Kidney Anatomy and Nephron Function

Renal Anatomy: Location and Orientation

• General Location and Protection

  • The kidneys are located in the abdominal cavity, flanking both sides of the vertebrae.

  • They are protected by the ribs; specifically, the 12th rib protects the superior portion of the kidney.

  • Anatomical surrounding structures include:

    • Posterior: Back muscles and the vertebral column.

    • Anterior: The stomach, intestines, and spleen.

    • Superior: The adrenal glands (suprarenal glands) sit directly on top of each kidney.

• Kidney Position and Visibility

  • The kidneys can be viewed through sagittal sections and transverse sections to understand their relationship with other organs.

  • The transverse section illustrates the levels of encapsulation and cushions that maintain the kidney's position within the abdomen.

External and Internal Kidney Structure

• Regional Anatomy

  • Renal Capsule: The outermost, hard fibrous covering made of dense irregular connective tissue. Its primary functions are to protect the kidney from infection and friction.

  • Cortex: The light-colored, granular, superficial region. It contains the majority of the kidney's blood vessels.

  • Medulla: The middle layer, characterized by dozens of cone-shaped medullary pyramids.

• The Medullary Pyramids and Drainage

  • Renal Pyramids: Triangularly shaped structures within the medulla.

  • Apex (Renal Papilla): The pointed portion of the pyramid that projects inward toward the hilum or the middle of the kidney.

  • Renal Columns: Areas of tissue that separate the medullary pyramids.

• The Renal Sinus and Hilum

  • Hilum: A midline opening on the medial side of the kidney where the renal artery and renal vein enter/exit, and where the renal pelvis is located.

  • Renal Sinus: The internal space within the hilum that houses the renal pelvis, renal artery, and renal vein.

• The Collection System

  • Minor Calyx: A small cup-like structure into which the apex of each renal pyramid drains.

  • Major Calyx: Large branches formed by the merging of 2 to 3 minor calyces. Each kidney typically has 2 to 3 major calyces.

  • Renal Pelvis: A flat, funnel-shaped tube that is the expanded upper end of the ureter. It collects urine from the major calyces and drains it into the ureter.

  • Ureter: The tube that drains urine from the renal pelvis to the urinary bladder.

Functional Units: The Nephron

• Definition: Nephrons are the tiny, functional units of the kidney responsible for filtering blood and forming urine.

• Components of the Nephron

  • Renal Corpuscle (Bowman’s Capsule): Contains a knot of capillaries known as the glomerulus. It is the site where blood filtration begins.

  • Renal Tubule: The sequence of tubes where filtered fluid (filtrate) flows, consisting of:

    • Proximal Convoluted Tubule (PCT): Continuous with the glomerular capsule; characterized by an epithelial layer with a brush border of microvilli to increase surface area for reabsorption.

    • Loop of Nephron (Loop of Henle): Consists of a descending limb and an ascending limb. It is named after the scientist who discovered it.

    • Distal Convoluted Tubule (DCT): Composed of epithelial cells rich in mitochondria, which provide the $ATP$ required for active tubular secretion.

• Collecting Duct:

  • Technically not part of an individual nephron, as it is shared by several nephrons.

  • It collects urine from the DCTs of multiple nephrons and opens at the apex (tip) of the medullary pyramid to drain into a minor calyx.

• Types of Nephrons

  • Cortical Nephrons (80%): Located primarily in the outer cortex. They have short loops of Henle that extend only a short distance into the medulla. Blood flows through them rapidly.

  • Juxtamedullary Nephrons (Remaining 20%): Located near the cortex-medulla junction. They have much longer loops of Henle that extend deep into the medulla. They are associated with the vasa recta and play a crucial role in maintaining acid-base balance and concentrating urine through a slower blood flow.

  • Macula Densa: Specialized cells within the PCT (associated with juxtamedullary nephrons) that assist in the filtration process.

Renal Blood Supply and Vasculature

• Path of Blood Flow (Inbound)

  • Abdominal Aorta: The primary source of blood.

  • Renal Artery: Branches directly off the aorta into the hilum.

  • Interlobar Artery: (Also referred to as interglomerular in some context).

  • Arcuate Artery.

  • Cortical Radiate Artery (Interlobular Artery).

  • Afferent Arteriole: Delivers blood directly into the glomerulus.

• Microcirculation

  • Glomerulus: A ball of capillaries where filtration occurs.

  • Efferenrt Arteriole: Transports blood away from the glomerulus that was not filtered.

  • Peritubular Capillaries: Surround the renal tubules in cortical nephrons for reabsorption and secretion.

  • Vasa Recta: Specialized capillary loops that surround the loops of Henle in juxtamedullary nephrons.

• Path of Blood Flow (Outbound)

  • Cortical Radiate Vein (Interlobular Vein).

  • Arcuate Vein.

  • Interlobar Vein.

  • Renal Vein: Depicted in blue, it exits the hilum and drains into the Inferior Vena Cava.

• Pressure Dynamics

  • Highest Pressure: Found in the renal artery because it must push blood through the extensive capillary network.

  • Lowest Pressure: Found in the renal vein as it returns blood to the venous system.

