16.1 Body System Anatomy & Systemic Physiology Overview for Radiographers

16.1 Body System Anatomy & Systemic Physiology Overview for Radiographers

Radiologic technology requires a deep structural and functional understanding of human anatomy and physiology. Accurate positioning, proper exposure technique, identification of normal anatomical landmarks, and recognition of pathological variations all depend upon a solid mastery of systemic anatomy. This section examines five core organ systems central to diagnostic radiography and fluoroscopy: the respiratory, cardiovascular, gastrointestinal, urinary, and skeletal systems.

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1. The Respiratory System & Mediastinal Anatomy

The respiratory system facilitates gas exchange between atmospheric air and circulating blood. Anatomically, it is divided into the upper respiratory tract (nasal cavity, pharynx, larynx) and the lower respiratory tract (trachea, bronchial tree, lungs).

Trachea and Bronchial Tree

The trachea is a fibrous, cartilaginous tube measuring approximately 10 to 12 cm in length and 2 to 2.5 cm in diameter. Originating at the lower border of the cricoid cartilage (C6 level), it descends anterior to the esophagus and bifurcates at the level of the carina (T4–T5 thoracic spine level).

• Right Primary Bronchus: Wider, shorter (approx. 2.5 cm), and more vertical than the left. Due to this steep alignment, aspirated foreign bodies enter the right main bronchus far more frequently.
• Left Primary Bronchus: Narrower, longer (approx. 5 cm), and more horizontal as it passes beneath the aortic arch to enter the left lung hilum.
• Secondary (Lobar) & Tertiary (Segmental) Bronchi: The right primary bronchus divides into 3 lobar bronchi (superior, middle, inferior), corresponding to the 3 lobes of the right lung. The left primary bronchus divides into 2 lobar bronchi (superior, inferior), matching the 2 lobes of the left lung.

Lungs and Pleural Cavities

The lungs are paired spongy, elastic organs situated within the pleural cavities of the thorax.

• Right Lung: Shorter and broader due to the right hepatic lobe inferiorly; possesses 3 lobes divided by the oblique and horizontal fissures.
• Left Lung: Longer and narrower, featuring a cardiac notch on its anterior border; possesses 2 lobes divided by a single oblique fissure.
• Pleura: Double-layered serous membrane. The parietal pleura lines the inner thoracic wall and diaphragm, while the visceral pleura closely adheres to the lung surface. The potential space between them—the pleural cavity—contains serous fluid that reduces friction during respiration.

Mediastinal Structures

The mediastinum is the central interpleural space of the thorax bounded anteriorly by the sternum and posteriorly by the thoracic spine. It contains:

1. Anterior Mediastinum: Thymus gland, lymph nodes, internal mammary vessels.
2. Middle Mediastinum: Heart, pericardium, ascending aorta, pulmonary trunk, superior and inferior vena cava, trachea, primary bronchi, phrenic nerves.
3. Posterior Mediastinum: Thoracic descending aorta, esophagus, thoracic duct, azygos and hemiazygos veins, sympathetic trunk.

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2. The Cardiovascular System & Systemic Circulation

The cardiovascular system propels oxygenated blood, nutrients, and hormones throughout the body while clearing metabolic waste products.

Heart Chambers and Valves

The heart is a hollow muscular pump located within the middle mediastinum, encased by the fibrous and serous pericardium.

• Right Atrium: Receives deoxygenated blood from the systemic circulation via the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus.
• Right Ventricle: Pumps deoxygenated blood through the pulmonary semilunar valve into the pulmonary trunk toward the lungs.
• Left Atrium: Receives oxygenated blood returning from the lungs via 4 pulmonary veins (2 right, 2 left).
• Left Ventricle: Thickest muscular chamber; pumps oxygenated blood through the aortic semilunar valve into the systemic aorta.
• Atrioventricular (AV) Valves: The tricuspid valve (3 cusps) guards the right AV orifice, while the bicuspid/mitral valve (2 cusps) guards the left AV orifice. Chordae tendineae anchored to papillary muscles prevent valve prolapse during ventricular systole.

Pulmonary vs. Systemic Circulation

1. Pulmonary Circuit: Right ventricle $\rightarrow$ Pulmonary trunk $\rightarrow$ Right/Left pulmonary arteries $\rightarrow$ Pulmonary capillary beds (alveolar gas exchange) $\rightarrow$ 4 Pulmonary veins $\rightarrow$ Left atrium.
2. Systemic Circuit: Left ventricle $\rightarrow$ Ascending aorta $\rightarrow$ Aortic arch $\rightarrow$ Thoracic/Abdominal aorta $\rightarrow$ Systemic arterial network $\rightarrow$ Systemic capillary beds $\rightarrow$ Systemic venous system (SVC/IVC) $\rightarrow$ Right atrium.

