Physiology Module 4.1: Cardiovascular System and Renal Physiology

Study Notes

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).

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).

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.

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.

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.

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).

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.

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.

Podcast