjacent cells.

• Active Transport of Na+:

  • Creates an electrical gradient.
  • Anions follow Na+ which creates an osmotic gradient.
  • Water follows, leaving behind a higher concentration of cations.
  • Cations move down concentration gradients.
  • Exchangers (NHE) and pumps (Na+-K+-ATPase) are involved.

Saturation of Renal Transport

• Saturation:
  • Saturation is the maximum transport rate when all carriers are occupied by substrate.
• Transport Maximum (Tm):
  • The transport rate at saturation.
• Renal Threshold:
  • The plasma concentration of a substance at which it first appears in urine.
• Glucose and other related factors:
  • Glucose in urine is also called glucosuria or glycosuria.
  • Peritubular capillary pressures favor reabsorption.

Secretion

• Definition:
  • Active movement of molecules from the extracellular fluid into the nephron lumen.
• Importance:
  • Secretion is important in homeostasis, specifically for K+ and H+.
  • Increasing secretion increases nephron excretion.

Excretion

• Definition:
  • Excretion is the sum of filtration minus reabsorption, plus secretion in the kidneys.
• Clearance:
  • Clearance is the rate at which a solute disappears from the body by excretion or metabolism.
  • Clearance of X = excretion rate of X/[X]plasma, expressed as ml plasma/min cleared of X.
• Key Substances:
  • Inulin is a plant polysaccharide that freely filters but neither reabsorbed or secreted and used to measure GFR
  • Creatinine is a breakdown product of phosphocreatine, its production and breakdown are relatively constant, but some secretion occurs in the urine.

Glomerular Filtration, Reabsorption and Excretion

• Renal Handling: how a substance is dealt with by the kidneys.
• Relationship: how filtration, reabsorption, and excretion of a substance relate to each other.
• Inulin clearance is equal to GFR. Glucose is normally completely reabsorbed.

Volume and Pressure Regulation

• Responses to Decreased/Elevated Volume and Pressure
  • Volume receptors in the atria and carotid/aortic baroreceptors stimulate homeostatic reflexes.
  • Cardiovascular system responds with changes in cardiac output and vasoconstriction or vasodilation, in order to stabilize blood volume and pressure.
  • Kidneys respond by excreting salts and water to adjust ECF volume.
  • Thirst and water consumption are also triggered.

Osmolarity Changes Through the Nephron

• Fluid Flow Changes: Fluid progressively becomes more concentrated as it passes through the descending limb of the loop of Henle, and into the collecting duct.
• Regulation: Variable reabsorption of water and solutes in the nephron sections are under hormonal control.

Vasopressin in Water Permeability

• Actions: Vasopressin leads to insertion of water pores (aquaporins) into the apical membrane of the collecting duct cells.
• Mechanism: Vasopressin binds to receptors, activating a cAMP second messenger system and triggering the insertion of aquaporins.

Renin-Angiotensin Pathway

• Renin-Angiotensin System (RAS): Controls blood pressure.
• Juxtaglomerular Cells: Secrete renin when blood pressure falls.
• Renin's Action: Converts angiotensinogen into angiotensin I.
• ACE (Converting Enzyme): Converts angiotensin I to angiotensin II.
• Angiotensin II's Role: Potent vasoconstrictor; raises blood pressure. This leads to a cascade of responses.

Aldosterone

• Primary Action: Aids in renal sodium reabsorption.
• Stimuli: RAS pathway, increases in potassium concentration, and possibly other factors.
• Regulation: It's released by the adrenal cortex in response to factors like potassium concentration, blood pressure.

Natriuretic Peptides

• ANP and BNP: Peptides produced and released in response to elevated blood volume or stretch of heart chambers.
• Effects: Influence kidney function by promoting sodium and water excretion, reduce blood volume, pressure.

Acid-Base Disturbances & Intercalated Cells

• Intercalated cells: specialized cells in the collecting duct that help maintain acid-base balance. (Type A and Type B)
• Acidosis: Type A intercalated cells actively excrete hydrogen ions.
• Alkalosis: Type B intercalated cells actively excrete bicarbonate ions.

Diabetes Mellitus

• Diabetes Mellitus: Characterized by high plasma glucose.
• Types: Type 1 (insulin deficiency), Type 2 (insulin resistance).
• Complications: Affect blood vessels, eyes, kidneys.
• Diagnosing Diabetes: Blood glucose measurements (fasting, glucose tolerance tests).

Metabolism

• Anabolism: Constructing larger molecules from smaller ones.
• Catabolism: Breaking larger molecules into smaller ones.
• Fed State/Absorptive State: After eating, glucose and other nutrients are absorbed and used.
• Fasting State/Postabsorptive State: In the absence of food, the body breaks down stored nutrients to meet its energy demands.

Fasted State Metabolism

• Glycogenolysis: Breakdown of glycogen (stored glucose) to release glucose.
• Lipolysis: Breakdown of triglycerides to produce fatty acids and glycerol.
• Protein Breakdown: Deamination to produce amino acids and energy.
• Acetyl CoA: Important intermediate in ATP production.
• Ketone Bodies: Produced when acetyl CoA can not be fully processed.

Digestion, Absorption: Fats, Carbohydrates & Proteins

• Fats: Bile salts emulsify fats, pancreatic lipase breaks down fats, absorbed as monoglycerides/fatty acids and chylomicrons.
• Carbohydrates: Starch/glycogen broken into monosaccharides by amylase and/or specific enzymes, absorbed into the blood.
• Proteins: Endopeptidases act at internal peptide bonds, exopeptidases at terminal peptide bonds. Absorbed as amino acids following digestion.

Energy Storage

• Fat and Glycogen: Provide energy storage in the body.
• Glycogen: The body's readily available energy source.
• Fat: Long-term energy storage, contains more energy, but slower to access.