Human Circulatory System: Heart, Blood Flow, and Pressure
Introduction
The basic function of the circulatory system is to transport nutrients and oxygen to cells and remove metabolic waste. The heart, a muscular pump, propels blood throughout the body. The circulatory system is divided into two main circuits: pulmonary circulation (heart-lungs-heart) and systemic circulation (heart-body-heart). Arteries carry blood away from the heart, and veins carry blood back to the heart, regardless of oxygen content.
Heart
The heart, located in the mediastinum, is protected by the rib cage. Its position changes slightly with diaphragm movement during breathing. The heart’s axis is generally directed downward and to the left. Externally, the heart is covered by the pericardium, consisting of a fibrous outer layer and a serous inner layer with parietal and visceral components. The pericardial cavity contains fluid to facilitate heart movement. The heart wall has three layers: endocardium, myocardium (muscle), and epicardium (visceral pericardium). Internally, the heart has four chambers: two atria and two ventricles. Blood enters the atria and is pumped out by the ventricles. Four valves (aortic, pulmonary, tricuspid, and mitral) regulate blood flow. The heart is nourished by the right and left coronary arteries.
Pulmonary Circulation
Pulmonary circulation transports blood between the heart and lungs. Deoxygenated blood from the right ventricle flows to the lungs via the pulmonary arteries. In the lungs, gas exchange occurs, and oxygenated blood returns to the left atrium via the pulmonary veins.
Systemic Circulation
Systemic circulation delivers oxygenated blood from the left ventricle to the body’s organs (except the lungs) and returns deoxygenated blood to the right atrium. Arterial blood is pumped into the aorta and distributed throughout the body. Venous blood returns to the heart via the vena cava. Blood pressure is the force exerted by blood on artery walls, originating from the heartbeat. The cardiac cycle includes systole (contraction, highest pressure) and diastole (relaxation, lowest pressure).
How to Measure Blood Pressure
Blood pressure is measured using a sphygmomanometer and a stethoscope. A cuff is inflated around the arm to temporarily stop blood flow in the brachial artery. As the cuff is deflated, the systolic pressure (maximum) is recorded when blood flow first resumes, and the diastolic pressure (minimum) is recorded when blood flow is unimpeded. Normal blood pressure is typically between 110-140 mmHg systolic and 60-80 mmHg diastolic.
Measures to Prevent Blood Pressure Complications
- Moderate sodium intake.
- Increase potassium intake.
- Engage in regular physical activity.
- Maintain a healthy weight.
- Minimize use of blood pressure-raising medications (e.g., oral contraceptives, anti-inflammatory drugs).
Heart Rhythm
The sinoatrial (SA) node, located in the right atrium, acts as the heart’s natural pacemaker. It generates electrical impulses that stimulate atrial contraction and the atrioventricular (AV) node. The AV node transmits impulses to the ventricles, causing ventricular contraction. The sympathetic and parasympathetic nervous systems regulate heart rate. The heart also has an atrioventricular fascicle system for additional stimulation.