Kidney Blood Flow and Filtration

• Blood flow through the kidney follows a unique pattern to maintain blood homeostasis.
• Renal arteries deliver about 1.2 L of blood per minute to the kidney for filtration.
• The sequence of blood vessels, from largest to smallest, is renal arteries, segmental arteries, interlobar arteries, arcuate arteries, and interlobular arteries.

Afferent Arterioles and Glomerulus

• Interlobular arteries branch into afferent arterioles, which supply the glomerulus.
• The glomerulus is a ball of capillaries where blood is filtered via glomerular filtration.
• Filtrate is produced at a rate of about 120 mL/min, known as the glomerular filtration rate.
• The glomerulus is not the primary site for gas and nutrient exchange in the kidneys.

Efferent Arterioles and Peritubular Capillaries

• Capillaries of the glomerulus drain into the efferent arteriole, unlike most capillary beds.
• Efferent arterioles branch into peritubular capillaries, which surround the nephron tubules.
• Peritubular capillaries receive oxygen and nutrients from the tubules and reabsorb substances back into the blood.
• Blood drains from peritubular capillaries through interlobular veins, arcuate veins, interlobar veins, and finally the renal vein.
• This blood flow pattern allows the kidneys to filter blood and reclaim most of the filtered fluid and solutes.

Nephron Structure

• Each kidney contains over a million nephrons, which are the functional units of the kidney.
• The nephron consists of the renal corpuscle and the renal tubule.

Renal Corpuscle

• The renal corpuscle consists of the glomerulus and the glomerular capsule.
• The glomerulus is a ball of looping, fenestrated capillaries allowing rapid fluid exit.
• The glomerular capsule has an outer parietal layer of simple squamous epithelium and an inner visceral layer of podocytes.
• Podocytes surround the glomerular capillaries.
• The capsular space lies between the parietal and visceral layers and receives fluid from the glomerular capillaries and the fluid is called filtrate.

Filtration Membrane

• Podocytes have extensions called foot processes that interlock to form filtration slits.
• The filtration membrane is formed by fenestrated glomerular endothelial cells, podocytes, and their shared basal lamina.
• The filtration membrane prevents large substances like blood cells and most proteins from exiting the glomerular capillaries.
• Water and small solutes like electrolytes, glucose, amino acids, and urea can cross the filtration membrane into the capsular space.

Renal Tubule

• From the capsular space, the filtrate enters the renal tubule where it is modified by tubular reabsorption and secretion.
• Tubular reabsorption returns substances from the filtrate back to the blood.
• About 99% of water and solutes in the filtrate are reclaimed during tubular reabsorption.
• Without tubular reabsorption, the entire blood volume would be lost in about 40 minutes.
• Tubular secretion moves substances from the blood into the filtrate.

Parts of the Renal Tubule

• The renal tubule consists of the proximal tubule, the nephron loop (descending and ascending limbs), and the distal tubule.
• The proximal tubule is the most metabolically active part of the nephron.
• Proximal tubule cells have microvilli to facilitate rapid reabsorption and secretion.
• The proximal tubule reabsorbs about 65% of water and electrolytes, about 90% of bicarbonate ions, and nearly 100% of glucose, amino acids, and other organic compounds.
• The nephron loop reabsorbs an additional 20–25% of the filtrate’s water and ions.
• Only the nephron loops of certain nephrons dip into the renal medulla.
• The distal tubule fine-tunes water, electrolyte, and acid-base balance, with about 85–90% of water and solutes already reclaimed.
• Hormone such as aldosterone and antidiuretic hormone control fine-tuning.

Collecting System

• Several distal tubules drain into one collecting duct
• The collecting system includes cortical collecting ducts and medullary collecting ducts.
• Medullary collecting ducts enlarge to form papillary ducts near the renal papilla.
• Papillary ducts drain urine into minor calyces, with about 1 mL/min of urine being drained.
• The renal tubule and collecting system reclaim about 99% of the water filtered at the glomerulus.

Juxtaglomerular Apparatus

• The macula densa is a group of tall, closely packed cells at the junction between the ascending limb of the nephron loop and the distal tubule.
• Juxtaglomerular cells (JG cells) are specialized cells located on the afferent arteriole near the macula densa.
• The juxtaglomerular apparatus (JGA) consists of the macula densa and JG cells.
• The JGA controls the flow of filtrate through the nephron and blood pressure within the glomerulus.