Circulatory and Respiratory Systems

Core Review

The Circulatory System

Heart Anatomy: The heart is a four-chambered muscular pump:

• Right atrium (RA): Receives deoxygenated blood from the body via the superior and inferior vena cavae.
• Right ventricle (RV): Pumps deoxygenated blood to the lungs via the pulmonary artery. The tricuspid valve (right AV valve) prevents backflow into the RA.
• Left atrium (LA): Receives oxygenated blood from the lungs via four pulmonary veins.
• Left ventricle (LV): Pumps oxygenated blood to the body via the aorta. The bicuspid (mitral) valve prevents backflow into the LA. The LV has the thickest myocardium because it must generate the highest pressure.

Semilunar valves (pulmonary and aortic) prevent backflow from the great arteries into the ventricles.

Blood Flow Path (memorize in order): Body (venae cavae) → RA → tricuspid valve → RV → pulmonary semilunar valve → pulmonary artery → lungs (gas exchange) → pulmonary veins → LA → bicuspid/mitral valve → LV → aortic semilunar valve → aorta → body.

Pulmonary Circuit: RV → pulmonary artery → lungs → pulmonary veins → LA. Low pressure, low resistance. Function: gas exchange. Systemic Circuit: LV → aorta → body tissues → venae cavae → RA. High pressure, high resistance. Function: deliver oxygen and nutrients.

Blood Vessel Structure:

Key distinction: arteries carry blood AWAY from the heart. This is not synonymous with "oxygenated." The pulmonary artery carries deoxygenated blood away from the RV to the lungs. The pulmonary veins carry oxygenated blood toward the LA.

Blood Components:

• Red blood cells (erythrocytes): Anucleate (in mammals), biconcave discs carrying hemoglobin. Contain carbonic anhydrase for CO₂ transport. Produced in red bone marrow; regulated by erythropoietin (EPO) from the kidneys in response to hypoxia.
• White blood cells (leukocytes): Defend against infection (neutrophils, lymphocytes, monocytes, eosinophils, basophils).
• Platelets (thrombocytes): Cell fragments from megakaryocytes; essential for blood clotting (hemostasis).

The Respiratory System

Ventilation (Breathing): Air enters through the nasal/oral cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. Gas exchange occurs ONLY at the alveoli — the rest of the respiratory tract is "anatomical dead space."

Inspiration (inhalation): ACTIVE process. The diaphragm contracts (flattens) and external intercostal muscles contract (raise ribs). Thoracic volume increases → intrathoracic pressure decreases → air rushes in along the pressure gradient.

Expiration (exhalation at rest): PASSIVE process. The diaphragm and external intercostals relax. Elastic recoil of the lungs and chest wall decreases thoracic volume → pressure increases → air flows out. FORCED expiration recruits internal intercostals and abdominal muscles.

Alveolar Gas Exchange: The alveoli are thin-walled sacs surrounded by pulmonary capillaries. The respiratory membrane (alveolar epithelium + capillary endothelium + fused basement membrane) is extremely thin (~0.5 μm), maximizing diffusion. Fick's Law governs gas exchange: rate = (surface area × concentration gradient × diffusion coefficient) / thickness. The large surface area (~70 m²) of alveoli and the thin respiratory membrane optimize this.

Oxygen Transport: ~98% of O₂ is transported bound to hemoglobin (Hb) as oxyhemoglobin (HbO₂). Each Hb can bind up to 4 O₂ molecules. The oxygen-hemoglobin dissociation curve is sigmoidal — cooperative binding (Hb's affinity for O₂ increases as more O₂ binds).

Factors shifting the curve to the RIGHT (decreased Hb affinity → more O₂ delivery to tissues — Bohr effect):

• ↑ CO₂, ↑ H⁺ (↓ pH), ↑ temperature, ↑ 2,3-BPG (bisphosphoglycerate)

Carbon Dioxide Transport: CO₂ is transported in three forms:

1. Bicarbonate (HCO₃⁻) — ~70%: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺, catalyzed by carbonic anhydrase in RBCs. HCO₃⁻ is exchanged for Cl⁻ (chloride shift) across the RBC membrane. In the lungs, the reverse reaction releases CO₂ for exhalation.
2. Carbaminohemoglobin (~23%): CO₂ binds to amino groups on hemoglobin.
3. Dissolved CO₂ in plasma (~7%): Directly dissolved.

Control of Breathing

The respiratory centers in the medulla oblongata and pons regulate the rate and depth of breathing. The primary stimulus is CO₂ (via its effect on pH): central chemoreceptors in the medulla detect ↑ CO₂ (→ ↑ H⁺ in CSF) and increase ventilation. Peripheral chemoreceptors (carotid and aortic bodies) detect ↓ O₂, ↑ CO₂, and ↓ pH. During exercise, increased CO₂ production is the main driver of increased breathing.

Common Traps

• "Veins carry deoxygenated blood": The pulmonary veins carry oxygenated blood from lungs to the left atrium. The rule is "veins carry blood TO the heart" — not about oxygenation.
• "The right ventricle pumps to the body": No — the RIGHT ventricle pumps to the LUNGS (pulmonary circuit). The LEFT ventricle pumps to the body (systemic circuit). The LV wall is much thicker.
• "O₂ is mostly dissolved in plasma": ~98% is bound to hemoglobin. Very little O₂ is dissolved. CO₂ is the one transported primarily as bicarbonate, not dissolved.
• "The diaphragm relaxes to inhale": No — the diaphragm CONTRACTS (descends) during inhalation. Relaxation causes passive exhalation.