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3. The Gastrointestinal System & Accessory Organs

The gastrointestinal (GI) system encompasses the alimentary canal and associated accessory digestive glands, responsible for ingestion, mechanical and chemical digestion, nutrient absorption, and waste elimination.

Alimentary Canal Segments

• Esophagus: Muscular tube (approx. 25 cm) passing through the esophageal hiatus of the diaphragm at T10 to join the stomach at the cardiac orifice.
• Stomach: J-shaped sac divided into the fundus, body (corpus), pyloric antral section, and pyloric canal. Bounded by the lower esophageal sphincter (cardiac sphincter) superiorly and the pyloric sphincter inferiorly.
• Small Intestine: Comprises the duodenum (C-shaped 25 cm segment curving around the pancreatic head), jejunum (proximal 2/5, feathery mucosal appearance on barium studies), and ileum (distal 3/5, smoother mucosa ending at the ileocecal valve).
• Large Intestine: Begins at the cecum and vermiform appendix, continuing as the ascending colon, hepatic (right colic) flexure, transverse colon, splenic (left colic) flexure, descending colon, sigmoid colon, rectum, and anal canal.

Accessory Digestive Organs

• Liver: Largest internal gland located in the right upper quadrant (RUQ). Performs bile synthesis, metabolic storage, and detoxification. Dual blood supply: hepatic artery (30% oxygenated) and hepatic portal vein (70% nutrient-rich blood from GI tract).
• Gallbladder: Pear-shaped sac on the inferior liver surface that stores and concentrates bile. Bile flows from hepatic ducts $\rightarrow$ common hepatic duct + cystic duct $\rightarrow$ common bile duct (CBD), joining the main pancreatic duct at the hepatopancreatic ampulla of Vater, regulated by the sphincter of Oddi.
• Pancreas: Retroperitoneal head, body, and tail. Exocrine portion secretes digestive enzymes via the pancreatic duct; endocrine islets of Langerhans secrete insulin and glucagon.
• Spleen: Lymphatic organ in the left upper quadrant (LUQ) lying adjacent to the left hemidiaphragm and gastric fundus; filters old red blood cells and acts as a blood reservoir.

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4. The Urinary System & Renal Microanatomy

The urinary system maintains fluid-electrolyte equilibrium, acid-base balance, and blood pressure regulation through urine excretion.

Kidneys and Nephron Architecture

The kidneys are paired retroperitoneal structures located between T12 and L3. The right kidney sits slightly lower than the left due to hepatic displacement. Each kidney features an outer renal cortex and an inner renal medulla containing 8 to 18 striated renal pyramids.

• The Nephron: Microscopic functional filtration unit (approx. 1 million per kidney). Consists of:
  1. Renal Corpuscle: Glomerulus (capillary tuft) surrounded by Bowman's capsule.
  2. Renal Tubule: Proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), distal convoluted tubule (DCT), and collecting duct.

Drainage Pathway and Excretory Organs

Urine flows from Collecting ducts $\rightarrow$ Renal papillae $\rightarrow$ Minor calyces $\rightarrow$ Major calyces $\rightarrow$ Renal pelvis $\rightarrow$ Ureters (retroperitoneal muscular tubes, approx. 25–30 cm long) $\rightarrow$ Urinary Bladder (muscular sac in the pelvic cavity featuring the triangular trigone demarcated by two ureteral orifices and one urethral orifice) $\rightarrow$ Urethra.

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5. The Skeletal System & Arthrology Classification

The adult human skeleton contains 206 bones, categorized structurally into axial and appendicular divisions.

Axial vs. Appendicular Skeleton

• Axial Skeleton (80 bones): Skull (cranium and facial bones), hyoid bone, vertebral column (7 cervical, 12 thoracic, 5 lumbar, sacrum, coccyx), and thoracic cage (sternum and 12 pairs of ribs).
• Appendicular Skeleton (126 bones): Upper extremities and pectoral girdle (scapulae, clavicles, humeri, radii, ulnae, carpals, metacarpals, phalanges) and lower extremities and pelvic girdle (hip/os coxae, femora, patellae, tibiae, fibulae, tarsals, metatarsals, phalanges).

Bone Classification by Morphology

1. Long Bones: Tubular shaft (diaphysis) and articular ends (epiphyses); e.g., humerus, femur, phalanges.
2. Short Bones: Cuboidal; e.g., carpals and tarsals.
3. Flat Bones: Broad, protective plates of compact bone enclosing cancellous bone (diploe); e.g., cranial bones, sternum, scapulae.
4. Irregular Bones: Complex, specialized shapes; e.g., vertebrae, facial bones, os coxae.
5. Sesamoid Bones: Embedded within tendons to modify angle of pull; e.g., patella, flexor hallucis sesamoids.

Functional and Structural Arthrology (Joints)

Joints are classified functionally by degree of movement and structurally by binding tissue:

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Body System Functional Anatomy Summary Table