FISIOLOGÍA RENAL: FILTRACIÓN, ABSORCIÓN, SECRECIÓN, EXCRECIÓN

Overview of Renal Physiology

Structure of the Kidney

• The kidney is externally surrounded by a capsule, with an inner cortex containing medullary rays and an even deeper medulla or pyramids.

• Between each pyramid lies the column of Bertin; further in are the minor and major calyces that converge to form the renal pelvis.

Functional Units of the Kidney

• Each kidney contains approximately one million nephrons, which are the functional units responsible for filtration.

• Nephrons can be categorized into cortical nephrons (majority, short loops) and juxtamedullary nephrons (long loops, crucial for hyperosmolarity).

Filtration Process in Nephrons

Components Involved in Filtration

• The nephron includes specialized structures such as proximal convoluted tubule, loop of Henle (thin descending and ascending limbs), distal convoluted tubule, and collecting duct.

• The glomerulus is a vascular structure essential for filtration; it consists of a parietal layer and visceral layer with podocytes having foot processes called pedicels.

Mechanisms Influencing Filtration

• Glomerular filtration involves crossing the glomerular basement membrane influenced by hydrostatic pressure within capillaries.

• Key opposing forces include oncotic pressure from plasma proteins and pressure from urinary space affecting overall filtration capacity.

Factors Affecting Glomerular Filtration Rate (GFR)

Passive Nature of Filtration

• Filtration is considered passive as it does not require direct energy consumption but relies on pressure gradients.

Conditions Impacting GFR

• Increased hydrostatic pressure during pregnancy raises GFR while decreased pressures due to hypotension or dehydration lower it.

• Obstruction in urinary flow can also elevate pressures within Bowman’s space, reducing GFR efficiency.

Regulation Mechanisms for Homeostasis

Importance of Pore Size and Charge

• Membrane pore size and negative charge prevent large proteins like albumin from being filtered out into urine.

Regulatory Mechanisms

• Myogenic response allows afferent arterioles to constrict or dilate based on blood flow changes; this serves as a primary defense mechanism against fluctuations in blood pressure.

Tubular Reabsorption Processes

Proximal Tubule Functionality

• The proximal tubule reabsorbs 60%-65% of filtered substances including water, sodium, potassium, bicarbonate, glucose, amino acids through various transport mechanisms.

Threshold Effects

• For example, glucose reabsorption saturates at levels above 180 mg/dL leading to glucosuria; similar thresholds exist for bicarbonate reabsorption.

Understanding the Final Constituents of Urine

Mechanisms of Potassium and Hydrogen Secretion

• The final constituents of urine involve potassium secretion by principal cells and hydrogen ion secretion by intercalated cells. The presence of antidiuretic hormone (ADH) makes the tubule permeable to water, allowing passive reabsorption of free water, thus concentrating urine.

Role of Hormones in Tubular Function

• The distal tubule's function is regulated by hormones and drugs, notably the sodium-potassium-chloride co-transporter affected by furosemide, a specific diuretic. This highlights the importance of sodium-chloride co-transport in early distal convoluted tubules.

Aldosterone's Impact on Electrolyte Balance

• Aldosterone, synthesized in the adrenal cortex's glomerular layer, plays a crucial role in sodium reabsorption accompanied by water retention, increasing extracellular fluid volume and blood pressure. It also promotes potassium secretion but less so for hydrogen ions.

Natriuretic Peptides and Their Functions

• There are three types of natriuretic peptides: atrial (ANP), brain (BNP), and C-type (CNP). ANP is released due to atrial distension from increased intravascular volume, antagonizing the renin-angiotensin-aldosterone system (RAAS), promoting sodium chloride excretion and renal vasodilation.

Vasopressin: Synthesis and Actions

• Vasopressin or ADH is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus. Its release occurs via neurohypophysis during cellular dehydration or significant hypovolemia (>10%), acting on V2 receptors to increase water permeability in collecting ducts.

Calcium Regulation Through Hormonal Action

Parathyroid Hormone Functions

• Parathyroid hormone regulates calcium levels through actions such as extracting calcium from bones, enhancing intestinal absorption via vitamin D activation, and influencing renal handling of calcium while reducing phosphate reabsorption.

Kidney’s Multifaceted Roles

• The kidneys filter waste products like urea and creatinine through glomerular filtration while also managing tubular secretion for substances like lactate. They maintain homeostasis for fluids and electrolytes through RAAS regulation alongside natriuretic peptides.

Acid-Base Balance Mechanisms

Renal Regulation of pH Levels

• The kidneys help regulate acid-base balance by adjusting bicarbonate levels; they can extract bicarbonate during acidosis while compensating with respiratory adjustments to manage carbonic acid levels effectively.

Ammonium Production for Acid Excretion

• Kidneys synthesize ammonium from amino acid metabolism to aid in acid excretion. This process involves filtering nonvolatile acids while reabsorbing nearly all filtered bicarbonate to normalize plasma pH levels over time.

Endocrine Functions Related to Calcium Absorption

Activation of Vitamin D Metabolites

Functions of Erythropoietin and Renin Production

Erythropoietin Production and Function

• Erythropoietin (EPO) is primarily produced in the kidneys (95%) with a minor contribution from the liver (5%). It plays a crucial role in response to hypoxia, which is its main stimulus.

• The stabilization and activation of hypoxia-inducible factors lead to the transcription of genes that code for erythropoietin, facilitating red blood cell production.

• EPO activates erythropoiesis in the bone marrow. Its production can decrease in cases of anemia, commonly seen in renal diseases.

• Conversely, EPO synthesis may increase during secondary polycythemia due to conditions like renal artery stenosis or other related diseases.

Renin Production and Regulation

• Another significant endocrine function involves renin production, secreted by juxtaglomerular cells in the kidney's external medulla.

• The primary stimulus for renin release includes factors such as low blood pressure detected by the macula densa and sympathetic nervous system activity.