Human Circulatory System and the Heart

• Animals are complex multicellular organisms, and their bodies need a way to transport nutrients and remove waste.
• The human circulatory system is a complex network of blood vessels for this purpose.
• This system supplies cells, tissues, and organs with oxygen and nutrients.
• It also removes carbon dioxide and waste compounds.
• The circulatory system includes vessels such as arteries, veins, and capillaries, along with a pump, the heart.
• Vertebrates have a closed-loop circulatory system, where blood is separated from the interstitial fluid.
• The circulatory and respiratory systems work together to obtain oxygen and discharge carbon dioxide.

The Heart

• The heart pumps blood through pulmonary circulation to the lungs.
• It also pumps blood through systemic circulation to the rest of the body, including the heart itself.
• The heart is asymmetrical, with the left side larger than the right.
• The asymmetry correlates with the different sizes of the pulmonary and systemic circuits.
• In humans, the heart is about the size of a clenched fist.
• The heart has four chambers: two atria and two ventricles.
• Each side of the heart has one atrium and one ventricle.

Blood Flow

• The right atrium receives deoxygenated blood from the systemic circulation.
• Major veins such as the superior vena cava (from the head and arms) and the inferior vena cava (from lower organs and legs) carry this deoxygenated blood.
• The deoxygenated blood then passes to the right ventricle through the tricuspid valve, which prevents backflow.
• The right ventricle contracts, pumping blood to the lungs for reoxygenation.
• The left atrium receives oxygen-rich blood from the lungs.
• This blood passes through the bicuspid valve to the left ventricle.
• The left ventricle pumps the blood into the aorta.
• The aorta is the major artery of the body.
• The aorta carries oxygenated blood to the organs and muscles.
• This pumping pattern is called double circulation, and it occurs in all mammals.

The Cardiac Cycle

• The main function of the heart is to pump blood through the body, which happens through the cardiac cycle.
• The cardiac cycle coordinates the flow of blood through the heart using electrochemical signals.
• These signals cause the heart muscle to contract and relax.
• Each cardiac cycle involves a contraction phase (systole) that pushes blood out.
• It's followed by a relaxation phase (diastole) where the heart fills with blood.
• The contraction of the atria forces blood through valves into the ventricles
• The closing of these valves during ventricular contraction produces a "lub" sound.
• The ventricles contract, forcing blood into the aorta and pulmonary arteries.
• The closing of the valves to these arteries from blood flowing back during ventricular relaxation produces a "dub" sound.
• The pumping action of the heart comes from cardiac muscle cells, known as cardiomyocytes.
• Cardiomyocytes pump rhythmically and involuntarily.
• Electrical impulses in the heart generate electrical currents that can be measured on the skin using electrodes.
• An electrocardiogram (ECG) records these electrical impulses of the cardiac muscle.

Blood Vessels

• The heart's blood is carried through the body through vessels.
• Arteries carry blood away from the heart.
• The aorta is the main artery of the systemic circulation.
• The aorta branches into major arteries that supply blood to different limbs and organs.
• The walls of the aorta and arteries near the heart are elastic and respond to pressure changes from the heart beat.
• Arteries farther from the heart have more muscle tissue in their walls to control blood flow rates.
• Major arteries branch into minor arteries, which further divide into smaller arterioles to reach muscles and organs.
• Arterioles lead to capillary beds composed of narrow tubes through which red blood cells pass.
• Capillaries are the sites for nutrient, waste, and oxygen exchange with tissues at the cellular level.
• Capillaries are also where fluid leaks into the interstitial space.
• Capillaries then converge into venules, which connect to minor veins, and then to major veins.
• Veins carry blood high in carbon dioxide toward the heart.
• Veins have thinner walls than arteries because of lower pressure.
• Veins also have valves along their length to prevent backflow of blood.