Cardiovascular System Revision: Heart Structure and Function

Cardiovascular System

• The heart is a hollow, muscular, cone-shaped organ located in the mediastinum of the thoracic cavity, between the sternum and vertebrae, and between the lobes of the lungs.
• The heart has four chambers: right atrium, left atrium, right ventricle, and left ventricle.
• The heart valves (tricuspid, bicuspid, pulmonary semilunar, and aortic semilunar) prevent backflow of blood between the chambers and blood vessels.

Heart Valves

• Tricuspid valve: between right atrium and right ventricle, prevents backflow of blood from the right ventricle to the right atrium.
• Bicuspid valve: between left atrium and left ventricle, prevents backflow of blood from the left ventricle to the left atrium.
• Aortic semilunar valve: between the left ventricle and the aorta, prevents backflow of blood from the aorta to the left ventricle.
• Pulmonary semilunar valve: between the right ventricle and the pulmonary trunk, prevents backflow of blood from the pulmonary trunk to the right ventricle.

Coronary Circulation

• Function: supplies the myocardium with oxygen and nutrients, and removes wastes.
• Left coronary artery: supplies the left atrium and posterior wall of the left ventricle.
• Right coronary artery: supplies the right atrium and nearly all the right ventricle.
• Stages of coronary circulation:
  1. Ascending aorta
  2. Left and right coronary arteries branch from the ascending aorta
  3. Capillaries in the myocardium
  4. Coronary veins take blood to the coronary sinus
  5. The coronary sinus transports blood to the right atrium

Blood Flow through the Heart

• Blood flow sequence:
  1. Superior and inferior vena cavae and coronary sinus
  2. Right atrium
  3. Tricuspid valve
  4. Right ventricle
  5. Pulmonary semilunar valve
  6. Pulmonary trunk
  7. Right and left pulmonary arteries to the lungs
  8. Oxygenated blood returns to the heart through the pulmonary veins
  9. Left atrium
  10. Mitral valve
  11. Left ventricle
  12. Aortic semilunar valve
  13. Ascending aorta
  14. Arch of the aorta
  15. Descending aorta

Pericardium

• Layers of the heart:
  1. Fibrous pericardium: most outer layer, anchors the heart to surrounding structures, and prevents overfilling.
  2. Serous pericardium: inner layer, parietal serous lines the fibrous pericardium, and visceral serous attaches to the heart.
  3. Pericardial fluid: lubricates the heart and prevents friction.
  4. Myocardium: middle layer, cardiac muscle contracts and relaxes to pump blood.
  5. Endocardium: inner layer, thin membrane that lines the valves and is continuous with the large blood vessels.

Cardiac Cycle

• Stages of the cardiac cycle:
  1. Atrial systole: atria contract and pump blood into the ventricles.
  2. Ventricular systole: ventricles contract and pump blood into the aorta and pulmonary trunk.
  3. Diastole: atria and ventricles relax, and the heart fills with blood.

ECG

• The ECG (electrocardiograph) measures the electrical activity of the heart.
• P wave: atrial depolarization
• QRS complex: ventricular depolarization
• T wave: ventricular repolarization
• ECG waves relate to the stages of the cardiac cycle and the opening and closing of the valves.

Pacemaker System

• Components of the pacemaker system:
  1. Sinoatrial (SA) node: spontaneously and rhythmically initiates depolarization.
  2. Atrioventricular (AV) node: receives impulses from the SA node and delays them.
  3. Atrioventricular bundle (Bundle of His): receives impulses from the AV node and distributes them to the ventricles.
  4. Right and left bundle branches: receive impulses from the Bundle of His and distribute them to the ventricles.
  5. Purkinje fibers: conduct impulses from the bundle branches to the ventricles.
• Gap junctions in the myocardium allow electrical impulses to spread throughout the heart.

Blood Pressure

• Blood pressure: force of blood on the walls of the arteries.
• Systolic blood pressure: higher pressure measured during left ventricular systole.
• Diastolic blood pressure: lower pressure measured during left ventricular diastole.
• Mean arterial pressure: average arterial pressure throughout one cardiac cycle.
• Functions of blood pressure:
  1. Pushes blood through blood vessels to the organs and tissues.
  2. Causes capillary exchange between blood and cells.
  3. Causes glomerular filtration of blood in the kidneys.

Control of Blood Pressure

• Baroreceptors: detect changes in blood pressure.
• Cardiac and vasomotor centers: located in the medulla oblongata, control heart rate and blood vessel diameter.
• Autonomic nervous system (ANS): influences heart rate and blood vessel diameter.
• Ways to quickly raise or lower blood pressure:
  1. Heart: increase or decrease heart rate and contractility.
  2. Arteries/arterioles: constrict or dilate blood vessels.
  3. Veins: increase or decrease venous return.

