Physiology Module 4.1: Cardiovascular System and Renal Physiology

Renal Physiology

• The kidneys are pair of bean-shaped organs located in the lower back region behind the intestines.
• They are the major excretory and osmoregulatory organs, responsible for producing and excreting urine through urination (micturition).

Nephron

• The basic functional unit of the kidney is the nephron, which performs filtration and collection.
• The nephron consists of the glomerulus, Bowman's capsule, and the proximal convoluted tubule (PCT).

Glomerular Filtration Rate (GFR)

• GFR is the volume of fluid filtered from the glomeruli into the Bowman's space per unit time.
• Increased GFR occurs when the afferent arteriole dilates and the efferent arteriole constricts, increasing hydrostatic pressure.
• Decreased GFR occurs when the afferent arteriole constricts and the efferent arteriole dilates, decreasing hydrostatic pressure.

Proximal Convoluted Tubule (PCT)

• The PCT is the longest part of the nephron, located in the cortex of the kidney.
• Approximately 80% of the glomerular filtrate is reabsorbed via selective reabsorption in the PCT.
• Glucose, amino acids, and mineral ions are reabsorbed into the blood through active transport.
• Urea is not actively reabsorbed.
• Sodium is reabsorbed into the bloodstream in exchange for H+ ions with the help of NHE3.
• Hydrogen ions in the lumen-urine bind to bicarbonate, forming carbonic acid, which is then converted to water and carbon dioxide by carbonic anhydrase.
• Water and carbon dioxide can permeate freely in the PCT.

Loop of Henle

• The loop of Henle is a hair-pin bend structure with a descending limb and an ascending limb, located in the medulla of the kidney.
• The descending limb is permeable to water and urea, while the ascending limb is relatively impermeable.
• The loop of Henle works by making the tissues in the medulla hypertonic to the filtrate, transporting chloride ions out of the filtrate and sodium ions freely.
• The deepest portion of the medulla has the lowest water potential.

Distal Convoluted Tubule (DCT)

• The DCT reabsorbs inorganic ions according to the body's needs, selectively reabsorbing sodium and bicarbonate.
• It can also secrete substances into the filtrate to maintain pH, among other functions.
• The walls of the DCT are permeable to water, but this is controlled by anti-diuretic hormone (ADH).

Collecting Duct

• The collecting duct is the final segment of the nephron, where multiple nephrons can share one collecting duct.
• Water is only permeable in the collecting duct if ADH is present.
• Epithelial sodium channels (ENaC) are responsible for sodium reabsorption, while the sodium-potassium exchanger/pump is responsible for potassium excretion.

Regulation of Water Reabsorption

• Vasopressin (ADH) binds to the V2 receptor, leading to the production of cAMP, which translocates aquaporins to the cell membrane of the collecting duct cells.
• The fusion of aquaporins in the cell membrane makes the collecting duct cells permeable to water, allowing water reabsorption.