Structure of the Nephron

Fig. 6.2 - Segments of a superficial and a juxtamedullary nephron (Costanzo Physiology, 7th Ed.)

1. Renal Corpuscle

Glomerulus

• A tuft of fenestrated capillaries arising from an afferent arteriole and drained by an efferent arteriole.
• Average diameter ~200 µm in humans.
• Contains three main cell types:
  • Fenestrated endothelial cells - line the capillary lumen, with pores 70-100 nm in diameter; allow passage of water and small solutes but retain blood cells.
  • Mesangial cells - lie in the central region between capillary loops; provide structural support and can contract to regulate filtration surface area.
  • Podocytes (visceral epithelial cells) - large, terminally differentiated cells that wrap foot processes around the capillaries. Adjacent foot processes are bridged by the filtration slit diaphragm, a key component of the filtration barrier.

Glomerular Filtration Barrier

1. Fenestrated capillary endothelium
2. Glomerular basement membrane (GBM) - contains lamina rara interna, lamina densa, and lamina rara externa; rich in negatively charged heparan sulfate, which repels albumin.
3. Filtration slit diaphragm (between podocyte foot processes)

Bowman's Capsule

• A double-walled cup surrounding the glomerulus.
• The parietal layer (flat epithelium) lines the outer wall.
• The visceral layer (podocytes) covers the capillaries.
• Bowman's space (urinary space) lies between these two layers and collects the glomerular filtrate.
• At the vascular pole, afferent and efferent arterioles enter/exit. At the urinary pole, Bowman's space is continuous with the proximal tubule.

2. Renal Tubule

Proximal Tubule

• Proximal Convoluted Tubule (PCT) - located in the cortex; highly convoluted. Cells have an extensive brush border (microvilli) on the luminal surface, massively increasing surface area for reabsorption. Rich in mitochondria. Reabsorbs ~65-70% of filtered Na⁺, water, glucose (entirely), amino acids, HCO₃⁻, phosphate, and uric acid.
• Proximal Straight Tubule (PST / pars recta) - descends into the outer stripe of the outer medulla.

Loop of Henle

Macula Densa

• A cluster of specialized cells at the end of the TAL, where it contacts the glomerular arterioles.
• Senses NaCl concentration in tubular fluid.
• Part of the juxtaglomerular apparatus (JGA), which regulates renin release and GFR via tubuloglomerular feedback.

Distal Convoluted Tubule (DCT)

• Located in the cortex; relatively short.
• Continues active NaCl reabsorption via the NCC (Na⁺-Cl⁻ cotransporter); site of action of thiazide diuretics.
• Responds to aldosterone (increases Na⁺ reabsorption) and parathyroid hormone (increases Ca²⁺ reabsorption).
• Impermeable to water in the absence of ADH.

Connecting Tubule (CNT)

• Bridges the DCT and collecting duct.
• Derived from metanephric blastema (same as nephron).
• Contains principal cells and intercalated cells.

3. Collecting Duct System

• Principal cells - respond to ADH and aldosterone; reabsorb Na⁺, water; secrete K⁺.
• Intercalated cells (A and B types) - regulate acid-base balance via H⁺-ATPase and Cl⁻/HCO₃⁻ exchangers.

4. Two Types of Nephrons

Sagittal and coronal sections of the kidney, showing cortex, outer medulla (outer/inner stripe), inner medulla, and papilla.

5. Renal Vasculature of the Nephron

Summary of Nephron Segments and Locations

• Costanzo Physiology, 7th Edition, pp. 251-252
• Brenner and Rector's The Kidney, 2-Volume Set, pp. 83-90

Structure and Function of the Heart

Fig. 9.1 - Structure of the heart and blood flow, showing chambers, valves, great vessels, and wall layers (Guyton & Hall Medical Physiology, 14th Ed.)

