Circulation Summary Notes PDF

**Circulation 2** **Heartbeat Overview** In vertebrates, the heartbeat originates in the heart itself. Some cardiac muscles are **autorhythmic**, meaning they contract and relax repeatedly without nervous system signals. Their contractions are coordinated by a group of autorhythmic cells in the...

**Circulation 2** **Heartbeat Overview** In vertebrates, the heartbeat originates in the heart itself. Some cardiac muscles are **autorhythmic**, meaning they contract and relax repeatedly without nervous system signals. Their contractions are coordinated by a group of autorhythmic cells in the right atrium, called the **sinoatrial (SA) node**, which acts as a pacemaker. The SA node produces electrical impulses which spread rapidly within heart tissue. These impulses generate currents that are measured in an electrocardiogram (ECG). **Heart Conduction System** 1. **SA node** sends signals that spread through the atria, causing both to contract in unison 2. **AV node** delays the signals (\~0.1 secs) to allow the atria to empty completely before the ventricles contract The atrioventricular (AV) node is another group of autorhythmic cells, located in the wall between the left and right atria. It is a relay point where the electrical impulses are delayed before spreading to the heart. 1. **bundle branches** carry signals to the heart apex 2. signals spread throughout the **ventricles**, causing them to **contract** **Control of Heart Rate** Physiological control can alter heart rate: - **nerve impulses** - signals from the **autonomic** nervous system - **hormones** - e.g. epinephrine, secreted by adrenal glands which speeds up SA node - **body temperature** - 1°C raises hr by \~10bpm **Autonomic nervous system** Consists of the **sympathetic** and **parasympathetic** nervous systems, which are largely responsible for this regulation, acting like the accelerator and brake in a car. Sympathetic nerves originate from **T1-T4 levels of the spinal cord** (release norepinephrine; act on entire heart). They **increase heart rate and force of contraction**. Parasympathetic nerves originate from **vagus nerves** (release acetylcholine; mostly act on SA node). They **reduce heart rate and force of contraction**. **The Cardiac Cycle** The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle. The **contraction** (pumping) phase is called **systole**. The **relaxation** (filling) phase is called **diastole**. **Phases:** 1. atrial and ventricular diastole - semilumar valves close ("dub", second heartbeat sound) 2. atrial systole and ventricular diastole 3. ventricular systole and atrial diastole - atrioventricular valves close ("lub", first heartbeat sound) **Key Terms:** **Cardiac output** = volume of blood pumped into the systemic circulation per minute; depends on heart rate and stroke volume **Heart rate** = beats per minute **Stroke volume** = amount of blood pumped in a single contraction **Blood pressure** = determined by cardiac output and peripheral resistance met by blood as it passes from the arteries into the capillaries **Blood vessels** All blood vessels contain a central lumen lined with an epithelial layer that lines blood vessels. This endothelium is smooth and minimises resistance. **Arteries** - Thick elastic walls to accommodate higher blood pressure - Transport blood from the heart to capillaries - Act as a pressure reservoir to ensure steady blood flow **Capillaries** - Smallest diameter blood vessel (flattened endothelial cells) with thin walls for efficient diffusion of nutrients, waste, etc in and out of tissues **Veins** - Contain one-way valves to maintain unidirectional blood flow - Transport blood from capillaries back to the heart - Act as a storage reservoir for blood (about 50-60% of blood volume is in veins - Blood pressure is very low in veins, so the flow of blood back to the heart is aided by skeletal muscle contractions **Clinical Implications** **Atherosclerosis** - Deposition of plaques, composed predominantly of fatty substances, in arterial wall - Can lead to myocardial infarction (heart attack) or stroke **Deep vein thrombosis (DVT)** - Pooling of blood in deep veins leading to clot formation, often due to inactivity **Blood Flow Velocity** - High resistance and large total cross-sectional area ---\> reduced velocity - High resistance ---\> reduced pressure - Blood flow resistance in narrow diameters of capillaries dissipates much of the pressure - Velocity of blood flow is lowest in capillary beds as a result of high resistance and large total cross-sectional area, allowing for material exchange - Pulse pressure = systolic-diastolic **Blood Pressure** - **Systolic pressure** = pressure in the arteries during ventricular systole; it is the highest pressure in the arteries - A pulse is the rhythmic bulging of artery walls with each heartbeat - **Diastolic pressure** = pressure in the arteries during diastole; it is lower than systolic pressure - Blood pressure for a healthy 20 year old at rest is about 120/70 mmHg **Regulation of Blood Pressure** - Vasoconstriction = contraction of smooth muscle in arterial walls; increases blood pressure - Vasodilation = relaxation of smooth muscles in arterial walls; causes blood pressure to fall