← Biology — Std 12
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Respiration and Circulation

Ch. 8Std 12

Easy Overview

Every cell in your body is a tiny engine that burns fuel (glucose) using oxygen, producing energy (ATP) and waste (carbon dioxide). But oxygen is in the air and cells are deep inside your body. How does oxygen get to them? And how does CO2 get out? That's the job of two coordinated systems — the respiratory system (bringing air in and out of your lungs) and the circulatory system (transporting gases, nutrients, and wastes around your body). This chapter covers both systems and how they work together. The respiratory system is your body's air-handling unit. Air enters through the nostrils, where hairs and mucus filter out dust. It passes through the nasal cavity (warmed and moistened), pharynx (throat), larynx (voice box — contains vocal cords), trachea (windpipe), bronchi (one to each lung), bronchioles (smaller branches), and finally into the alveoli — tiny air sacs where gas exchange happens. Your lungs contain about 300 million alveoli, providing a surface area of about 70-100 square meters — roughly the size of a tennis court packed into your chest. Each alveolus is surrounded by a dense network of capillaries, and the walls are just one cell thick, allowing rapid diffusion of gases. Breathing (ventilation) is the mechanical process of moving air in and out. Inhalation is active: the diaphragm contracts and flattens, and the external intercostal muscles lift the ribs up and out. This increases thoracic volume, decreasing pressure below atmospheric, and air rushes in. Exhalation is normally passive: muscles relax, the diaphragm returns to its dome shape, thoracic volume decreases, pressure increases, and air flows out. This is negative pressure breathing. Gas exchange occurs by simple diffusion across the alveolar-capillary membrane. Oxygen diffuses from alveoli (high pO2 about 100 mmHg) into blood (low pO2 about 40 mmHg). CO2 diffuses in the opposite direction. In blood, oxygen is transported mainly bound to hemoglobin in red blood cells (about 98.5%). Each hemoglobin can carry four oxygen molecules. The oxygen-hemoglobin dissociation curve shows that hemoglobin's affinity for oxygen depends on pO2, pH (Bohr effect), and temperature. Carbon dioxide is transported as bicarbonate ions (70%), carbaminohemoglobin (20-25%), and dissolved in plasma (5-10%). The circulatory system is the transport network. The heart has four chambers: right atrium receives deoxygenated blood from the body; right ventricle pumps it to the lungs; left atrium receives oxygenated blood from the lungs; left ventricle pumps it to the body. The left ventricle has the thickest wall because it generates the highest pressure. Valves prevent backflow. The cardiac cycle has three phases: atrial systole, ventricular systole, and joint diastole — one cycle takes about 0.8 seconds. The heart's rhythm is set by the SA node (pacemaker), spreading to AV node, Bundle of His, and Purkinje fibers. Blood vessels include arteries (carry blood away from heart, thick walls, high pressure), veins (return blood to heart, thin walls, valves), and capillaries (exchange points). Blood pressure is about 120/80 mmHg. Blood contains plasma (55%) and formed elements (45%) — RBCs, WBCs, and platelets.

The respiratory system — structure and passage

The respiratory system consists of upper tract (nostrils, nasal cavity, pharynx, larynx) and lower tract (trachea, bronchi, bronchioles, alveoli). The nasal cavity warms and humidifies air. The pharynx is shared with the digestive tract. The larynx houses vocal cords; the epiglottis closes during swallowing. The trachea has C-shaped cartilage rings keeping it open, then branches into left and right bronchi, which divide into bronchioles (smooth muscle, no cartilage). Terminal bronchioles end in clusters of alveoli. The right lung has three lobes, the left has two (to accommodate the heart).

Alveoli — the gas exchange units

Alveoli are tiny, thin-walled sacs (200-300 micrometers diameter) at the ends of bronchioles. Their walls are squamous epithelium (type I pneumocytes) — one cell thick for rapid diffusion. Type II pneumocytes secrete surfactant (a phospholipid-protein mixture) that reduces surface tension, preventing alveolar collapse. Each alveolus is wrapped in capillaries. The alveolar-capillary membrane (0.2-0.5 micrometers thick) is where O2 and CO2 diffuse. With 300 million alveoli providing 70-100 square meters total surface area, gas exchange is extraordinarily efficient.

Mechanism of breathing — ventilation

Breathing is driven by pressure differences. The pleural cavity has negative pressure keeping lungs inflated. Inhalation: diaphragm contracts (flattens), external intercostals contract (ribs move up and out). Thoracic volume increases, intrapleural pressure drops to -6 to -8 mmHg below atmospheric, air flows in. Exhalation: muscles relax, diaphragm domes up, ribs move down. Lung volume decreases, pressure increases, air flows out. Tidal volume (normal breath) is about 500 mL. Forced breathing uses accessory muscles — scalene and sternocleidomastoid for inhalation; internal intercostals and abdominals for exhalation.

