- Blood circulates through arteries, veins, and capillaries in a closed system.
- Tissue fluid fills the spaces between cells and facilitates exchange between blood and cells.
- Lymph is transported through lymph vessels and supports immune function.
- Arteries carry oxygenated blood (bright red), while veins carry deoxygenated blood (dark red), although veins may appear blue from the outside.
- Non-circulating fluids, like synovial fluid and vitreous humour, are confined to specific organs and do not move through the circulatory system.
Definitions [5]
Definition: Closed Blood Circulatory System
A closed blood circulatory system is one in which blood flows entirely within blood vessels, as seen in humans and other vertebrates.
Definition: Open Blood Circulatory System
An open blood circulatory system is found in animals like insects, where blood flows freely through body cavities without confined blood vessels.
Define a cardiac output.
- Cardiac output is the volume of blood that the heart pumps out in a minute. It is computed by multiplying heart rate (the number of beats per minute) by stroke volume, which is the amount of blood pumped by each ventricle per minute.
- A healthy person's heart beats 72 times per minute, pumping out roughly 70 millilitres of blood with each beat. As a result, the average cardiac output is five litres, or 5000 mL.
Define a cardiac cycle.
- The heart's cyclical repetition of successive events, known as the cardiac cycle, is made up of the contraction and relaxation of the ventricles and atria during diastole.
- A cardiac cycle lasts about 0.8 seconds. The trial systole (0.1 second), ventricular systole (0.3 second), and full cardiac diastole (0.4 second) are the phases of the heart cycle.
Define portal vein.
A portal vein is one that starts with capillaries and also ends in capillaries.
Key Points
Key Points: Fluids in Our Body
Key Points: Human Circulatory System
- Arteries carry oxygenated blood from the heart to tissues, while veins bring deoxygenated blood back; capillaries connect them.
- Exception: Pulmonary artery carries deoxygenated blood, and pulmonary vein carries oxygenated blood.
- The heart is fist-sized, located in the thorax between the lungs, and protected by the pericardium.
- The heart has four chambers separated by septa, with valves (tricuspid, bicuspid/mitral, semilunar) ensuring one-way blood flow.
- Heart sounds: “Lub” is due to closure of the tricuspid and bicuspid valves, and “Dub” is due to closure of the semilunar valves.
- The SA node (pacemaker) generates impulses (70–75/min) and controls the heartbeat through the AV node and the bundle of His.
Key Points: Cardiac Cycle
- The cardiac cycle starts with joint diastole, where all chambers are relaxed, and blood flows into the ventricles through open AV valves.
- The SA node generates an impulse, causing atrial systole, pushing extra blood into the ventricles.
- The impulse passes through the AV node and bundle of His, causing ventricular systole.
- During ventricular systole, tricuspid and bicuspid valves close (lub sound) and semilunar valves open to pump blood into the arteries.
- In ventricular diastole, ventricles relax, semilunar valves close (dub sound), and AV valves reopen for filling.
- Each cycle pumps about 70 mL of blood (stroke volume), and cardiac output is about 5 L/min (stroke volume × heart rate).
Key Points: Regulation of Cardiac Activity
- The heart is myogenic - its activities are intrinsically regulated by nodal tissue (SAN and AVN), meaning it generates its own impulse.
- A neural centre in the medulla oblongata moderates cardiac function through the ANS.
- Sympathetic nerves (ANS) increase heart rate, ventricular contraction strength and cardiac output.
- Parasympathetic nerves (ANS) decrease heart rate, slow conduction speed and reduce cardiac output. (AIPMT 2007)
- Adrenal medullary hormones also increase cardiac output.
Key Points: Disorders of Circulatory System
- Hypertension (high BP) is when blood pressure is above 120/80 mm Hg; ≥140/90 mm Hg is serious and can damage the heart, brain, and kidneys.
- Coronary Artery Disease (CAD) or atherosclerosis occurs due to fat, cholesterol, and calcium deposits, narrowing the arteries supplying the heart.
- Angina (angina pectoris) is chest pain caused by reduced oxygen supply to the heart muscles due to restricted blood flow.
- Heart failure is when the heart cannot pump enough blood to meet the body's needs; it is different from a heart attack and cardiac arrest.
- Coronary thrombosis is the formation of a blood clot in the coronary arteries, often in the left anterior descending artery.
- These disorders mainly affect blood flow and oxygen supply, leading to serious cardiovascular problems.