Blood Vessels

• Types of blood vessels:
  1. Arteries: carry blood away from the heart.
  2. Arterioles: deliver blood to capillaries.
  3. Capillaries: site of nutrients exchange.
  4. Veins: carry blood towards the heart.
• Structure of blood vessels:
  1. Elastic arteries: large lumen, can stretch and recoil without damage.
  2. Muscular arteries: thicker wall, carry blood to furthest destinations from the heart.
  3. Arterioles: variable contract/relax of smooth muscle, affect peripheral resistance.
  4. Capillaries: super thin walls, involved in microcirculation.
  5. Veins: return blood to the heart, have one-way valves to prevent backflow.

Note: This is a summary of the text in bullet points. It is not a comprehensive review of the cardiovascular system.### Blood Vessels

• Venules:
  • Form by uniting capillaries
  • Transport blood to veins
  • Walls consist of endothelial cells, smooth muscle cells, and fibroblasts
  • WBCs move out of the CVS to inflamed areas through pores in venules
• Veins:
  • Have thin walls and one-way valves
  • Act as blood reservoirs
  • Can be drawn when BP drops
  • Contraction of skeletal muscle assists blood flow
  • Proportion of blood: 65% in veins, 15% in arteries, and capillaries

Structure of Blood Vessels

• Hollow interior called lumen
• Walls consist of three layers:
  1. Tunica intima (lined by endothelium)
• Consists of simple squamous epithelium
• Smooth to reduce friction
• Larger vessels also have a basement membrane and connective tissue 2. Tunica media (composed of smooth muscle and elastic fibers)
• Circular smooth muscle vasodilates and vasoconstricts to change vessel diameter 3. Tunica externa (has a network of collagen fibers)
• Protects and reinforces the vessel
• Anchors the vessel to surrounding structures
• Nerve fibers and elastic fibers in larger vessels

Location of Specific Arteries

• Aorta: ascending, arch, descending, thoracic, and abdominal
• Carotid: external carotid (tissues of head, place to palpate), internal carotid (brain and eye), and temporal artery
• Abdominal aorta: left and right common iliac arteries
• Upper limbs: axillary, brachial, radial, and ulnar arteries
• Lower limbs: popliteal, posterior tibial, and dorsalis pedis arteries

Venous Return

• Blood flows out of organs via venules
• Venules to veins
• Drains to inferior and superior vena cava
• Blood returned to right atrium
• Risk of low venous return: reduces blood movement into the heart, leading to inadequate blood supply to the heart

Blood and Haemostasis

• Functions of blood:
  • Transportation: oxygen, nutrients, metabolic waste, and hormones
  • Regulation: pH, body temperature, and water content of cells
  • Protection: from diseases, excess bleeding, and antibodies
• Total blood volume: 5,000-6,000 mL in males, 4,000-5,000 mL in females

Red Blood Cells

• Characteristics: biconcave discs, no nucleus or organelles
• Functions: carries oxygen, contains haemoglobin, and helps regulate blood viscosity
• Haemoglobin: protein with four haem groups, each with Fe2+ that can bind to one oxygen molecule
• Anaemia: lack of haemoglobin, leading to inadequate oxygen supply to tissues

White Blood Cells

• Functions: defend against pathogens, toxins, and cancerous cells
• Types:
  • Neutrophils: phagocytose particles and pathogens
  • Monocytes: differentiate into macrophages and dendritic cells
  • Eosinophils: combat parasites and helminths
  • Basophils: key role in inflammatory reactions

Platelets

• Functions: involvement in blood clotting and coagulation
• Fragments of cells enclosed in a piece of cell membrane, no nucleus

Production of RBCs

• Stimulus: hypoxaemia (low oxygen levels in the blood)
• Production process: erythropoietin (EPO) production in the kidneys, stem cell division in red bone marrow, haemoglobin production, and nucleus ejection

Homeostasis of RBC Levels

• Negative feedback system: kidney cells detect low oxygen levels, produce EPO, and stimulate RBC production
• Rate of RBC production is increased after blood loss, inadequate RBC production, or low oxygen availability

Plasma

• Components: water, proteins, electrolytes, regulatory molecules, dissolved gases, nutrients, and waste products
• Functions: acts as a solvent, regulates blood pH, and transports blood components

Blood Tests

• CBC: complete blood count, screens for anaemia and infection
• HCT/PCV: haematocrit, percentage of total blood made up by red blood cells
• Hb: haemoglobin concentration in blood
• MCV: mean cell volume, average volume of each red blood cell
• MCH: mean cell haemoglobin, average amount of haemoglobin in each red blood cell

Haemostasis

• Definition: rapid, localised prevention of bleeding in damaged vessels
• Stages: vascular spasm, platelet plug formation, and coagulation
• Functions: prevents haemorrhage, promotes healing, and regulates blood clotting

Respiratory System

• Functions: external gas exchange, pH regulation, sense of smell, voice production, and aids venous and lymph return
• Structures: upper respiratory tract (nose, pharynx, and larynx) and lower respiratory tract (trachea, bronchi, and alveoli)

Upper Respiratory Tract

• Nose: external cavity, nasal cavity, and nasal mucosa
• Pharynx: nasopharynx, oropharynx, and laryngopharynx
• Larynx: connects pharynx to trachea, composed of hyaline cartilage

