Understanding Blood Vessels, Heart Anatomy, and Cardiac Cycle
Blood Vessels: Arteries, Veins, and Capillaries
There are three types of blood vessels:
- Arteries
- Veins
- Capillaries
Arteries
Arteries are blood vessels with thick, elastic walls. They carry blood from the heart to the organs and branch off into smaller arteries.
Veins
Veins are blood vessels with thinner and less elastic walls than arteries. They have valves that prevent blood from flowing backward. They channel blood from the organs to the heart and are formed by the joining of small veins called venules.
Capillaries
Capillaries are microscopic blood vessels that reach all the cells. They join the arterioles and venules to form a closed circuit. Their walls are very thin and allow nutrients, gases, and waste to be exchanged between the blood and the cells.
If the heart stopped working, the cells would not receive the oxygen or nutrients they need to carry out vital functions.
Heart Anatomy
The heart is a muscular, hollow organ located in the rib cage between the lungs. It is divided into two halves, right and left, and is separated lengthwise by the septum. Each half has two cavities: an atrium and a ventricle.
The Atria (Upper Chambers)
The pulmonary veins that stem from the lungs end in the left atrium. The venae cavae, through which blood from the rest of the body’s veins flows, end in the right atrium.
The Ventricles (Lower Chambers)
These are joined to the atria via a set of valves that prevent blood from flowing backward. The left valve is the mitral valve, and the right one is the tricuspid valve. The pulmonary artery leaves the right ventricle and carries blood to the pulmonary alveoli for gas exchange. The aorta artery leaves the left ventricle and distributes oxygenated blood around the body. The large arteries that leave the heart have valves at the point of exit called semilunar valves, which prevent blood from flowing back into it.
The Cardiac Cycle
The heart has a special muscle tissue called cardiac muscle, which allows it to contract and dilate to pump blood. The sequence of these contraction and dilation movements is called the cardiac cycle.
During a cardiac cycle, blood enters the heart through the veins and leaves it through the arteries. However, the blood vessels remain full of blood at all times, as this is a continuous cycle.
Stages of the Cardiac Cycle
The cardiac cycle has three stages: diastole, atrial systole, and ventricular systole.
Diastole
In the first stage of the cardiac cycle, the atria are relaxed and dilated. This allows blood to enter the heart: it flows to the right atrium from the venae cavae and to the left atrium from the pulmonary veins.
Atrial Systole
In this stage, the atria contract, and blood flows into the ventricles: it flows from the right atrium to the right ventricle through the tricuspid valve and from the left atrium to the left ventricle through the mitral valve. These valves prevent blood from flowing back into the atria.
Ventricular Systole
In this stage, the ventricles contract, forcing blood into the arteries through the semilunar valves: blood leaves the right ventricle through the pulmonary artery and the left ventricle through the aorta. The blood flows intermittently and is perceived as the arterial pulse.