Renal Physiology and Acid-Base Balance: Study Notes for Anatomy & Physiology

Renal Physiology

Intrinsic Controls: Renal Autoregulation

Renal autoregulation ensures stable glomerular filtration rate (GFR) despite fluctuations in systemic blood pressure. Two main mechanisms are involved:

• Myogenic Mechanism: The smooth muscle in the afferent arteriole responds to changes in blood pressure. Increased pressure causes vasoconstriction, while decreased pressure leads to vasodilation, stabilizing GFR.

• Tubuloglomerular Feedback: The macula densa cells of the juxtaglomerular apparatus detect changes in NaCl concentration in the distal tubule. High NaCl leads to vasoconstriction of the afferent arteriole, reducing GFR; low NaCl causes vasodilation, increasing GFR.

Example: If systemic blood pressure rises, the myogenic mechanism constricts the afferent arteriole to prevent excessive GFR.

Extrinsic Controls: Neural and Hormonal Mechanisms

Extrinsic controls override intrinsic mechanisms during extreme conditions (e.g., severe blood loss):

• Sympathetic Nervous System: Activates vasoconstriction of renal vessels, reducing GFR to conserve blood volume.

• Renin-Angiotensin-Aldosterone Mechanism: Low blood pressure triggers renin release, leading to angiotensin II formation, which constricts arterioles and stimulates aldosterone release, increasing Na+ and water reabsorption.

Key Point: Changes in these controls can increase or decrease GFR, affecting urine output and blood pressure.

Tubular Reabsorption

Reabsorption is the process by which substances are moved from the filtrate back into the blood. There are two main routes:

• Transcellular Route: Substances pass through the tubular cell membranes.

• Paracellular Route: Substances move between tubular cells through tight junctions.

Transport of Sodium: Sodium reabsorption is primarily via primary active transport across the basolateral membrane using the Na+/K+ ATPase pump. Sodium movement creates gradients for secondary active transport of other substances (e.g., glucose, amino acids).

• Co-transport: Substances move in the same direction as Na+ (symport).

• Counter-transport: Substances move in the opposite direction (antiport).

Obligatory vs. Facultative Water Reabsorption:

• Obligatory: Occurs in the proximal tubule and descending limb of the loop of Henle, where water follows solute reabsorption.

• Facultative: Occurs in the collecting duct, regulated by antidiuretic hormone (ADH).

Capillaries Involved: Peritubular capillaries reabsorb substances from the proximal and distal tubules; vasa recta are involved with the loop of Henle.

Permeability: Different segments of the renal tubule have varying permeability to water and solutes. For example, the descending limb is permeable to water but not solutes, while the ascending limb is permeable to solutes but not water.

Hormonal Regulation of Reabsorption

Several hormones regulate reabsorption in specific parts of the renal tubule:

• ADH (Antidiuretic Hormone): Increases water reabsorption in the collecting duct.

• Aldosterone: Increases Na+ reabsorption in the distal tubule and collecting duct.

• ANP (Atrial Natriuretic Peptide): Decreases Na+ reabsorption, increasing urine output.

• Parathyroid Hormone: Increases Ca2+ reabsorption in the distal tubule.

Tubular Secretion

Tubular secretion is the process of moving substances from the blood into the filtrate, occurring mainly in the proximal and distal tubules. It is important for removing excess ions (e.g., K+, H+), metabolic wastes, and drugs, and for regulating acid-base balance.

Regulation of Urine Concentration and Volume

The kidneys regulate urine concentration and volume through countercurrent mechanisms:

• Countercurrent Multiplier: Located in the loop of Henle, it creates an osmotic gradient in the medulla by actively transporting NaCl out of the ascending limb.

• Countercurrent Exchanger: The vasa recta preserves the gradient by exchanging water and solutes without dissipating the medullary osmotic gradient.

Gradient Effects: The multiplier creates the gradient; the exchanger preserves it. The collecting duct uses this gradient to concentrate urine.

Hydration States:

• Overhydrated: ADH secretion decreases, leading to dilute urine.

• Dehydrated: ADH secretion increases, leading to concentrated urine.

Nitrogenous Wastes in Urine

Urine contains several nitrogenous wastes:

• Urea: Produced from amino acid catabolism.

• Uric Acid: Produced from nucleic acid metabolism.

• Creatinine: Produced from creatine phosphate in muscle metabolism.

Urine Transport, Storage, and Elimination

Urine is transported from the kidneys to the bladder via the ureters, stored in the bladder, and eliminated through the urethra.

• Ureter Transport: Urine moves by peristaltic contractions of smooth muscle.

• Urinary Bladder Muscle: The detrusor muscle contracts during urination.

• Sphincters: The internal urethral sphincter (involuntary) and external urethral sphincter (voluntary) control urination.

• Micturition Events: Stretch receptors signal the brain, leading to detrusor contraction and sphincter relaxation for urine release.

Acid-Base Balance

Normal pH of Body Fluids

The normal pH of arterial blood is 7.35–7.45. Deviations indicate:

• Acidosis: pH < 7.35

• Alkalosis: pH > 7.45

Causes of pH Changes

Body pH changes are primarily due to metabolic processes (e.g., CO2 production, lactic acid, ketone bodies) and, to a lesser extent, dietary intake.

Physiological Buffering Systems

The body maintains pH through buffering systems:

• Respiratory System: Regulates CO2 (and thus H+) via changes in ventilation.

• Renal System: Regulates H+ and HCO3- excretion or reabsorption.

Respiratory Regulation of H+

The respiratory system controls blood pH by altering the rate and depth of breathing:

• Increased CO2 (from hypoventilation) lowers pH (acidosis).

• Decreased CO2 (from hyperventilation) raises pH (alkalosis).

Equation:

$\mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \leftrightarrow \mathrm{H}_2\mathrm{CO}_3 \leftrightarrow \mathrm{H}^+ + \mathrm{HCO}_3^-$

Renal Regulation of H+

The kidneys regulate acid-base balance by excreting or reabsorbing H+ and HCO3-:

• Secretion of H+ into the filtrate.

• Reabsorption or generation of HCO3-.

Equation:

$\mathrm{HCO}_3^- + \mathrm{H}^+ \rightarrow \mathrm{H}_2\mathrm{CO}_3 \rightarrow \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O}$

Table: Comparison of Buffering Systems

System	Speed	Main Action
Chemical Buffers	Seconds	Bind/release H+
Respiratory System	Minutes	Removes CO2
Renal System	Hours	Excretes H+, reabsorbs HCO3-

Additional info: Expanded explanations and equations were added for clarity and completeness.