Unit K – Heart Structure and Function



Unit K – Heart Structure and Function

 

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In the last unit, we discussed the pulmonary and systemic circuits. In terms of the heart, it is the right side that feeds the pulmonary circuit and the left that feeds the systemic circuit. We also discussed the fetal circulatory system, in which there are two cardiac structures (foramen ovale and ductus arteriosus) that allow the blood to bypass the pulmonary circuit.

Parts of the Heart:

ATRIA – receiving chambers of the heart.

■ Right atrium: Collects deoxygenated blood from anterior and posterior vena cavas

■ Left atrium: Collects oxygenated blood from Pulmonary Veins

VENTRICLES – sending chambers of the heart

Right ventricle: pumps blood through the pulmonary circuit.

■ blood is forced through the pulmonary SEMILUNAR VALVE and into the pulmonary trunk which branches to form the pulmonary arteries. These arteries take the blood to the lungs.

■ When the right ventricle contracts, the blood pressure in it forces the AV valve (tricuspid valve) to close

Left ventricle: pumps blood through the systemic circuit.

■ when the left ventricle contracts, the bicuspid valve closes and blood is forced through the aortic SEMILUNAR VALVE and enters the aorta.

■ The left ventricle has a thicker muscle layer as it has to push the blood further.

**Both the pulmonary valve and the aortic valve are SEMI-LUNAR VALVES.

Atrioventricular Valves: flaps of tissue that separate the atria from the ventricles.

■ These valves prevent the blood from flowing in the reverse direction when the ventricles contract.

■ Right side “Tricuspid”—three cusps, or flaps

■ Left side “bicuspid” – two cusps.

 Chordae Tendineae: Strong, Fibrous strings that support the A.V. Valves

■ Keeps the valves from inverting with the force of blood flow during the contraction of the ventricles.

 

Pulmonary Trunk: Receives blood from the right ventricles

■ Branches off to form the Pulmonary Artery

 

Septum: wall of the Heart that separates the left and right sides of the Heart

■ keeps the pulmonary and systemic systems separate from each other.

Coronary Arteries & Veins:

■ the very first branches of the aorta take the blood to the coronary arteries. These take blood into the heart muscle itself.

■ The coronary veins return the blood to the vena cava as they enter the right atrium.

Cardiac Cycle and Intrinsic Beat

Contraction of the heart is a two step process:

Systole – Contraction of the Heart

Diastole – Relaxation of the Heart

 

Each Heartbeat (Cardiac Cycle) Consists of:

|Time |Atria |Ventricles |

|0.15 seconds |Atrial systole |Ventricular diastole |

|0.30 seconds |Atrial diastole |Ventricular systole |

|0.40 seconds |Atrial diastole |Ventricular diastole |

| | |

|Total time for 1 heart beat = 0.85seconds |(Average time for a person with a heart rate of 70 beats per minute |

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■ The ventricles have a stronger and longer contraction because blood must be pumped throughout the body.

 

■ The lub-dubb sound of the heart is due to the closing of the valves:

First the atrioventricular, then the semi-lunar.

 

 

The beat of the heart is said to be intrinsic. It will beat without any nervous system stimulation. (It can be removed from the body and still continue beating)

 

 

The beat is controlled by a special type of tissue called Nodal Tissue, which has both muscular and nervous tissue characteristics.

 

There are two locations of Nodal Tissue in the Heart:

1. SA Node (Sinoatrial Node)

• Found in the upper wall of the right atrium.

• The SA Node (also called the pacemaker) initiates the heartbeat and sends out an excitation impulses every 0.85 seconds. The impulse causes both Atria to contract. The impulses are sent to the AV Node Via the bundle of His.

2. AV Node (Atrioventricular Node)

• Found at the bottom of the right atrium near the Septum.

• When the impulse reaches the AV Node, an impulse is sent from the AV Node, Down the Purkinje Fibers causing both ventricles to contract.

