1. Structure of the human heart
The heart is a hollow, muscular organ between the lungs, enclosed in a double-layered pericardium with pericardial fluid (reduces friction, protects).
- Four chambers: two thin-walled auricles (atria) above, which receive blood; two thick-walled ventricles below, which pump it out. The left ventricle has the thickest wall because it pumps blood to the whole body.
- A muscular septum separates the right (deoxygenated) and left (oxygenated) halves.
- Valves: tricuspid (three flaps) between right auricle and right ventricle; bicuspid or mitral (two flaps) between left auricle and left ventricle; semilunar valves (three half-moon pockets) at the bases of the pulmonary artery and the aorta. Chordae tendineae join the tricuspid and bicuspid flaps to papillary muscles, so the flaps cannot turn back into the auricles.
| Main vessel | Joins | Blood |
|---|---|---|
| Superior and inferior venae cavae | Body → right auricle | Deoxygenated |
| Pulmonary artery | Right ventricle → lungs | Deoxygenated |
| Pulmonary veins | Lungs → left auricle | Oxygenated |
| Aorta | Left ventricle → body | Oxygenated |
Key terms
- Auricle
Upper, thin-walled chamber of the heart that receives blood.
- Ventricle
Lower, thick-walled chamber that pumps blood out.
- Chordae tendineae
Cords holding the flaps of the tricuspid and bicuspid valves.
- Semilunar valves
Pocket valves at the bases of the aorta and pulmonary artery.
2. Working of the heart: the cardiac cycle
Systole = contraction; diastole = relaxation. One cardiac cycle at 75 beats per minute lasts 0.8 s:
- Auricular systole (0.1 s): auricles contract and push blood into the ventricles through the open tricuspid and bicuspid valves.
- Ventricular systole (0.3 s): ventricles contract; the tricuspid and bicuspid close (first sound, lubb); the semilunar valves open and blood enters the pulmonary artery and aorta.
- Joint diastole (0.4 s): all chambers relax; the semilunar valves close (second sound, dub); blood from the veins fills the auricles and flows on into the ventricles.
Path of blood: venae cavae → right auricle → tricuspid → right ventricle → semilunar valve → pulmonary artery → lungs → pulmonary veins → left auricle → bicuspid → left ventricle → semilunar valve → aorta → body.
At rest the heart pumps about 70 mL per beat, about 5.04 litres a minute at 72 beats a minute.
Key terms
- Systole
Contraction of a heart chamber.
- Diastole
Relaxation of a heart chamber.
- Cardiac cycle
One complete sequence of contraction and relaxation of the heart.
3. Double circulation
In mammals blood passes through the heart twice in one complete round of the body:
- Pulmonary circulation: right ventricle → pulmonary artery → lungs (blood takes up oxygen) → pulmonary veins → left auricle.
- Systemic circulation: left ventricle → aorta → all organs → venae cavae → right auricle.
Advantages: oxygenated and deoxygenated blood are kept separate; the left ventricle sends blood to the body at high pressure, while the lungs receive blood at lower pressure. Warm-blooded, active mammals need this rich supply of oxygen.
Key terms
- Double circulation
Blood passes through the heart twice in one round of the body.
- Pulmonary circulation
Heart → lungs → heart.
- Systemic circulation
Heart → body → heart.
4. Arteries, veins and capillaries
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Carries blood | Away from the heart | Towards the heart | Between arterioles and venules |
| Wall | Thick, muscular and elastic | Thin, less muscle | One cell thick |
| Lumen | Narrow | Wide | Very narrow |
| Valves | Absent (except semilunar at the base) | Present | Absent |
| Pressure and flow | High, in jerks (pulse) | Low, smooth | Falling, slow |
| Blood | Oxygenated (except pulmonary artery) | Deoxygenated (except pulmonary veins) | Exchange takes place |
| Position | Mostly deep | Often near the surface | In all tissues |
- Arteries withstand high pressure; their elastic walls stretch with each beat.
- Veins return blood at low pressure; valves prevent backflow, and muscles squeeze the veins. Weak valves in leg veins cause varicose veins.
- Capillaries are the site of exchange: oxygen and food diffuse out, carbon dioxide and wastes diffuse in.
Key terms
- Artery
Vessel carrying blood away from the heart.
- Vein
Vessel carrying blood towards the heart.
- Capillary
Microscopic vessel with a wall one cell thick, where exchange occurs.
5. Pulse and blood pressure
Pulse: the rhythmic expansion of an artery as each surge of blood is pumped by the left ventricle. It is felt where an artery lies near the surface over a bone: the wrist (thumb side), the side of the neck, the temple. Pulse rate = heart rate (about 72 per minute at rest). Veins show no pulse.
Blood pressure: the pressure of blood on the walls of the arteries, measured with a sphygmomanometer in mm of mercury (mm Hg).
- 120/80 mm Hg is normal for a young adult.
- Systolic (upper figure, 120): highest pressure, while the ventricles contract.
- Diastolic (lower figure, 80): lowest pressure, while the ventricles relax.
- Pressure persistently above about 140/90 mm Hg is high blood pressure (hypertension).
| Reading | Meaning |
|---|---|
| 120 | Systolic: ventricles contract |
| 80 | Diastolic: ventricles relax |
Key terms
- Pulse
Rhythmic expansion of an artery with each heartbeat.
- Systolic pressure
Arterial pressure during ventricular contraction.
- Diastolic pressure
Arterial pressure during ventricular relaxation.
- Sphygmomanometer
Instrument for measuring blood pressure.
6. Blood vessels of the liver and kidney
| Organ | Vessels bringing blood | Vessel taking blood away |
|---|---|---|
| Liver | Hepatic artery (oxygenated, from the aorta); hepatic portal vein (from the intestine, rich in digested food) | Hepatic vein (to the inferior vena cava) |
| Kidney | Renal artery (from the aorta; contains urea) | Renal vein (to the inferior vena cava; least urea) |
Portal system: a vein that begins in the capillaries of one organ and ends in the capillaries of another. The hepatic portal vein begins in the capillaries of the intestine and ends in the capillaries of the liver.
Significance of the hepatic portal system: the liver receives absorbed food before the rest of the body, so it can store excess glucose as glycogen, store other nutrients, remove harmful substances (toxins, drugs) and break down excess amino acids (forming urea), keeping the blood that reaches the heart steady in composition.
Key terms
- Hepatic portal vein
Carries blood from the intestine to the liver.
- Portal system
A vein running from the capillaries of one organ to those of another.
- Renal vein
Carries cleaned blood from the kidney.
Exam tips
Common mistakes
- Drawing the heart's right side on the right of the page: in a front view it is on your left.
- Saying "arteries carry oxygenated blood": arteries carry blood away from the heart (the pulmonary artery carries deoxygenated blood).
- Swapping lubb and dub: lubb = tricuspid and bicuspid close; dub = semilunar valves close.
- Confusing the hepatic vein (leaves the liver) with the hepatic portal vein (intestine to liver).
- Naming vessels other than the main ones of the heart, liver and kidney (not required).
How marks are usually awarded
- 1 mark: a term (systole, portal system), a valve or vessel, an odd one out with its category.
- 2 marks: two differences, a reading explained, a feature with its reason.
- 3 marks: the path of blood with valves, a labelled L.S. of the heart, T.S. of an artery and a vein.