Lower Respiratory Tract

• Trachea: has C-shaped rings, lined with mucous membrane, and contains smooth muscle and elastic tissue
• Bronchi: primary, secondary, and tertiary bronchi
• Bronchioles: lined with simple cuboidal epithelium, and contains smooth muscle and elastic tissue
• Alveoli: lined with simple squamous epithelium, rich supply of blood capillaries, and 300 million in total

Pleura

• Visceral pleura: covers the lung
• Parietal pleura: lines the wall of the thoracic cavity
• Pleural cavity: contains serous fluid, reduces friction in breathing movements, and causes the membranes to adhere to each other### Respiratory System
• Mucosa at the carina (branch point of the trachea into the bronchi) is extremely sensitive, triggering violent coughing when touched
• Coughing helps expel mucus during smoking, preventing pathogens and dust from entering the lungs
• Breathing and gas exchange:
  • Inspiration: diaphragm and external intercostal muscles contract, increasing thoracic cavity volume, and air flows into the lungs
  • Expiration: diaphragm and external intercostal muscles relax, decreasing thoracic cavity volume, and air flows out of the lungs
• Chemical stimuli that modify breathing:
  • Changes in PCO2 levels
  • Changes in H+ concentration
  • Large decreases in PO2
• Inflation reflex prevents over-inflation of the lungs

Factors Affecting Airflow and Breathing

• Surface tension in the alveoli
• Compliance of the lungs
• Airway resistance:
  • Caused by friction between air and air passages
  • Increased by obstructing airways or reducing diameter
  • Smooth muscle in bronchiole walls sensitive to neural controls and chemicals
• Factors affecting the work of breathing:
  • Ease: open, unobstructed airway; minimal resistance to airflow; ability to move diaphragm and ribs; compliant lungs; clear alveolar-capillary interface
  • Difficulty: obstruction to airflow; constriction from spasm (e.g., asthma); injury or obstruction to diaphragm/ribs; stiff lungs; alveoli contain secretions/fluid (e.g., pneumonia)

Respiratory Terms and Concepts

• Lung compliance: ease with which the lungs and thoracic wall can be expanded
• Airway resistance: resistance to air flow in respiratory passageways
• Surface tension vs. surfactant:
  • Surface tension: thin watery liquid film in alveoli; prevents alveoli from collapsing; makes the lungs recoil during expiration
  • Surfactant: reduces surface tension; prevents alveoli from collapsing; reduces energy required to expand the lungs during inspiration
• Lung volumes and terms:
  • Tidal volume (VT): amount of air that moves in or out of the lungs with each respiratory cycle
  • Vital capacity (VC): total amount of air exhaled after maximal inhalation
  • Total lung capacity (TLC): maximal volume of gas in the lungs after a maximal inhalation
  • Residual volume (RV): maximum volume of air that can be inhaled after normal inspiration
  • Forced expiratory volume in 1 second (FEV1): maximum amount of air that can be forcibly exhaled during the first second following maximal inhalation
• Internal respiration: occurs between systemic capillaries and body tissues
• External respiration: occurs between the alveolus and pulmonary capillaries

Gas Exchange and Transport

• Gas exchange:
  • Ventilation of the lungs
  • Diffusion of O2 from alveoli into pulmonary capillaries
  • Perfusion of systemic capillaries with oxygenated blood
  • Diffusion of O2 from systemic capillaries into cells
• Oxygen transport:
  • 98.5% transported by hemoglobin (Hb); 1.5% dissolved in plasma
  • Hb releases O2 to tissues with low PO2
• Carbon dioxide transport:
  • 70% carried as bicarbonate ions; 7% dissolved in plasma
  • CO2 diffuses from cells into blood, then transported to lungs for exhalation

Arterial Blood Gas Sampling and Respiratory System Disorders

• Arterial blood gas sampling: measures H+, HCO3-, and calculates pH; helps determine the level of hypoxaemia, hypercapnia, and metabolic disturbance
• Hypoxia: absence or depletion of oxygen within cells and tissues
• Hypoxaemia: depletion of oxygen within the blood
• Hypercapnia: rise in carbon dioxide level in the blood
• Smoking: damages and destroys cilia, impairing the lungs' ability, and reducing the blood's capacity to carry oxygen

Control of Breathing

• Involuntary control is controlled by the autonomic nervous system in response to changes in pH or CO2 levels in the blood or pH of CSF
• Medulla oblongata and Pons contain respiratory center, controlling rate and depth of breathing
• Sympathetic and parasympathetic nervous systems influence breathing

Effects of Aging on the Respiratory System

• Respiratory function decreases with age
• Airways and tissues of the respiratory system become less elastic and more rigid
• Elasticity of the lungs decreases
• Vital capacity declines
• Decrease in blood oxygen level
• Older person has reduced ability to perform vigorous exercises
• Sleep apnea is more common
• The older person is more susceptible to pulmonary disorders like pneumonia and bronchitis