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• An electrocardiogram registers the voltage changes across the surface of the heart as it beats. The letters PQRST are the standard labels used to identify the parts of the EKG

 

• The P curve records the simultaneous contraction of the atria as they drive the blood out into their ventricles.

 

• The QRS is the contraction of the ventricles as they drive the blood out into their respective arteries.

 

• The T marks the recovery of the Ventricles (restoration of the normal lectrical condition, preparing them for the next contraction)

Autonomic Control of the Heart

• The rate of the heart can also be controlled by the nervous system.

• The heart rate center is located in the Medulla Oblongata of the brain.

• The SA Node is connected to the brain by the Vagus nerve

• This nerve pathway called the Autonomic Nervous System (not under conscious control) has two systems that affects the Heart Rate:

 

1. Parasympathetic System – Causes the heart beat to slow down.

2. Sympathetic System - Causes the heart beat to increase during times of stress.

• Factors such as 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 to slowly, or if blood pressure is low, the brain will signal the SA Node to speed up its contraction.

Blood Pressure

• Ventricles pump a volume of blood (approx 70 ml) each time they contract. Arteries must have elastic, expandable walls.

• The force of blood against the blood vessel walls is simply known as Blood Pressure

• Blood pressure is not constant.

• By taking your pulse, you can feel that blood pressure fluctuates with each heartbeat. The pulse you feel is actually a pressure wave that travels from the heart throughout the arteries.

• Pulse – As blood is pumped through arteries, the arterial walls swell, and then recoil. This swelling can be felt in any artery that runs close to the surface.

 

• Blood Pressure is normally measured along the brachial artery of the arm. A reading of 120/80 is quite normal.

 

120 --- Systolic Readings as ventricles contract

80 --- Diastolic Readings as the heart relaxes

 

• A number of things can affect blood pressure. High blood pressure would put constant strain on the tissues (a range of capillary beds including the heart muscle) that are being fed by the blood. This may cause capillaries to burst (if this happens in the brain, you will have a stroke). The potential for tissue damage is greater the longer the blood pressure remains high. There are times that an elevated blood pressure is normal (as when you’re doing physical activity). The key is that the blood pressure should return to a normal, lower level. The heartbeat centre is in the brain (medulla oblongata) and it controls the pulse rate via the autonomic nervous system. It can speed up the heart (via sympathetic nerves) or slow it down (via parasympathetic nerves).

• Low blood pressure is not particularly a good thing either. Proper kidney function can only be maintained if there is sufficient pressure for filtration (see Unit O). This could result from genetics or low blood levels due to anemia or blood loss.

HYPERTENSION is the condition known as high blood pressure (ie: 150/100)

HYPOTENSION is the condition known as low blood pressure (ie: 100/60).

Luckily the body can adjust blood pressure. Monitored by the hypothalamus (part of the brain), the body can dilate (widen) arterioles thus lower blood pressure in them, or constrict (narrow) them to raise the blood pressure.

What are the factors that affect blood pressure?

1. Vessel diameter: controlled by vasoconstricters and vasodilators

2. Blood viscosity: determined by the amount of water in the blood

3. Total blood vessel length: more adipose tissue = increased blood vessel length

4. Vessel elasticity: affected by plaques (fatty deposits); atherosclerosis (hardening of arteries = decreased elasticity).

Atherosclerosis - accumulation of soft masses of fatty material, esp. cholesterol, beneath inner linings of arteries. These protrude and interfere with blood flow and increase blood pressure.

5. Blood volume: determined by the amount of salt/water in the blood

Diet and lifestyle are often to blame for elevated blood pressure. ). High salt intake from diet or kidneys will increase BP. Excessive salt will retain water in the body – greater fluid volume leads to greater blood pressure

6. Cardiac output: controlled by vagus (increased) and sympathetic nerves (decreased)

7. Age: as you get older, there is a loss of elasticity in the blood vessels.

8. Stress: constricts blood vessels which means increased pressure to move the blood.

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POTATOE CHIP

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