Cardiac Cycle: Systole and Diastole

Cardiac Cycle

• The cardiac cycle consists of systole (contraction) and diastole (relaxation) phases.
• The cardiac cycle phases are isovolumic contraction, ejection, isovolumic relaxation, rapid inflow, diastasis, and atrial systole.

Systole

Isovolumic Contraction

• All heart valves are closed.
• The closing of the AV valves marks the beginning of systole, producing the first heart sound, "lub".

Ejection

• Semilunar valves open, allowing blood to be ejected.
• AV valves remain closed.

Diastole

Isovolumic Relaxation

• All heart valves are closed.
• The closing of the semilunar valves marks the beginning of diastole, producing the second heart sound, "dubb".

Rapid Inflow

• Semilunar valves are closed.
• AV valves open.
• The ventricles fill to 70-80% capacity.

Diastasis

• Semilunar valves are closed.
• AV valves are open.
• There is little change in ventricular volume.

Atrial Systole

• Semilunar valves remain closed.
• AV valves are open.
• Atrial contraction adds 20-30% more volume to the ventricles.

Blood Pressure and Cardiac Output

• Mean Arterial Pressure (MAP) = Diastolic Pressure + 1/3 (Systolic-Diastolic)
• Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR)
• Fick Principle formula: CO = VO2 / (Ca - Cv)
  • VO2 = oxygen consumption (ml of pure gaseous oxygen per minute).
  • Ca = oxygen content of arterial blood.
  • Cv = oxygen content of mixed venous blood.

Nervous System Effects on Heart

• The sympathetic nervous system increases blood pressure through vasoconstriction and increases heart rate.
• The parasympathetic nervous system slows heart rate through the release of acetylcholine.

Cardiac Output Factors

• Cardiac output is the amount of blood the heart pumps in one minute.
• Factors: heart rate (HR), stroke volume (SV), preload (EDV), afterload, and contractility.

Stroke Volume

• Stroke volume (SV) is the amount of blood ejected by the left ventricle in one contraction.
• SV = EDV (End-Diastolic Volume) - ESV (End-Systolic Volume)

Heart Rate

• Heart rate (HR) is the number of times the heart beats per minute (bpm).
  • The sympathetic nervous system increases heart rate.
  • The parasympathetic nervous system decreases heart rate.
  • Other factors: Autonomic Nervous System activity, hormones, temperature, electrolyte levels, physical activity, stress, emotions.

End-Diastolic Volume (EDV) and End-Systolic Volume (ESV)

• EDV is the volume of blood in the ventricles at the end of ventricular diastole, just before contraction.
• ESV is the volume of blood in the ventricles at the end of ventricular systole.

Heart Sounds

• The first heart sound (S1) is caused by the closure of the AV valves (tricuspid and mitral) at the beginning of ventricular systole.
• The second heart sound (S2) is caused by the closure of the semilunar valves (aortic and pulmonary) at the beginning of ventricular diastole.

Heart Valve Locations for Auscultation

• Pulmonary valve: Left 2nd intercostal space (near sternum)
• Aortic valve: Right 2nd intercostal space (near sternum)
• Mitral valve: Left 5th intercostal space (midclavicular line)
• Tricuspid valve: Left 4th or 5th intercostal space (near sternum)

Heart Chamber Activity

Systole

• Ventricles contract, increasing pressure and ejecting blood into the aorta and pulmonary artery.
• Atria relax and begin to fill with blood.

Diastole

• Ventricles relax and fill with blood from the atria.
• Atria contract at the end of diastole to push the remaining blood into the ventricles.
• Ventricular filling occurs during ventricular diastole

Blood vessels

• The three tunics found in arteries and veins are:
  • Tunica intima (innermost layer)
  • Tunica media (middle layer)
  • Tunica externa (outermost layer)

Blood vessel Components

• Tunica intima is composed of endothelium (simple squamous epithelium).
• Tunica media is composed of smooth muscle.
• Capillaries contain only the tunica intima.
• Arterioles have the greatest resistance due to their small diameter.

Blood vessel information

• Exchange of oxygen, carbon dioxide, nutrients, and waste products happens in capillaries through diffusion, filtration, and osmosis.

Aorta and Arteries

• Major arterial branches from the aortic arch:
  • Brachiocephalic trunk (right side only, gives rise to the right common carotid and right subclavian arteries)
  • Left common carotid artery
  • Left subclavian artery
• The common iliac artery divides into the internal iliac artery (supplies pelvic organs) and the external iliac artery (continues as the femoral artery)

Vessels of the arm

• Subclavian artery → Axillary artery → Brachial artery → Radial & Ulnar arteries forming the superficial and deep palmar arches in the hand.

Intestines

• Superior mesenteric artery (SMA) supplies the midgut
• Inferior mesenteric artery (IMA) supplies the hindgut

Celiac Trunk Supply

• Left gastric artery supplies the stomach and lower esophagus.
• Common hepatic artery supplies the liver, gallbladder, stomach, and duodenum.
• Splenic artery supplies the spleen, pancreas, and stomach.

Fetal Circulation

• Umbilical vein carries oxygenated blood from the placenta to the fetus.
• Umbilical arteries carry deoxygenated blood from the fetus back to the placenta.
• Ductus venosus becomes the ligamentum venosum (in the liver).
• Ductus arteriosus becomes the ligamentum arteriosum (connects the pulmonary trunk to the aorta).
• Foramen ovale becomes the fossa ovalis (in the interatrial septum).

Blood Vessel Changes

• Ascending aorta becomes the aortic arch
• Thoracic aorta descends through the thorax
• Abdominal aorta passes through the diaphragm
• Common iliac arteries bifurcate at L4.

Circle of Willis Arteries

• Arteries that contribute to the formation of the circle of Willis are:
  • Anterior cerebral artery
  • Middle cerebral artery
  • Posterior cerebral artery

Limb Vessels

Arteries

- Arteries of the upper limb: Subclavian → Axillary → Brachial → Radial & Ulnar → Palmar arches.
- Arteries of the lower limb: External iliac → Femoral → Popliteal → Anterior & Posterior Tibial → Dorsalis Pedis & Plantar Arches.

Veins

- Veins of the upper limb: Cephalic, Basilic, Brachial, Axillary, Subclavian.
- Veins of the lower limb: Great Saphenous, Small Saphenous, Femoral, Popliteal.

Hepatic Portal Vein

• Hepatic portal vein is formed by the superior mesenteric vein and the splenic vein.

Systemic Veins

• Great saphenous vein is the largest superficial vein in the body.

Cardiovascular System Functions

• Venous valves prevent backflow of blood
• Blood flows from high to low pressure.

Factors that effect Blood flow

• Increased pressure difference increases blood flow.
• Increased resistance decreases blood flow.

Fetal Circulation Vessels

• Umbilical vein carries oxygenated blood from placenta
• Ductus venosus bypasses the liver to the inferior vena cava
• Foramen ovale allows blood to bypass the lungs
• Ductus arteriosus connects the pulmonary artery to the aorta, to bypass the lungs
• Umbilical arteries carry deoxygenated blood