LO1 of Unit 1 – Cardiovascular System (2021)
The Structure and Function of
the Circulatory System
Learning outcomes
By the end of this section, you will be able to:
1.1 Describe the function of the heart
1.2 Describe how blood moves through the four chambers of the heart
1.3 Describe systemic and pulmonary circulation
1.4 Describe the structure and functions of blood vessels
1.5 Explain what blood pressure is
1.6 Identify blood pressure classifications
1.1
The function of the heart
The heart is located between the right and left lung, behind the sternum. The rib cage protects it.
The heart comprises cardiac muscle tissue and functions as a pump, pumping blood to the body and the lungs. When healthy, the average heart rate (or pumping rate) is between 60-80bpm. The heart has its blood supply, which arrives via the coronary arteries.
Click on the image to zoom
Location of the heart/ribs
1.2
How blood moves through
the four chambers of the heart
The heart is divided into right and left sides separated by a muscular partition called the septum. The septum ensures that blood from the heart’s left side cannot directly enter the chambers on the heart’s right side.
The heart has four chambers. The two upper chambers are called atria-one on the left and the other. The two lower chambers called ventricles are larger and have thicker, more muscular walls than the atria. Furthermore, the left ventricle wall is larger and more muscular than the right ventricle due to the need to eject blood from the heart to all body tissues forcefully. A thicker, more muscular wall enables the left ventricle to produce forceful contractions under higher pressures.
Blood enters the heart following a one-way direction from the atria to the ventricles.
Image of the heart/blood flow between chambers 1.3
Systemic and pulmonary circulation
Blood moves through the four chambers of the heart in a one-way cycle. With the pulmonary vein, oxygenated blood travels from the lungs to the left atrium.
Blood then passes from the left atrium to the left ventricle through the mitral or bicuspid valve. Valves open and close to allow blood to enter the ventricles and stop blood backflow when they contract.
When the heart contracts, oxygenated blood is pushed out from the left ventricle into the aorta (largest artery) and away from the heart. Oxygenated blood flows through the blood vessels to the tissues of the body. Circulation from the heart to the body is called systemic circulation.
Deoxygenated blood enters the right atrium via the superior and inferior vena cava (largest vein) on return to the heart. The blood passes from the right atrium to the right ventricle through the tricuspid valve—the ventricles contract causing the pulmonary valve to open pushing deoxygenated blood into the pulmonary artery. The blood now travels to the lungs via the pulmonary artery where carbon dioxide is removed, and oxygen re-enters as part of the respiration mechanism. Blood travelling from the heart to the lungs is called the pulmonary circulation.
Systemic and pulmonary circulation What is diffusion?
Diffusion is the passive movement of molecules (i.e. oxygen and carbon dioxide) moving from an area of high concentration (the lungs and capillaries) to a place of low concentration (the lungs and capillaries).
The aorta divides into smaller arteries and, eventually, into the smallest arteries called arterioles, which diverge into capillary beds.
Diffusion of gases O2 and CO2 and nutrient exchange occurs between capillaries and cells.
The capillaries converge again into small veins called venules connected to minor veins and then to major veins to take blood high in carbon dioxide back to the heart.
Image of capillaries/diffusion 1.4
The structure and functions of blood vessels
Consisting of smooth muscle tissue, blood vessels function to distribute nutrients and remove waste from body tissues. The vessels begin and end at heart, forming two closed systems. There are three basic types of blood vessels.
- Arteries
- Veins
- Capillaries
Arteries
Our arteries function to transport blood away from the heart and are under high pressure. Arteries mainly carry oxygenated blood; however, there is an exception: the pulmonary artery transports deoxygenated blood from the heart’s right ventricle to the lungs.
Arteries have thick muscular walls (smooth muscle) which are elastic, allowing them to stretch when the heart contracts forcefully, pushing blood out from the left ventricle into the aorta. In this way, the pressure the blood exerts on the artery wall is not excessive. However, arterial pressure is critical because it maintains blood flow through the capillary beds.
Artery Capillaries
Being only one cell thick, capillaries are the smallest type of blood vessel, so they’re incredibly thin. Blood pressure within the capillaries is very low (otherwise you would bleed to death), declining along their length from less than 35mmHg to about 18mmHg.
The thin walls allow oxygen, nutrients and carbon dioxide to exchange (via diffusion) between the capillaries and the body’s cells and from the body’s cells to the capillaries.
Capillaries Veins
The role of veins is to carry mainly deoxygenated blood towards the heart. However, the pulmonary vein is the exception, carrying oxygenated blood from the lungs to the left atrium.
Venous pressure is low, only about 18mmHg from the venules to the right atrium. Due to the low pressure, veins have thin walls and a large lumen. The low pressure in veins means blood would struggle to return to the heart without a sufficient mechanism to aid return. Fortunately, several mechanisms assist venous return to the right atrium, which enable them to deal with low blood pressure flow. One striking feature is that veins below the heart level are lined with non-return valve structures. Once blood is pumped through the valve system, they shut, stopping any backflow of blood which would otherwise pool in the lower extremities.
Veins 1.5
Understanding blood pressure
Blood pressure is defined as measuring pressure in the heart and arteries during the heart’s contracting phase. Blood pressure is measured in millimetres of mercury (mmHg) and written as two numbers, for example, 120/80mmHg. The first or higher number represents the systolic blood pressure. It is measured as the heart contracts when blood is pumped into the arteries. The lower number is the measurement of diastolic blood pressure and represents the blood pressure when the heart is resting and relaxing between contractions. The systolic reading is more variable of the two readings and affected by exercise, stress, diet etc. The diastolic reading is usually constant, and therefore, variations can indicate changes in heart health (atherosclerosis etc.).
Blood pressure is essential in maintaining the cardiac cycle. If blood flow pressure around the body were to drop far below what’s deemed normal, the blood would struggle to transport oxygen to muscles and other vital tissues and organs. On the other hand, if blood pressure were to become too high, the extra strain is placed on the arteries (and your heart) leading to blood vessel damage. Damage to blood vessels increases the likelihood of a heart attack and stroke.
1.6
Blood pressure classifications
Classification of blood pressure levels of the British Hypertension Society
Causes of hypertension
- Drugs (non-steroidal anti-inflammatory drugs, oral contraceptives, steroids, liquorice, sympathomimetics, some cold cures)
- Renal disease (present, past, or family history, proteinuria or haematuria: palpable kidney(s)—polycystic, hydronephrosis, or neoplasm)
- Renovascular disease (abdominal or loin bruit)
- Pheochromocytoma (paroxysmal symptoms)
- Conn’s syndrome (tetany, muscle weakness, polyuria, hypokalaemia)
- Coarctation (radio-femoral delay or weak femoral pulses).
- Cushing’s (general appearance)
Contributory factors
- Overweight
- Excess alcohol (> 3 units/day for men; > 2 units/day for women)
- Excess salt intake
- Lack of exercise
- Environmental stress
Complications of hypertension or target organ damage
- Stroke, transient ischaemic attack, dementia, carotid bruits
- Left ventricular hypertrophy or left ventricular strain on electrocardiogram
- Heart failure
- Myocardial infarct, angina, coronary artery bypass graft, or angioplasty
- Peripheral vascular disease
- Fundal haemorrhages or exudates, papilledema
- Proteinuria
- Renal impairment (raised serum creatinine)
Risk factors for cardiovascular disease
- Smoking
- Diabetes
- Ratio of total cholesterol: HDL cholesterol
- Family history
- Age
- Sex