Human Heart Physiology Quiz

Cardiovascular System, Biological Fluids, Renal Function

• CFR.5 The Heart: Structure, physiology, cardiac cycle, arterial blood pressure, cardiac failure

Learning Outcomes

• Describe the features of the human heart and its role in systemic and pulmonary circulatory systems
• Identify the external features of the human heart, major arteries, veins, compartments, cardiac muscle, orientation and location in the body
• Describe the internal features of the heart: atria, ventricles, septum, bicuspid and tricuspid valves, semilunar valves, sinoatrial node
• Explain the cardiac cycle stages (systole, diastole, stroke volume, cardiac output, Frank-Starling law)
• Discuss blood pressure measurements, and different levels detected in different parts of the systemic circulation (aorta vs vena cava)

The Heart

• The heart circulates blood
• Found in the thorax, within its own pericardial cavity
• Attached to the walls of the pericardium where great vessels enter/exit
• Pericardium is lined by mesothelium (simple squamous epithelium), forming part of the pericardial membrane
• Epicardium is the outer covering of the heart, covered by mesothelium

The Heart Structure (Diagram)

• External and internal structures labelled
• Atria, ventricles, septum, valves identified
• Aorta, superior and inferior vena cava, pulmonary arteries and veins

Structure of the Heart (Diagram)

• Diagram of the heart's internal structure with numbered labels
• Include major components and vessels in labelled diagram

Systemic and Pulmonary Circulation (Diagram)

• Diagram illustrating the pathways of blood flow in both circulatory systems in the body
• Label major vessels and organs in both circuits

Cardiac Muscle Physiology

• Coordination of muscular contraction is vital for proper heart function
• Cardiac muscle is striated, but arranged into individual cells separated by areas of intercalated discs
• Intercalated discs have numerous gap junctions which allow free diffusion of ions between cells
• Cardiac muscle is myogenic, meaning it contracts rhythmically and continuously without external stimulation
• Functions as a syncytium (action potentials spreading from cell to cell)
• Refractory period: 0.25 - 0.3 seconds; no further contraction during this period

Nervous Control of the Heart - CV Center

• Heart rate regulated by the cardiovascular/ cardiac centre in the medulla oblongata which responds to pressure receptors in the walls of blood vessels
• Input to the centre includes from higher brain centres, proprioceptors, baroreceptors and chemoreceptors
• Output to effectors increase nerve impulses leading to heart rate changes
• Blood vessels vasoconstrict/vasodilate

Heart Rate

• Varies widely in different mammals; generally lower in larger animals
• Adult human heart: ~72 beats per minute
• Can rise to 195 beats per minute under stress
• Trained athletes tend to have lower resting heart rates (~50 beats per min)

Physiology of the Heart

• Contraction begins spontaneously in the sinoatrial (SA) node in the right atrium near the superior vena cava entry
• SA node (pacemaker)
• Consists of modified cardiac muscle mixed with nerve fibres of the autonomic nervous system (sympathetic and parasympathetic/vagus nerve)

Physiology of the Heart

• Heart beat originates spontaneously at the SA node due to its myogenic nature
• SA node generates 100 beats per minute
• Autonomic nervous system can 'slow down' or 'speed up' heart rate (change frequency)
• Sympathetic fibres increase heart rate by up to 100%
• Parasympathetic fibres decrease heart rate by up to 30%

Excitatory and Conductive System

• SA node initiates the heartbeat, and sends signals via the conducting system to stimulate contractions

• Bundle Of His/atrioventricular bundle (start of the system)

• Right bundle branch

• Left bundle branch

• Purkinje fibres (end of system): conduct impulses and allow both ventricles to contract simultaneously at ~4m/s

Cardiac Cycle

• Contraction (systole) and relaxation (diastole) of heart chambers; period from end of one contraction to the end of the next is the cardiac cycle
• Typically 70 beats per minute in healthy adults (slowed by parasympathetic nervous system)
• One complete cycle ~0.8 seconds
• Human heart beats approximately 2.6 billon times in an average lifespan

Cardiac Cycle

• Starts with atrial contraction initiating by the SA node
• Blood normally flows continuously into the atria from vena cavae and pulmonary vein
• ~70% of atrial blood flows directly into the ventricles while ~30% is pushed into ventricles by atrial systole
• Delay in transmission through the AV node causes a pause between atrial systole and ventricular systole

Cardiac Cycle

• During ventricular diastole, the ventricles fill to a volume of 120-130ml
• Ventricular Systole: Pressure rise leads to atrioventricular valve closure, followed by semilunar valve opening
• 70ml ejected per contraction, the stroke volume;
• remaining volume is end-systolic volume (~50-60 ml).

Cardiac Output

• Volume of blood ejected into the aorta (or right ventricle) per minute
• Calculated as Stroke Volume x Heart Rate
• Normal approx. 5 Litres/min
• Can increase to 25 Litres/min under stress

Frank-Starling Law of the Heart

• Intrinsic ability of the heart to adapt to changing loads of inflowing blood
• Ventricles are elastic and can expand to accommodate a larger volume of blood
• All blood coming to heart is pumped
• Relationship between Ventricle end-diastolic pressure (LVEDP) and Stroke Volume (SV)

Blood Pressure

• Left ventricle drives blood around arteries
• Measured using a sphygmomanometer
• Typical reading in healthy adults: 120/80 mmHg (systolic/diastolic)
• Systolic pressure: highest blood pressure reading
• Diastolic pressure: lowest blood pressure reading

Blood Pressure

• Mean Arterial Pressure (MAP) = Diastolic + (1/3rd Systolic - Diastolic)
• Aorta: 100mmHg
• Arterioles: 85 mmHg
• Capillaries: 25mmHg
• Vena Cavae: 0 mmHg

Blood Pressure - Systemic Circulation

• Pulse pressure: difference between systolic and diastolic pressure readings
• Pulse pressure decreases as blood passes through smaller arteries and disappears in capillaries (~1mmHg)

Blood Pressure

• Maximal systolic pressure ends with a sharp drop (incisura/dicrotic notch) during ventricular diastole due to backflow
• The rapid closure of aortic semilunar valves causes a brief rebound (notch)

Learning Resources

• Sherwood. Human Physiology (Chapters 9 & 10)
• Solomon. Biology (Chapter 44)
• Chiras. Human Biology (6th Edition, Chapters 5 & 6)