1. Overview and Position

• Apex - formed by the inferolateral part of the left ventricle, pointing forward, downward, and to the left. It lies deep to the left 5th intercostal space, 8-9 cm from the midsternal line - the site of the apex beat.
• Base - the posterior face, formed mainly by the left atrium and the proximal great veins (pulmonary veins, superior and inferior vena cava). Fixed posteriorly opposite vertebrae T5-T8.
• Surfaces: anterior (sternocostal - mainly right ventricle), diaphragmatic/inferior (left + right ventricle), left pulmonary (left ventricle + left atrium), right pulmonary (right atrium).

2. Coverings: Pericardium and Heart Wall

Pericardium

• Fibrous pericardium - tough outer layer; prevents over-distension and anchors the heart to surrounding structures (diaphragm, sternum).
• Serous pericardium - divided into:
  • Parietal layer - lines the fibrous pericardium
  • Visceral layer (epicardium) - directly covers the heart surface
  • Pericardial space - potential space between parietal and visceral layers, containing 15-50 mL of serous fluid that reduces friction.

Heart Wall Layers (from inside out)

3. Cardiac Chambers

Right Atrium

• Receives deoxygenated blood from:
  • Superior vena cava (from head, neck, upper limbs)
  • Inferior vena cava (from trunk, lower limbs, abdominal organs)
  • Coronary sinus (from the heart wall itself)
• Internal anatomy: divided by the crista terminalis into:
  • Sinus of venae cavae (posterior, smooth-walled) - receives the venae cavae
  • Atrium proper (anterior) + right auricle - has musculi pectinati (ridge-like pectinate muscles) on its wall
• Contains the fossa ovalis (oval depression on the interatrial septum - remnant of the fetal foramen ovale)
• Opens into right ventricle via the right atrioventricular (tricuspid) valve

Right Ventricle

• Crescent-shaped in cross-section; wraps around the left ventricle anteriorly
• Wall is thinner than the left ventricle (generates lower pressure for the pulmonary circuit)
• Interior features:
  • Trabeculae carneae - muscular ridges and bridges on the inner wall
  • Papillary muscles (anterior, posterior, septal) - cone-shaped muscles attached to valve cusps via chordae tendineae
  • Septomarginal trabecula (moderator band) - carries part of the right bundle branch to the anterior papillary muscle
  • Conus arteriosus (infundibulum) - smooth-walled outflow tract leading to the pulmonary valve
• Ejects blood into the pulmonary trunk → lungs

Left Atrium

• Receives oxygenated blood from the four pulmonary veins (two from each lung)
• Posterior half has smooth walls (derived from incorporated pulmonary vein origins)
• Anterior half contains musculi pectinati; continuous with the left auricle
• Valve of foramen ovale - thin depression in the interatrial septum
• Opens into the left ventricle via the mitral (bicuspid) valve

Left Ventricle

• Conical, longer chamber; forms the apex of the heart
• Has the thickest myocardium (~8-12 mm vs. ~3-5 mm in RV) to generate sufficient pressure for the systemic circulation
• Interior features:
  • Fine, delicate trabeculae carneae
  • Two papillary muscles (anterior and posterior) - larger than those in the right ventricle
  • Aortic vestibule - smooth-walled outflow tract leading to the aortic valve
  • Interventricular septum - mostly muscular (thick); a small membranous part at the top
• Ejects blood into the aorta → systemic circulation

4. Heart Valves

Atrioventricular (AV) Valves

Semilunar Valves

5. Wall Structure: Cardiac Muscle (Myocardium)

Histology

• Striated (like skeletal muscle) due to organized actin-myosin sarcomeres
• Branching - form a latticework
• Connected end-to-end by intercalated discs
  • Fascia adherens - mechanical coupling between cells
  • Desmosomes - structural anchoring
  • Gap junctions - allow rapid ion diffusion, making the myocardium function as an electrical syncytium

Functional Syncytium

Left Ventricular Fiber Architecture

• Subepicardial fibers spiral leftward; during systole they rotate the apex counterclockwise (when viewed from below)
• Subendocardial fibers spiral rightward in the opposite direction
• Midwall fibers run circumferentially
• The net result is a twisting/wringing motion during systole, maximizing stroke volume. At end-systole the LV is like a loaded spring, and it untwists (recoils) during diastole to facilitate rapid ventricular filling.

