BIOS5130 Human Physiology and Disease - The Kidneys
Functions of the Kidneys:
The kidneys play a vital role in maintaining the body’s homeostasis through multiple functions, including:
Regulation of Extracellular Fluid (ECF) Volume and Blood Pressure: Through renal mechanisms, the kidneys adjust the volume of blood and maintain its pressure. An elevated blood volume can increase blood pressure, which can lead to hypertension.
Osmolarity Regulation: Kidneys help maintain the balance of solute concentrations in body fluids by regulating the excretion and reabsorption of water and various ions.
Ion Balance: The kidneys play an important role in balancing crucial ions such as sodium, potassium, and calcium, which are essential for various bodily functions.
Regulation of pH: They maintain the acid-base balance in the blood, ensuring that the pH remains within the normal range (7.35-7.45) by excreting hydrogen ions and reabsorbing bicarbonate from urine.
Waste Excretion: Key metabolic waste products such as urea, creatinine, and toxins are excreted through urine.
Hormonal Functions: The kidneys also produce and secrete hormones like erythropoietin, which stimulates red blood cell production in response to low oxygen levels and renin, which helps regulate blood pressure.
Anatomy of the Urinary System:
Detailed anatomical structures of the urinary system include:
Gross Anatomy: Includes kidneys, ureters, bladder, and urethra, which are involved in urine formation, transport, storage, and elimination.
Nephron Anatomy: The nephrons, the functional units of the kidneys, consist of:
Bowman’s Capsule: Encloses the glomerulus and is the site of initial filtration.
Proximal Tubule: Reabsorbs the majority of water, ions, and nutrients from the filtrate.
Loop of Henle: Creates a concentration gradient essential for water reabsorption.
Distal Tubule & Collecting Ducts: Involved in the fine-tuning of water and electrolyte reabsorption, influenced by hormones such as aldosterone and antidiuretic hormone (ADH).
Renal Pelvis: Acts as a funnel for urine flowing from the nephrons into the ureters.
Overview of Kidney Function
Key Processes:
Filtration: Blood is filtered in the renal corpuscle, separating waste from useful substances.
Reabsorption: Valuable substances and water are reabsorbed back into the bloodstream.
Secretion: Unnecessary substances are secreted from the blood into the nephron.
Excretion: Process through which urine is expelled from the body.
Micturition: The nervous system controls the voluntary and involuntary expulsion of urine from the bladder.
Kidney Anatomy
Position and Structure
Kidneys Position: Retroperitoneally located at the lower rib level, protected by the rib cage and surrounded by a cushioning layer of fat.
Kidney Structure: Each kidney has a distinct structure with a right and left kidney extending from the inferior vena cava and aorta. The adrenal glands are situated on the superior end of each kidney. The ureters transport urine from each kidney to the urinary bladder.
Gross Anatomy of the Kidneys
Kidney Structure: Comprised of an outer cortex (where nephrons reside) and an inner medulla.
Urine Flow: From the nephrons, urine flows into the renal pelvis and through the ureters to the bladder for storage.
Nephron Anatomy
Structure: Nephrons are the functional units of the kidneys, with millions present in each kidney, composed of tubules and associated blood vessels.
Function: Nephrons filter fluid from the blood, modifying it through reabsorption and secretion processes to form urine. Each part of the nephron has specialized functions crucial for fluid balance.
Kidney Function Overview
Filtration: Fluid filters from blood into nephron lumen through the renal corpuscle. The resulting filtrate is termed as such and is processed further along the nephron.
Reabsorption: Essential materials return to the blood mainly in peritubular capillaries, reclaiming significant amounts of water and solutes.
Secretion: Blood materials enter the tubule lumen for excretion.
Filtration Mechanism
Filtration Fraction: ~20% of plasma flowing through the glomerulus is filtered, while less than 1% is typically excreted.
Filtration Barriers: Selective filtration occurs due to structures that take size and charge into account, ensuring only appropriate substances pass.
Filtration Barriers Description
Components: Made up of podocyte foot processes forming filtration slits, glomerular capillary endothelium, and a basement membrane, working together as a filtration barrier.
Glomerular Filtration Rate (GFR)
Definition: It is the volume of fluid filtered from the kidney's blood into the nephron per unit of time.
Influences: Factors affecting GFR include net filtration pressure, renal blood flow, and the surface area available for filtration. GFR is tightly regulated to remain consistent despite changes in blood pressure and flow.
GFR Regulation
Mechanisms: Includes myogenic response (vascular smooth muscle responding to pressure changes) and tubuloglomerular feedback (sensors detecting sodium levels in the filtrate). Renin secretion by macula densa cells also plays a role in regulating arteriole dynamics.
Juxtaglomerular Apparatus (JGA)
Components: Comprises the macula densa and granular cells; functions to regulate GFR and kidney function based on detected signals in the distal tubule.
Process of Reabsorption
Types: Active reabsorption (requires energy) and passive reabsorption (follows gradients), employing various transport processes at tubule cell membranes.
Renal Handling of Common Solutes
Includes sodium, chloride, potassium, calcium, glucose, urea, and PAH, demonstrating the kidneys' regulatory functions in maintaining homeostasis in the body’s electrolytes and waste products.
Micturition Process
Urine Composition: Remains largely unaltered as it moves through the collecting duct.
Control Mechanism: Involves both internal and external sphincters, with reflex control from the spinal cord, plus voluntary control from higher brain functions.
Conclusion
Acknowledgment: Thank you for participating in this enriching module on the kidneys' anatomy, physiology, and their critical role in maintaining homeostasis within the human body.