Renal Physiology and Functions

Renal Physiology

• The urinary system consists of two kidneys, two ureters, a urinary bladder, and a urethra.
• Kidneys have excretory, homeostatic, and endocrine functions.
• Excretory functions include removing excess water, drugs, toxins, urea, creatinine, uric acid, sodium, potassium, and hydrogen or bicarbonate ions.
• Homeostatic functions include acid-base regulation and water balance.
• Endocrine functions include producing renin and angiotensin to regulate arterial blood pressure and synthesize erythropoietin, as well as activating Vitamin D3.
• The functional unit of the kidney is the nephron.
• Nephrons are categorized as cortical or juxtamedullary.
• Cortical nephrons account for 85% of all nephrons, having glomeruli located in the outer two-thirds of the cortex and are responsible for maintaining homeostasis and regulating sodium.
• Juxtamedullary nephrons account for 15% of all nephrons and have glomeruli located near the medulla, concentrating urine.

Learning Objectives

• Students will be able to state the functions of the kidney.
• Students will be able to describe the mechanism of urine formation.
• Students will be able to define glomerular filtration rate.
• Students will be able to discuss the role of the kidney in water balance and acid-base regulation.

Urine Formation

• Kidneys filter excess water, ions, chemicals, and drugs.
• The least amount of urine to accomplish this is 400 ml per day.
• Urine formation is a three-step process:
  • Filtration of plasma by glomerular capillaries, removing plasma proteins. All substances need to be filtered before elimination.
  • Tubular reabsorption and secretion modify the glomerular filtrate. Essential elements (water, sodium, potassium, calcium, phosphate, glucose, and amino acids) are reabsorbed, while waste products (urea, hydrogen ions, chemicals, and drugs) are excreted.
• Glomerular filtration rate (GFR) is the volume of fluid filtered by all glomerular capillaries per minute.
• Normal GFR for a healthy young adult is 125 ml/min.
• Forces involved in glomerular filtration include capillary hydrostatic pressure (filtering force), osmotic pressure of plasma (reabsorbing force), and Bowman's capsule hydrostatic pressure (reabsorbing force).

Kidney Functions

• Renin-angiotensin pathway: The kidney produces renin, an enzyme, that initiates a cascade resulting in angiotensin II, which is crucial for regulating blood pressure.
• Erythropoietin: A hormone produced by the kidneys that stimulates red blood cell production in the bone marrow.
• Active Vitamin D3: The kidneys activate Vitamin D3, enabling calcium absorption in the intestines.

Glomerular Filtration Rate (GFR)

• GFR is the rate at which fluid is filtered through the glomeruli.
• A healthy young adult has a GFR of 125 ml/min.
• GFR declines with age.
• Forces involved in GFR are capillary hydrostatic pressure, osmotic pressure of plasma in Bowman's capsule, and hydrostatic pressure in Bowman's capsule.

Regulation of Hydrogen Ion Concentration

• Hydrogen ion (H+) concentration affects pH.
• A pH scale measures acidity, with 7 being neutral.
• Body fluids have a range of normal pH.
• Sources of acid production include volatile acids (CO2), fixed acids (sulfuric and phosphoric acids), and organic acids (lactic and acetoacetic acids).
• pH control systems maintain blood pH by using buffers, respiration, and the kidneys.

pH Control Systems

• Chemical buffer system: Buffers maintain pH by absorbing or releasing H+.
• Respiratory system: Eliminating CO₂ maintains pH by its effect on H₂CO₃.
• Renal system: The kidney is the most effective buffer system. By secreting H+ and reabsorbing bicarbonate, the kidneys precisely maintain plasma pH.

Renal Regulation of pH

• Kidneys regulate pH by acting on plasma bicarbonate levels.
• These involved tasks include hydrogen secretion, filtered bicarbonate reabsorption, and active bicarbonate synthesis.

Explore the intricacies of renal physiology, focusing on the structure and functions of the urinary system. This quiz covers kidney functions, including excretion, homeostasis, and endocrine roles, as well as the types of nephrons and their significance in kidney operation.