Kidney Coverings and Anchoring

1. Renal Fibrous Capsule: The exterior-most layer that prevents infection and mechanical friction.

2. Perirenal Fat (Adipose Capsule): A fatty mass that cushions the kidney and helps attach it to the body wall.

3. Renal Fascia: The outer layer of dense fibrous connective tissue that anchors the kidney in its abdominal position.

4. External Perirenal Fat: Located external to the renal fascia for additional cushioning.

Physiological Functions of the Kidneys

• Regulation of Body Fluids: Controls the volume, composition, and pH of body fluids.

• Waste Removal: Removes metabolic wastes from the blood, including nitrogenous and sulfur-containing products of protein metabolism.

• Erythropoiesis Control: Helps regulate the rate of red blood cell synthesis.

• Blood Pressure Regulation: Plays a central role in maintaining systemic blood pressure.

• Ion Absorption: Regulates the absorption of calcium ions.

• Homeostasis: Renal failure or malfunction has a massive impact on the homeostasis of the entire body and other organ systems.

Physiology of Urine Formation

• The Three-Step Process

  1. Glomerular Filtration: Substances move from the blood in the glomerular capillaries into the glomerular capsule (Bowman’s capsule). This is a process of bulk flow.

  2. Tubular Reabsorption: Substances move from the renal tubules back into the interstitial fluid and then into the peritubular capillaries. Approximately $70\%$ of filtrate is reabsorbed in the PCT.

  3. Tubular Secretion: Substances move from the plasma of the peritubular capillaries into the fluid of the renal tubules for excretion. This process requires active transport ($ATP$).

• Filtration Criteria

  • Filtration depends on the molecular weight and the charge of the molecule.

  • Glomerular Slits (Podocytes): These allow molecules to move through the capillaries into the capsule if they meet the weight, size, and ionic balance criteria.

  • Substances too large or of the wrong charge stay in the blood and exit via the efferent arteriole.

Glomerular Filtration Rate (GFR)

• Definition: The amount of filtrate produced in the kidneys every minute.

• Standard Values:

  • $\text{Average GFR} = 125\,ml/min$

  • $\text{Daily Filtrate} = 180\,L/day$

  • Humans reabsorb all but $1\%$ of the fluid filtered back into the blood.

• Net Filtration Pressure (NFP)

  • NFP is the driving force for filtration. It is the favoring force minus the opposing forces.

  • Favoring Force: Glomerular capillary hydrostatic pressure (blood pressure inside the capillaries).

  • Opposing Forces: Capsular hydrostatic pressure and glomerular capillary colloid osmotic pressure.

• Factors Altering GFR:

  • Increased Renal Blood Flow: Increases filtration.

  • Decreased Plasma Proteins: Makes blood thinner, increasing GFR but potentially causing edema.

  • Hemorrhage: Decreases blood pressure and blood flow to the kidney, thereby decreasing GFR.

  • Obstruction: If urine cannot be output, it backs up through the urethra, bladder, ureters, pelvis, and calyces, filling the capsule with fluid and stopping filtration due to high capsular pressure.

Hormone Regulation and Renal Effects

• Renin-Angiotensin-Aldosterone System (RAAS)

  1. Angiotensinogen: Produced by the liver.

  2. Renin (Angiotensinogenase): An enzyme produced by the kidneys that converts angiotensinogen into Angiotensin I.

  3. ACE (Angiotensin Converting Enzyme): Produced by the lungs; converts Angiotensin I into Angiotensin II.

  4. Angiotensin II Effects: Causes vasoconstriction, increases Aldosterone secretion, increases ADH secretion, and increases thirst.

• Antidiuretic Hormone (ADH)

  • Made in the Hypothalamus and released by the Posterior Pituitary.

  • Released when the concentration of water in blood decreases (increased osmotic pressure).

  • Effect: Increases the permeability of the DCT and collecting ducts to water, allowing water to be reabsorbed by osmosis.

  • Low ADH: Results in dilute urine (proper hydration).

  • High ADH: Results in concentrated, dark urine (dehydration).

  • Alcohol Interference: Alcohol suppresses ADH production, leading to uncontrollable urination and dehydration (the cause of hangovers).

• Other Hormones

  • Atrial Natriuretic Peptide (ANP): Affects sodium levels and increases GFR.

  • Aldosterone: Affects solute concentration, specifically promoting the secretion of potassium.

  • Parathyroid Hormone (PTH): Increases calcium reabsorption in the thick ascending loop of Henle and the distal tubules, reducing urinary calcium excretion.

Mechanisms of Transport in the Renal Tubules

• Reabsorption Pathways

  • Transcellular: Through the cells.

  • Paracellular: Between the cells.

• Transport Methods

  1. Primary Active Transport: Uses energy to move molecules against a gradient (e.g., the sodium-potassium pump: uses $1$ molecule of $ATP$ to move $2$ potassium ions in and $3$ sodium ions out).

  2. Secondary Active Transport: Uses the energy from an established gradient.

     • Symphort (Cotransport): Molecule moves in the same direction as sodium (e.g., glucose).

     • Antiport (Counter-transport): Molecule moves in the opposite direction of sodium (e.g., hydrogen or potassium moving against sodium in the countercurrent mechanism).

  3. Pinocytosis: "Cell drinking"; a form of endocytosis used to intake fluids or large substances like proteins.

  4. Passive Transport: Simple diffusion based on a concentration gradient; requires no energy.

Substance Comparison: Plasma, Filtrate, and Urine

• Note on Glucose: In cases of diabetes, blood is supersaturated with sugar, and glucose may be excreted in the urine. Normally, the body reabsorbs all glucose because it was energetically costly to break down from complex carbohydrates.