Definition
Blood is made of plasma (the liquid part, carrying dissolved nutrients, hormones, waste, and proteins), red blood cells (RBCs), white blood cells (WBCs), and platelets.
Red blood cells are mainly responsible for carrying oxygen, thanks to the haemoglobin protein they contain, which binds oxygen in the lungs and releases it in body tissues. In adults, red blood cells are formed in the bone marrow.
White blood cells defend the body against infection. They're divided into granulocytes (containing visible granules, e.g. neutrophils, eosinophils, basophils) and agranulocytes (no visible granules, e.g. lymphocytes, monocytes).
Basophils (a type of granulocyte) release histamine, which causes the symptoms of allergic reactions (like swelling and itching).
Lymphocytes (a type of agranulocyte) are the white blood cells that make antibodies against pathogens, as part of the body's specific immune response.
Platelets are cell fragments involved in blood clotting. A person with a low platelet count is likely to face difficulty in clotting blood, meaning even minor cuts may bleed excessively or for longer than normal.
Definition
When fibrinogen (a plasma protein) helps blood clot, it converts into fibrin and separates out from the blood along with trapped cells to form the clot; what remains of the liquid (plasma minus fibrinogen and clotting factors) is called serum.
Definition
All blood vessels (arteries, veins, and capillaries) share a common inner lining layer: the endothelium (a thin layer of cells lining the inside of every blood vessel, providing a smooth surface for blood flow).
Arteries have thick, muscular, elastic walls (to withstand and smooth out the high pressure pulses from the heart) and carry blood away from the heart, usually oxygenated (with the pulmonary artery as the key exception, carrying deoxygenated blood to the lungs).
Veins have thinner walls than arteries and contain valves (to prevent backflow, since blood in veins moves at much lower pressure), carrying blood towards the heart, usually deoxygenated (with the pulmonary vein as the key exception, carrying oxygenated blood from the lungs to the heart).
Capillaries have extremely thin walls (just one cell thick) and are where the actual exchange of materials (oxygen, carbon dioxide, nutrients, waste) between blood and body tissues occurs, since their thin walls and slow blood flow allow efficient diffusion.
Definition
Comparing the three vessel types: arteries have thick, muscular walls and high-pressure flow away from the heart; veins have thinner walls, valves, and low-pressure flow towards the heart; capillaries have single-cell-thick walls, the slowest flow, and are the only site of direct exchange with tissues.
Definition
The heart has four chambers: two atria (upper, thinner-walled, receiving blood) and two ventricles (lower, thicker-walled, pumping blood out). Atria have thinner walls than ventricles because they only need to push blood a short distance into the ventricles just below them, while ventricles must generate enough pressure to pump blood much farther (to the lungs or the whole body).
The left ventricle has the thickest walls of all four chambers, since it must generate enough pressure to pump oxygenated blood all the way around the entire body (a much longer, higher-resistance circuit than the right ventricle's short trip to the nearby lungs).
Oxygenated and deoxygenated blood are kept separate in the heart (via the septum dividing the left and right sides) so that blood delivered to the body remains as fully oxygenated as possible — mixing the two would reduce the oxygen-carrying efficiency of blood sent out to the tissues.
The pericardium is a protective double membrane sac surrounding the heart, containing a small amount of fluid that reduces friction as the heart beats and cushions it against physical shock.
Example 1 — tracing blood flow from the vena cava
Trace the flow of blood from the vena cava to various body parts.
- Vena cava right atrium (through the tricuspid valve) right ventricle pulmonary artery lungs (gas exchange, blood becomes oxygenated) pulmonary vein left atrium (through the bicuspid/mitral valve) left ventricle aorta arteries capillaries throughout the body veins back to the vena cava.
Answer: Vena cava right atrium right ventricle lungs left atrium left ventricle aorta body
Definition
The bicuspid (mitral) valve sits between the left atrium and left ventricle; if it's damaged, blood flow from the left atrium into the left ventricle (and, since a leaking valve lets blood flow backward, the efficient forward pumping of oxygenated blood out to the body) is affected.
