Unit 4: Circulation
1. Circulatory system of human
The human circulatory system, also called the cardiovascular system, is responsible for transporting nutrients, oxygen, hormones, and waste products throughout the body. The human system is a closed, double circulatory system.
- Closed: The blood is always contained within a network of vessels (heart, arteries, veins, capillaries).
- Double: The blood passes through the heart twice for each complete circuit around the body.
Components of the System
- The Heart: A four-chambered muscular pump.
- Atria (2): Upper chambers (right and left) that receive blood.
- Ventricles (2): Lower chambers (right and left) that pump blood out.
- Blood Vessels:
- Arteries: Carry blood Away from the heart. Usually carry oxygenated blood (exception: pulmonary artery).
- Veins: Carry blood To the heart. Usually carry deoxygenated blood (exception: pulmonary veins).
- Capillaries: Microscopic, one-cell-thick vessels where the exchange of gases and nutrients occurs between blood and tissues.
Double Circulation
- Pulmonary Circulation: The right side of the heart pumps deoxygenated blood to the lungs (via the pulmonary artery). The blood gets oxygenated and returns to the left side of the heart (via the pulmonary veins).
- Systemic Circulation: The left side of the heart pumps oxygenated blood to the rest of the body (via the aorta). The body cells use the oxygen, and deoxygenated blood returns to the right side of the heart (via the vena cava).
2. Body fluids- blood and lymph
Blood
Blood is a fluid connective tissue. It consists of plasma and formed elements.
- Plasma (approx. 55%): The straw-colored liquid matrix. It is mostly water, but also contains dissolved proteins (like albumin, fibrinogen for clotting), glucose, hormones, and salts.
- Formed Elements (approx. 45%):
- Red Blood Cells (RBCs or Erythrocytes): Contain hemoglobin, the protein that transports oxygen. They are biconcave discs and have no nucleus when mature.
- White Blood Cells (WBCs or Leukocytes): Part of the immune system; they fight infection.
- Platelets (Thrombocytes): Cell fragments that are essential for blood clotting.
Lymph
Lymph is a clear-to-white fluid that is part of the lymphatic system.
- Formation: When blood flows through capillaries, some of the plasma leaks out into the surrounding tissues. This fluid is called interstitial fluid.
- Function: Most of this fluid seeps back into the blood capillaries, but some is collected by a separate network of vessels called lymphatic vessels. Once inside these vessels, the fluid is called lymph.
- Lymph is eventually returned to the bloodstream. It also plays a key role in immunity, as it is filtered through lymph nodes, which are packed with white blood cells.
3. Blood Groups
Blood groups are determined by the presence or absence of specific molecules, called antigens, on the surface of Red Blood Cells. The two most important systems are the ABO and Rh systems.
ABO System
This system is based on two antigens: A and B.
| Blood Group |
Antigens on RBC |
Antibodies in Plasma |
Can Receive From (Donor) |
| A |
A |
Anti-B |
A and O |
| B |
B |
Anti-A |
B and O |
| AB |
A and B |
None |
A, B, AB, O (Universal Recipient) |
| O |
None |
Anti-A and Anti-B |
O (Universal Donor) |
Rh System
- This is based on the presence or absence of the Rh antigen (also called D antigen).
- Rh Positive (Rh+): You have the Rh antigen.
- Rh Negative (Rh-): You do not have the antigen.
- This is important in blood transfusions and pregnancy (e.g., an Rh- mother carrying an Rh+ baby can develop antibodies that may harm subsequent Rh+ babies).
Transfusion Rule: You cannot give a person an antigen they do not already have. If you give Type A blood (with A antigens) to a Type B person (with anti-A antibodies), their blood will clump together (agglutinate), which is fatal.
4. Blood Pressure
Blood pressure is the force (pressure) that circulating blood exerts on the walls of the arteries. It is measured in millimeters of mercury (mmHg) and given as two numbers.
Blood Pressure = Systolic / Diastolic (e.g., 120/80 mmHg)
- Systolic Pressure (the top number): The pressure in the arteries when the heart's ventricles contract (beat) and pump blood out. This is the highest pressure.
- Diastolic Pressure (the bottom number): The pressure in the arteries when the heart is resting (relaxing) between beats and filling with blood. This is the lowest pressure.
A normal resting blood pressure for an adult is around 120/80 mmHg.
5. Some disorders of circulatory system
- Hypertension (High Blood Pressure): Chronically elevated blood pressure (e.g., consistently 140/90 or higher). It forces the heart to work harder and can damage artery walls, leading to heart attack or stroke.
- Atherosclerosis: The "hardening of the arteries." It is a disease where plaque (made of cholesterol, fats, and other substances) builds up inside the arteries, narrowing them and restricting blood flow.
- Heart Attack (Myocardial Infarction): Occurs when the blood supply to a part of the heart muscle is blocked (usually by a blood clot in a plaque-narrowed coronary artery). This causes the heart muscle cells to die from lack of oxygen.
- Stroke: Occurs when the blood supply to part of the brain is blocked or when a blood vessel in the brain bursts. This causes brain cells to die.