Circulatory System
Circulatory System
Function of the Circulatory System: The circulatory system transports oxygen, nutrients, and other substances throughout the body, and removes wastes from tissues.
The Heart
Heart Structure:
Between the walls of the heart, two thin layers of epithelial and connective tissue form around a muscle layer called the myocardium.
Powerful contractions of the myocardium pump blood through the circulatory system.
The heart is divided into 4 chambers, a wall called the septum separates the right side and the left side of the heart to prevent oxygen-poor and oxygen-rich blood from mixing.
Each upper chamber, or atrium, receives blood from the body.
Each lower chamber, or ventricle, pumps blood out of the heart.
Blood Flow Through The Heart:
Blood from the body enters the heart through the right atrium.
Blood from the lungs enters the heart through the left atrium.
When the atria contract, blood flows into the ventricles.
Flaps of connective tissue called valves are located between the atria and the ventricles.
These valves open once the blood from the atria moves to the ventricles.
When the ventricles contract, the valve closes to prevent blood from flowing back to the atria.
The valves are located at the exits of each ventricle.
The Heart’s Blood Supply:
The heart muscle needs a constant supply of oxygen and nutrients.
Although, the heart gets very little oxygen and nutrients from the blood it pumps through its chambers.
Instead a pair of blood vessels called coronary arteries, branching from the aorta and run through the heart tissue, supply blood to the heart muscle.
These coronary arteries are very narrow and so if they are blocked, the heart will run out of oxygen.
Circulation:
The heart has two pumps, one to push blood to the lungs and one to push blood to the rest of the body.
These two pathways of blood through the body are called pulmonary circulation and systemic circulation.
The right side of the heart pumps oxygen-poor blood from the heart to the lungs through what is called the pulmonary circulation.
In the lungs, carbon dioxide diffuses from the blood, and oxygen is absorbed by the blood, making oxygen-rich blood that flows to the left side of the heart.
The left side of the heart pumps oxygen-rich blood to the rest of the body through what is called the systemic circulation.
Cells absorb the blood and load the blood with carbon dioxide, returning the oxygen-poor blood back to the right side of the heart.
Heartbeat:
The heart must beat in an orderly and coordinated way to act as an efficient pump.
Two networks of muscle fiber coordinate the heartbeats – one in the atria and one in the ventricles.
Once the muscle fibers are stimulated, the entire network contracts.
Atria Contract
Each contraction begins in the sinoatrial node (SA node) – located in the right atrium.
The SA node “sets the pace” for the heart, also being known as the pacemaker.
When the SA node fires, an electrical impulse spreads throughout the entire network of muscle fibers and the atria contracts.
Ventricles Contract
The impulse from the SA node is then transferred to the atrioventricular node (AV node).
The impulse is delayed for a fraction of a second while the atria contract and pump blood to the ventricles.
The AV node then produces impulses that spread throughout the ventricles, causing them to contract and pump blood out of the heart.
Control of Heart Rate:
Your heart rate varies depending on how much oxygen your body needs and how much carbon dioxide your body needs to release (e.g. Your heartbeat can increase to 200 beats per minute while performing vigorous exercises.)
The nervous system doesn’t control the heartbeats, but they do influence the activity of the SA node.
Neurotransmitters released by the sympathetic nervous system increase heart rate.
Neurotransmitters released by the parasympathetic nervous system decrease heart rate.
Blood Vessels
As blood flows through the circulatory system, it moves through three types of blood vessels – arteries, capillaries, and veins.
Arteries:
Arteries are large vessels that carry blood from the heart to the tissues of the body.
Except the pulmonary arteries, all arteries carry oxygen-rich blood.
They have thick, elastic walls which helps them withstand the pressure produced by the heart when it contracts and pumps blood through them.
The three layers of tissue that are found in their walls are the connective tissue, smooth muscle, and endothelium.
Capillaries
The smallest blood vessels are called the capillaries.
They have extremely thin walls which allows blood and nutrients to diffuse from blood into tissues, and carbon dioxide and other waste products to move from tissues to the blood.
Veins
After blood passes through the capillaries, it returns back to the heart through veins.
Blood must flow against gravity through the large veins in your arms and legs.
They are located near your skeletal muscles.
When you move, your skeletal muscles squeeze the veins which pushes blood towards the heart.
Many veins contain valves, the one farthest from the heart ensures a one directional flow of the blood.
Blood Pressure:
When the heart contracts, it produces a wave of fluid in the arteries, known as blood pressure.
Although blood pressure falls when the heart relaxes between beats, it still remains under pressure due to the elasticity of the arterial walls.
The body regulates blood pressure in a number of ways.
Sensory receptors in blood vessels detect blood pressure and send impulses to the brain.
When the blood pressure is high, the autonomic nervous system releases neurotransmitters that relax the smooth muscles in blood vessel walls. (Vice versa for when the blood pressure is low).
The kidneys also regulate blood pressure by affecting the volume of blood.
Triggered by the hormones produced by the heart and other organs, the kidneys can remove water from the blood and eliminate it as in urine when the blood pressure is high; it conserves more water when the blood pressure is low.