Respiratory volumes and capacities

Spirometry measures lung volumes. Tidal Volume (TV): about 500 mL per normal breath. Inspiratory Reserve Volume (IRV): additional air forcefully inhaled after normal inspiration (about 3000 mL). Expiratory Reserve Volume (ERV): additional air forcefully exhaled after normal expiration (about 1200 mL). Residual Volume (RV): air remaining after maximal exhalation (about 1200 mL). Capacities: Inspiratory Capacity = TV + IRV. Vital Capacity = IRV + TV + ERV (about 4800 mL). Total Lung Capacity = VC + RV (about 6000 mL). FEV1 (Forced Expiratory Volume in 1 second) diagnoses obstructive lung diseases.

Transport of oxygen — hemoglobin's job

Oxygen is transported mainly bound to hemoglobin (Hb) in RBCs. Each Hb (with 4 heme groups, each containing Fe2+) carries up to 4 O2 molecules. The reaction: Hb + 4O2 = Hb(O2)4. The oxygen-hemoglobin dissociation curve is sigmoid due to cooperativity — binding one O2 increases affinity for the next. Factors shifting curve right (releasing O2 more readily): increased CO2 (Bohr effect), decreased pH (more H+), increased temperature, increased 2,3-BPG. In lungs (high pO2, low CO2, higher pH), Hb loads O2. In tissues (low pO2, high CO2, lower pH), Hb unloads O2.

Transport of carbon dioxide — the return journey

CO2 is transported in three forms. (1) As bicarbonate (HCO3-) — about 70%. In RBCs, carbonic anhydrase speeds: CO2 + H2O to H2CO3 to H+ + HCO3-. HCO3- diffuses into plasma (chloride shift: Cl- enters RBC). (2) As carbaminohemoglobin (Hb-CO2) — about 20-25%. CO2 binds directly to amino groups of hemoglobin. (3) Dissolved in plasma — about 5-10%. In the lungs, these reactions reverse: HCO3- + H+ to H2CO3 to CO2 + H2O. CO2 diffuses into alveoli and is exhaled. This is why breathing rate increases when blood CO2 rises.

The heart — your personal pump

Your heart has four chambers: two atria (receiving rooms) and two ventricles (pumping rooms). Right side pumps deoxygenated blood to the lungs (pulmonary circulation). Left side pumps oxygenated blood to the body (systemic circulation). The left ventricle has the thickest wall — it must generate enough pressure to push blood throughout the body. Valves: atrioventricular (tricuspid on right, bicuspid/mitral on left) prevent backflow into atria; semilunar valves (pulmonary and aortic) prevent backflow into ventricles. The lub-dub sound is valves closing.

Cardiac cycle — one heartbeat in detail

One heartbeat equals one cardiac cycle. Atrial systole (0.1 sec): atria contract, pushing blood into ventricles. Ventricular systole (0.3 sec): ventricles contract, AV valves close (lub sound), semilunar valves open, blood ejected into arteries. Joint diastole (0.4 sec): all chambers relaxed, semilunar valves close (dub sound), heart fills passively. Total cycle: about 0.8 seconds at 72 beats per minute. Cardiac output = heart rate x stroke volume (about 5 L/min at rest). During exercise, both rate and stroke volume increase.

Blood vessels — the highway system

Arteries carry blood away from the heart. They have thick, elastic, muscular walls to handle high pressure. Arteries branch into arterioles, then into capillaries — thin-walled (one cell thick) vessels where all exchange happens. Capillaries merge into venules, then veins, which return blood to the heart. Veins have thin walls, larger lumen, and valves to prevent backflow. Blood pressure is highest in arteries (systolic about 120 mmHg, diastolic about 80 mmHg) and lowest in veins (about 0-5 mmHg). Pulse is the rhythmic expansion of arteries.

ECG — the heart's electrical activity

An electrocardiogram (ECG) records the electrical activity of the heart. The P wave represents atrial depolarization (contraction). The QRS complex represents ventricular depolarization (contraction). The T wave represents ventricular repolarization (relaxation). The PR interval (0.12-0.20 sec) is the time for impulse to travel from SA node to ventricles. The QT interval represents total ventricular activity. ECG is used to diagnose arrhythmias, heart attacks, and other cardiac conditions. A normal ECG shows regular rhythm with specific wave patterns.