6. Cardiac Conduction System

Fig. 3.86 - Conduction System of the Heart, (A) right chambers, (B) left chambers (Gray's Anatomy for Students)

Components (in order of excitation)

• Located at the junction of the superior vena cava and right atrium, at the upper end of the crista terminalis
• Spontaneously depolarizes at 60-100 beats/min (intrinsic rate)
• Excitation spreads across both atria as a wave → causes atrial contraction
• Simultaneously stimulates the AV node

• Located near the opening of the coronary sinus, close to the attachment of the septal cusp of the tricuspid valve, in the interatrial septum
• Introduces a critical delay (~0.1 second) before passing the impulse to the ventricles - allows atria to complete contraction and fill the ventricles before ventricular systole begins
• Intrinsic rate if SA node fails: 40-60 beats/min (junctional rhythm)

• Direct continuation of the AV node
• Runs along the lower border of the membranous interventricular septum
• The only electrical connection between atria and ventricles (all other tissue is electrically insulated by the fibrous skeleton)
• Splits into right and left bundle branches

• Right bundle branch - runs subendocardially on the right side of the interventricular septum → enters the septomarginal trabecula → reaches the base of the anterior papillary muscle → disperses into Purkinje fibers
• Left bundle branch - passes to the left side of the muscular septum → descends toward the apex, giving off branches → continuous with Purkinje network

• Fine network spreading throughout both ventricular walls
• Conduction velocity: up to 4 m/sec (vs. 0.3-0.5 m/sec in ordinary ventricular muscle)
• Ensure near-simultaneous excitation of all ventricular muscle from apex upward toward the outflow tracts - maximizing ejection efficiency
• Also innervate papillary muscles first, so they contract slightly before the valve cusps are pushed closed

Intrinsic Pacing Hierarchy

7. Cardiac Action Potential

Ventricular Muscle Action Potential

• Triggers the contractile mechanism (Ca²⁺-induced Ca²⁺ release from sarcoplasmic reticulum)
• Creates a long absolute refractory period - preventing tetanic contractions (which would be fatal in the heart)

8. Coronary Circulation

9. Cardiac Innervation

10. Cardiac Function Summary

Dual Pump Arrangement

Output

• At rest: ~5 L/min (cardiac output = heart rate × stroke volume)
• During exercise: can increase to 20-25 L/min
• Stroke volume at rest: ~70 mL
• Heart rate at rest: ~70 bpm

Skeletal vs. Cardiac Muscle - Key Differences

• Gray's Anatomy for Students, pp. 226-250 (Heart structure, chambers, valves, conduction system)
• Guyton and Hall Textbook of Medical Physiology, 14th Ed., pp. 122-125 (Cardiac muscle anatomy, action potentials)
• Barash, Cullen, and Stoelting's Clinical Anesthesia, 9th Ed., pp. 839-840 (Cardiac anatomy and physiology overview)The full detailed answer is above. Here is a quick summary of what was covered:

1. Position and orientation - apex, base, surfaces, mediastinal location
2. Pericardium and wall layers - fibrous/serous pericardium, endocardium, myocardium, epicardium
3. Four chambers - right atrium (crista terminalis, fossa ovalis), right ventricle (trabeculae, papillary muscles, moderator band), left atrium (pulmonary veins), left ventricle (thick wall, interventricular septum)
4. Four valves - tricuspid, mitral (AV valves with chordae tendineae), pulmonary, aortic (semilunar valves; coronary arteries arise from aortic sinuses)
5. Cardiac muscle histology - striated, intercalated discs, gap junctions, functional syncytium
6. Conduction system - SA node → AV node (delay) → Bundle of His → right/left bundle branches → Purkinje fibers (4 m/sec)
7. Action potential phases (0-4) with ionic basis - the plateau (Phase 2) explained and its importance
8. Coronary circulation - RCA, LAD, LCx territories; diastolic filling
9. Autonomic innervation - sympathetic (β1, ↑rate/force) vs. parasympathetic (vagus, ↓rate)
10. Cardiac output and function - dual pump, ~5 L/min at rest, key differences from skeletal muscle