Mechanics of Breathing, Gas Exchange, and Transport: Study Guide

Mechanics of Breathing

Muscle Contraction and Airflow

The process of breathing involves the coordinated contraction of skeletal muscles to move air into and out of the lungs. The primary muscles responsible are the diaphragm and intercostal muscles.

• Inspiration: Diaphragm contracts and moves downward, external intercostal muscles lift the rib cage, increasing thoracic volume and decreasing pressure to draw air in.

• Expiration: Usually passive; diaphragm and intercostals relax, thoracic volume decreases, and air is expelled.

• Accessory Muscles: Used during forced breathing (e.g., exercise).

Example: During exercise, accessory muscles such as the sternocleidomastoid and abdominal muscles assist in deeper and faster breathing.

Respiratory System Anatomy

The respiratory system consists of conducting airways (nose, pharynx, larynx, trachea, bronchi, bronchioles) and respiratory airways (alveoli) where gas exchange occurs.

• Primary Respiratory Muscles: Diaphragm and intercostal muscles.

• Accessory Muscles: Sternocleidomastoid, scalene, abdominal muscles.

Hering-Breuer Reflex

The Hering-Breuer reflex is a protective mechanism that prevents over-inflation of the lungs by inhibiting inspiration when stretch receptors in the lungs are activated.

• Protective Value: Prevents lung damage due to excessive stretching.

Resistance to Airflow

Resistance to airflow in the lungs is determined by airway diameter, lung volume, and the viscosity of air.

• Greatest Source of Variable Resistance: Bronchioles, due to their ability to constrict or dilate.

• Factors Affecting Resistance: Smooth muscle tone, inflammation, mucus production.

Respiratory System Overview

The respiratory system is responsible for gas exchange, regulation of blood pH, and protection from inhaled pathogens.

• Primary Functions: Oxygen uptake, carbon dioxide removal, acid-base balance.

Obstructive and Restrictive Pulmonary Disorders

Pulmonary disorders are classified as obstructive (e.g., asthma, COPD) or restrictive (e.g., fibrosis).

• Obstructive Disorders: Increase airway resistance, reduce airflow.

• Restrictive Disorders: Reduce lung compliance, limit lung expansion.

Driving Force for Gas Movement

Movement of oxygen and carbon dioxide is driven by partial pressure gradients between alveoli and blood.

• Oxygen: Moves from alveoli (high PO2) to blood (low PO2).

• Carbon Dioxide: Moves from blood (high PCO2) to alveoli (low PCO2).

Ventilation and Perfusion

Ventilation refers to air movement in the lungs; perfusion refers to blood flow in pulmonary capillaries. The body adjusts both to optimize gas exchange.

• Ventilation-Perfusion Matching: Ensures efficient oxygen uptake and carbon dioxide removal.

Gas Exchange and Transport

Bronchioles and Arterioles Response

Bronchioles and arterioles adjust their diameter to optimize airflow and blood flow, respectively, in response to changes in gas concentrations.

• Bronchioles: Dilate in response to increased CO2, constrict with decreased CO2.

• Arterioles: Constrict in response to low oxygen, dilate with high oxygen.

Partial Pressures of Gases

Partial pressure is the pressure exerted by a single gas in a mixture. It drives diffusion of gases across membranes.

• Oxygen (O2): Partial pressure in alveoli is higher than in blood, promoting diffusion into blood.

• Carbon Dioxide (CO2): Partial pressure in blood is higher than in alveoli, promoting diffusion into alveoli.

Oxygen and Carbon Dioxide Transport

Oxygen is primarily transported bound to hemoglobin; carbon dioxide is transported as dissolved gas, carbaminohemoglobin, and bicarbonate.

• Hemoglobin: Binds oxygen reversibly; affinity changes with pH, CO2, temperature.

• Bicarbonate Formation: CO2 + H2O → H2CO3 → HCO3- + H+

$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow HCO_3^- + H^+$

2,3-DPG and Hemoglobin

2,3-Diphosphoglycerate (2,3-DPG) binds to hemoglobin, reducing its affinity for oxygen and facilitating oxygen release to tissues.

• Effect: Enhances oxygen delivery during hypoxia or anemia.

Fetal vs. Adult Hemoglobin

Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, allowing efficient oxygen transfer from mother to fetus.

• Comparison: Fetal hemoglobin binds oxygen more tightly, facilitating uptake at lower oxygen pressures.

Hypoxia and Cyanosis

Hypoxia is a deficiency of oxygen in tissues; cyanosis is a bluish discoloration due to low oxygen saturation.

• Causes: Respiratory, circulatory, or hemoglobin abnormalities.

Renal Physiology

Kidney Functions and Structure

The kidneys regulate fluid balance, electrolyte concentration, and waste removal. They are surrounded by a capsule and vascular network.

• General Functions: Filtration, reabsorption, secretion, excretion.

• Regions: Cortex (outer), medulla (inner), pelvis (collects urine).

Processes in Urine Formation

Urine formation involves four main processes:

• Filtration: Movement of water and solutes from blood into nephron.

• Reabsorption: Return of useful substances to blood.

• Secretion: Addition of waste products from blood to nephron.

• Excretion: Removal of urine from body.

Glomerular Filtration Rate (GFR) Regulation

GFR is controlled by blood pressure, neural, and hormonal mechanisms.

• Sympathetic Stimulation: Constricts afferent arterioles, reducing GFR during stress.

Renin-Angiotensin Mechanism

The renin-angiotensin system regulates blood pressure and fluid balance.

• Activation: Triggered by low blood pressure or sodium; renin converts angiotensinogen to angiotensin I, then to angiotensin II.

• Effects of Angiotensin II: Vasoconstriction, aldosterone release, increased sodium reabsorption.

Protein Transport in Urine

Proteins are normally reabsorbed in the proximal tubule; presence in urine indicates pathology.

Tubular Secretion and Reabsorption

Tubular secretion adds substances to filtrate; reabsorption returns substances to blood.

• Mechanisms: Active and passive transport processes.

Atrial Natriuretic Peptide (ANP)

ANP is released in response to atrial stretch and promotes sodium and water excretion to lower blood pressure.

• Stimulus: Increased blood volume.

• Effect: Inhibits sodium reabsorption, increases urine output.

Aldosterone Regulation

Aldosterone is released in response to angiotensin II, high potassium, or low sodium, and acts on distal tubules to increase sodium reabsorption and potassium excretion.

• Target Cells: Principal cells of the distal nephron.

• Stimuli: Angiotensin II, increased plasma K+.

Summary Table: Key Processes in Renal Physiology

Process	Main Location	Function
Filtration	Glomerulus	Removes water and solutes from blood
Reabsorption	Proximal tubule, loop of Henle, distal tubule	Returns useful substances to blood
Secretion	Distal tubule, collecting duct	Adds waste products to filtrate
Excretion	Collecting duct, ureter	Removes urine from body

Additional info: Some explanations and terminology have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.