Blood composition — the river of life

Blood consists of plasma (55%) and formed elements (45%). Plasma is 90% water plus proteins (albumin, globulins, fibrinogen), nutrients, hormones, and wastes. Formed elements: (1) Red blood cells (erythrocytes) — biconcave discs, no nucleus, contain hemoglobin, live about 120 days. (2) White blood cells (leukocytes) — fight infection; five types: neutrophils (60%, bacteria), lymphocytes (30%, immunity), monocytes (6%, macrophages), eosinophils (3%, allergies), basophils (1%, inflammation). (3) Platelets (thrombocytes) — cell fragments involved in blood clotting.

Blood clotting — plugging the leak

When a blood vessel is injured, three steps occur: (1) Vasoconstriction — blood vessel narrows to reduce blood flow. (2) Platelet plug formation — platelets adhere to exposed collagen and aggregate, forming a temporary plug. (3) Coagulation — a cascade of clotting factors (thromboplastin, prothrombin, thrombin, fibrinogen) converts fibrinogen to fibrin, forming a stable clot. Vitamin K is essential for producing several clotting factors. Clotting disorders: hemophilia (genetic deficiency of factor VIII or IX) and thrombosis (abnormal clot formation).

Lymphatic system — the secondary circulation

The lymphatic system is a network of vessels, lymph nodes, and lymphoid organs (spleen, thymus, tonsils). Lymph (excess interstitial fluid) enters lymphatic capillaries, flows through lymph nodes (filtered by lymphocytes), and returns to the blood via the thoracic duct. Functions: (1) Returns excess fluid and proteins to blood. (2) Transports absorbed fats from intestines (lacteals). (3) Defends against infection (lymph nodes produce lymphocytes and filter pathogens). Lymph nodes swell during infection as lymphocytes multiply.

Respiratory and circulatory disorders

Common respiratory disorders: Asthma (bronchial constriction triggered by allergens), Chronic Obstructive Pulmonary Disease (COPD — includes emphysema and chronic bronchitis, mainly from smoking), Pneumonia (lung infection with fluid in alveoli), Tuberculosis (bacterial infection causing lung lesions), and Emphysema (destruction of alveolar walls reducing surface area). Circulatory disorders: Hypertension (high blood pressure, risk for heart attack and stroke), Atherosclerosis (plaque buildup in arteries), Myocardial infarction (heart attack — blockage of coronary artery), and Angina pectoris (chest pain from reduced blood flow to heart).

Key Points

  • •Air pathway: nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, alveoli
  • •Alveoli are gas exchange sites with huge surface area (70-100 m2) and thin walls
  • •Breathing: diaphragm + intercostal muscles change chest volume, creating pressure gradient
  • •Tidal volume ~500 mL; Vital capacity ~4800 mL; Residual volume ~1200 mL
  • •Hb + O2 to oxyhemoglobin (in lungs); O2 released in tissues (low pO2, high pCO2)
  • •CO2 transported as bicarbonate (70%), carbaminohemoglobin (20-25%), dissolved (5-10%)
  • •Heart: 4 chambers; SA node, AV node, Bundle of His, Purkinje fibers conduct impulse
  • •Cardiac cycle: atrial systole (0.1s), ventricular systole (0.3s), joint diastole (0.4s)
  • •ECG: P wave (atrial depolarization), QRS (ventricular depolarization), T wave (ventricular repolarization)
  • •Arteries carry blood away from heart, veins return blood, capillaries exchange materials
  • •Blood pressure: systolic ~120 mmHg, diastolic ~80 mmHg
  • •Blood: plasma (55%) + formed elements (45%) — RBCs, WBCs (5 types), platelets
  • •Blood clotting: vasoconstriction, platelet plug, coagulation cascade (fibrinogen to fibrin)
  • •Lymphatic system returns interstitial fluid, transports fats, defends against infection
  • •Respiratory disorders: asthma, COPD, pneumonia, TB, emphysema
  • •Circulatory disorders: hypertension, atherosclerosis, myocardial infarction, angina

Practice Questions

  • Trace the path of air from the nostrils to the alveoli. What happens at each step?
  • Explain the mechanism of breathing. Why is human breathing called negative pressure breathing?
  • How is oxygen transported in the blood? What factors affect hemoglobin's affinity for O2?
  • Describe the structure of the human heart with a labeled diagram.
  • What is the cardiac cycle? Explain atrial systole, ventricular systole, and diastole.
  • Draw and label a normal ECG trace. What do the P wave, QRS complex, and T wave represent?
  • Differentiate between arteries, veins, and capillaries in structure and function.
  • Describe the process of blood clotting. What happens in hemophilia?