How Does The Blood Flow Through The Heart
The human circulatory system also known as the cardiovascular system consists of the heart, arteries, veins, and capillaries. The heart pumps oxygen and nutrient-rich blood contained within the system around a circuit of vessels, supplying all of the body’s tissues with the blood that is critical for sustaining life.
The process of diffusion is the random movement of molecules from a region of higher concentration to a region of lower concentration is not fast enough to support the oxygen and nutrient demands of a large multicellular organism like a human.
Diffusion only works in short distances. While humans do rely on diffusion between the blood and the atmosphere in the lungs, and between the blood and the cells in the capillaries, the delivery of blood to these exchange sites must be rapid and efficient.
For these reasons, blood is transported
throughout the human body by the process of bulk flow. Through this process, air and blood move from regions of higher pressure to regions of lower pressure. In the human circulatory system, the heart is the pump that generates the pressure gradients that drive the bulk flow of blood. Such a system allows for the rapid transport of molecules in respiratory gases and nutrients over long distances, in order to reach all of the body’s tissues.
The Structure And Function Of The Circulatory System
The circulatory system consists of the blood, a fluid connective tissue, contained completely within a network of blood vessels that is connected to the heart.
The heart and its system of delivering blood is composed of two separate circuits. The pulmonary circuit (supplied by the right side of the heart), receives blood returning to the heart from the body and pumps it to the lungs. This circuit serves to exchange carbon dioxide in the blood with oxygen from the lungs.
The systemic circuit (supplied by the left side of the heart) takes the freshly oxygenated blood and delivers it to the entire body.
In both circuits, the blood travels through a series of blood vessels. Blood is pumped out of the heart into large muscular arteries that branch into smaller arteries, then arterioles, followed by intricate networks of tiny capillaries.
The capillaries are the sites of exchange between the blood and nearby cells. After leaving the capillaries, the blood is collected into venules and then veins of increasing size, before being returned to the heart. In both systems, arteries take blood away from the heart, and veins bring blood toward the heart.
The Flow Of Blood Through The Cardiovascular System
One of the best ways to understand the design of the human circulatory system is to take a ride with a red blood cell through the entire circuit. Let’s start in the left ventricle, the larger muscular chamber of the left side of the heart.
When the heart beats, the red blood cell gets forcibly ejected from the left ventricle into the aorta. From there, the blood cell travels into one of many large arteries that branch into progressively smaller arteries. Hence, each vessel the red blood cell enters will eventually lead to multiple exit points as it branches.
Soon, the red blood cell moves from a small systemic artery into a systemic arteriole with a smaller diameter.
The arteriole leads to a systemic capillary bed in some tissue in the body where the vessels are so small that the red blood cell can barely squeeze through.
In this systemic capillary, the red blood cell gives up some of its load of oxygen (O2) molecules to nearby cells for use in the process of cellular respiration. Carbon dioxide (CO2), a waste product of cellular respiration, diffuses from these cells into the blood cell.
After this exchange of gases, the blood cell enters a venule, then a small vein, and then a larger vein. Eventually, the blood cell reaches the large veins that deposit the oxygen-poor blood into the right atrium. This is the end of the systemic circuit.
The pulmonary circuit (where the red blood cell once again becomes oxygenated) begins when the blood is pumped from the right atrium into the right ventricle and leaves via the pulmonary arteries to travel to the lungs.
Again, there is a significant degree of branching of both the larger and then the smaller pulmonary arteries. The red blood cell progresses from a small pulmonary arteriole into a pulmonary capillary, which is wrapped around a small portion of the lung surface.
The carbon dioxide diffuses out of the capillary and into the air within the lungs while oxygen is diffusing in the opposite
direction and binding to the hemoglobin molecules packed within the red blood cells.
Once again, pulmonary venules and then successively larger veins collect the blood as it leaves its capillary bed. Soon after entering one of the large pulmonary veins, the blood cell is deposited into the left atrium and finally the left ventricle, where it first began its journey.
There is no rest for the red blood cell. for blood to accomplish its function, it must remain in motion. As soon as it becomes stationary, it’s store of oxygen and nutrients quickly becomes depleted and the cell becomes saturated with waste products. To keep the body’s blood in motion, the heart pumps about 8,000 liters of blood per day.
This is equivalent to 4,000 regular two-liter soda bottles. It is difficult to say how quickly an individual blood cell will travel through the circulatory system.
It would depend on which specific body tissue a specific blood cell circulates
through. Flow through individual organs and tissues varies from minute to minute based on the changing oxygen demands of tissues and on the type and degree of human activity taking place at that time. The total flow of blood through the system remains fairly constant and is typically about 5.25 liters/minute, close to the total volume of blood contained within the system.
Flow is far more rapid in the arteries than it is within the capillaries. Arteries are delivery vessels, while capillaries are sites of exchange, a process that requires time.
I hope this was helpful.