Bio 6.6 PDF - The Circulatory System

# Unit 6: The Circulatory System ## The Cardiac Cycle: PQRST The cardiac cycle encompasses one atrial and ventricular **systole** and one atrial and ventricular **diastole**. During systole and diastole, an **electrical signal** is transmitted through the heart muscle. These electrical impulses can...

# Unit 6: The Circulatory System ## The Cardiac Cycle: PQRST The cardiac cycle encompasses one atrial and ventricular **systole** and one atrial and ventricular **diastole**. During systole and diastole, an **electrical signal** is transmitted through the heart muscle. These electrical impulses can be detected using an **Electrocardiogram (ECG)**, which registers **voltage changes** across the surface of the heart. This results in a characteristic line called a **PQRST curve**. When a contraction (**systole**) occurs, the heart is in its **depolarization** phase. When relaxation (**diastole**) occurs, the heart is in its **repolarization** phase. Each letter in the PQRST curve can be assigned to the depolarization or systole and repolarization or diastole of the atria and ventricles. - **The P Curve (atrial depolarization)** records the simultaneous contractions (systole) of the **atria** as they drive the blood down into the ventricles. The peak is **small** as the atria do not contract with much force. - The **PR segment** represents the **slow conduction** of the electrical impulse through the **AV Node**. - **The QRS curve (ventricular depolarization, atrial repolarization)** is the contraction (systole) of the **ventricles** as they drive the blood out into their respective **arteries**. Note the much **higher peak** in the QRS phase of the cardiac cycle in the diagram. This is due to the much **longer & stronger** contraction of the ventricles pushing blood out of the heart. - **The T curve (ventricular repolarization)** marks the **(reset) recovery** phase of the ventricles, which restores the normal electrical condition and prepares them for the **next contraction**. ## The Heart Rate Center The heart rate center is in the **Medulla Oblongata** of the brain. The SA Node is connected to the brain by the **Vagus nerve**. This nerve pathway, which is part of the **Autonomic nervous system** (not under conscious control), has two systems that affect the heart rate: 1. **Parasympathetic n.s.** : Causes the heart rate to decrease. 2. **Sympathetic n.s.** : Causes the heart rate to increase. Factors such as the **need for oxygen** or the **blood pressure level** determine which of these systems become active. When the brain perceives that the blood is getting delivered to the tissues too slowly, or if blood pressure is low, the **brain will signal the SA Node to speed up**, thus increasing the rate of contraction. A diagram of the heart with the SA Node and AV Node is shown. ## Blood Pressure Blood pressure is defined as the force pushing against the blood vessel walls generated by blood in the vessel. Blood pressure constantly changes depending on the **external environment** as the body will adjust the blood pressure to maintain **homeostasis**. Blood pressure is composed of two numerical values: 1. **Systolic pressure**: Blood pressure generated when the ventricles **contract**. 2. **Diastolic pressure**: Blood pressure when the heart is at rest. ## Measuring Blood Pressure Normal blood pressure is measured along the **brachial artery**, which is in the arm. A machine called a **sphygmomanometer** measures this pressure. Normal human blood pressure should be maintained around **120/80 mm Hg**. The top number refers to **systolic pressure**, while the bottom number refers to **diastolic pressure**. **Pulse**: as blood is pumped through arteries, the walls swell and recoil. We detect the recoil as the pulse! - **Hypertension**: 140 or 175/ 90 or 90 - **Hypotension**: 90/ 60