The 'lubb' sound of a heartbeat is produced by the closing of the atrioventricular valves (bicuspid and tricuspid), as the ventricles begin to contract.
Definition
Systole is the phase when heart chambers contract, pumping blood out. Diastole is the phase when heart chambers relax, filling with blood — a full heartbeat cycles through both phases in sequence.
Definition
The hepatic portal vein carries blood from the alimentary canal (digestive tract) to the liver — an unusual vessel, since it connects one capillary bed (in the gut, absorbing digested nutrients) directly to another (in the liver), letting the liver process and regulate absorbed nutrients before they reach the rest of the body.
Definition
Myocardial infarction (heart attack) is due to blockage of blood flow in a coronary artery (the vessels supplying the heart muscle itself with oxygenated blood) — this deprives part of the heart muscle of oxygen, causing that tissue to die if the blockage isn't cleared quickly. It's commonly caused by fatty deposits (atherosclerotic plaques) narrowing or blocking a coronary artery; treatment includes emergency procedures to restore blood flow (like angioplasty) and long-term medication/lifestyle changes.
Angina is chest pain or discomfort caused by reduced (but not completely blocked) blood flow to the heart muscle, without death of heart tissue — essentially a warning sign that the heart isn't getting quite enough oxygen, often during exertion.
Coronary heart disease (CHD) is the general condition of narrowed/blocked coronary arteries (usually from atherosclerosis), which can lead to angina or, if a blockage becomes complete, a myocardial infarction.
Arteriosclerosis is the general hardening and loss of elasticity of artery walls with age. Atherosclerosis is more specific: the build-up of fatty deposits (plaques, mainly cholesterol) inside artery walls, which narrows the artery and can also make it harder/less elastic — atherosclerosis is essentially one common specific cause contributing to arteriosclerosis.
Remember
If blood cholesterol levels are never regulated, cholesterol progressively builds up as plaques inside artery walls (atherosclerosis) — over time, this narrows arteries (restricting blood flow and raising blood pressure), makes artery walls stiffer and less elastic, and significantly raises the risk of a plaque fully blocking a coronary artery (causing a heart attack) or a brain artery (causing a stroke).
Three ways smoking harms the heart and blood vessels: nicotine raises heart rate and blood pressure, forcing the heart to work harder; carbon monoxide in smoke reduces the blood's oxygen-carrying capacity, straining the heart further to deliver enough oxygen; and chemicals in smoke damage the inner lining of blood vessels, promoting the fatty plaque build-up of atherosclerosis.
Regular exercise strengthens the circulatory system by making the heart muscle itself stronger and more efficient (pumping more blood per beat), improving blood vessel flexibility, and helping regulate healthy cholesterol and blood pressure levels.
Lifestyle choices that help prevent cardiovascular disease include regular exercise, a balanced low-fat/low-cholesterol diet, avoiding smoking, and managing stress and blood pressure — each directly counters one of the mechanisms that damage the heart and blood vessels over time.
Quick Recap
- ●Blood: plasma RBCs (O via haemoglobin) WBCs (granulocytes e.g. basophils/histamine; agranulocytes e.g. lymphocytes/antibodies) platelets (clotting)
- ●Vessels: arteries (thick, high pressure, away from heart), veins (thin, valves, towards heart), capillaries (one cell thick, site of exchange) — all share an endothelium lining
- ●Heart: 4 chambers (atria thin, ventricles thick, left ventricle thickest); blood: vena cava right atrium right ventricle lungs left atrium left ventricle aorta
- ●Valves prevent backflow (bicuspid/tricuspid 'lubb' sound); systole (contract/pump) and diastole (relax/fill); pericardium protects and cushions the heart
- ●Myocardial infarction: blocked coronary artery, tissue dies. Angina: reduced flow, no tissue death. CHD: narrowed coronary arteries generally
- ●Atherosclerosis (fatty plaque build-up) vs arteriosclerosis (general hardening); unmanaged cholesterol narrower, stiffer arteries heart attack/stroke risk
- ●Smoking harms the heart via nicotine (raises heart rate/BP), CO (reduces O delivery), and vessel-lining damage; exercise and healthy lifestyle protect against CVD
