Blood is a vital bodily fluid that transports oxygen and nutrients throughout the body and carries away waste products. The majority of blood in the human body is contained within the blood vessels, which make up the circulatory system. There are three main types of blood vessels: arteries, veins and capillaries.
Arteries
Arteries carry blood away from the heart to the tissues and organs of the body. They have thick, elastic walls which can withstand the high pressure of blood being pumped directly from the heart. Arteries repeatedly branch into smaller arteries and eventually into arterioles before reaching the capillaries. The two largest arteries in the body are the aorta and pulmonary artery.
The aorta originates from the left ventricle of the heart and branches off to supply oxygenated blood to the rest of the body through systemic circulation. The aorta arcs up and over the heart before descending through the chest and into the abdominal cavity, where it splits into the common iliac arteries supplying the lower limbs. Along its path, the aorta gives rise to the major arteries supplying the head, arms, and trunk.
The pulmonary artery emerges from the right ventricle and branches into the left and right pulmonary arteries, delivering deoxygenated blood to the lungs for gas exchange. After becoming oxygenated, the blood returns to the heart through the pulmonary veins into the left atrium.
Due to their direct connection to the pumping action of the heart, arteries experience surges of blood with every contraction. This creates a pulse that can be felt in certain superficial arteries like those near the surface of the wrist or neck. The elastic walls expand to accommodate the blood volume and recoil to maintain flow in between heartbeats.
Structure of Arteries
The structure of arteries enables them to cope with the pressure exerted by the pumping heart. They have three distinct layers:
- Tunica intima (innermost layer) – Made up of endothelial cells resting on a thin basal lamina. Endothelial cells provide a smooth, slippery surface that enables unobstructed blood flow.
- Tunica media (middle layer) – Comprised of smooth muscle and elastic tissue that gives arteries their ability to pulsate and adjust blood flow based on pressure changes.
- Tunica externa (outer layer) – Made up of connective tissue with some elastic fibers. Anchors the vessel within surrounding tissues.
The relative thickness and composition of these layers varies between arteries. Large, elastic arteries near the heart have a substantial tunica media allowing them to expand. More peripherally located arteries have a thicker tunica externa and more smooth muscle in the tunica media to regulate blood flow.
Veins
Veins return blood to the heart after the tissues have extracted oxygen and nutrients. They contain valves to prevent backflow and have thin, expandable walls that require less structural reinforcement compared to arteries.
Some key examples of veins include:
- Superior and inferior vena cava – Large veins that return deoxygenated blood from the body into the right atrium of the heart.
- Pulmonary veins – Return freshly oxygenated blood from the lungs to the left atrium.
- Portal vein – Transports blood from the digestive organs to the liver for processing before returning it to the heart.
- Renal veins – Carry blood away from the kidneys.
While arteries experience pulsatile flow, blood flows steadily through veins due to their distance from the heart and the presence of valves. The tunica media in veins is much thinner than in arteries, providing less structural support. Smaller veins have microscopic one-way valves to prevent backflow while larger veins contain visible semilunar valves for this purpose.
Structure of Veins
Like arteries, veins also have three layers:
- Tunica intima – Made of endothelial cells and supported by a thin layer of connective tissue.
- Tunica media – Thin layer of smooth muscle, collagen and elastin. Much thinner than in arteries.
- Tunica externa – Made of connective tissue; thicker than the media layer.
The tunica media contains less elastic tissue and smooth muscle compared to arteries. While the tunica externa is made of connective tissue in both types of vessels, in veins it is relatively thicker and provides more structural support.
Capillaries
Capillaries are tiny blood vessels that connect the arterial and venous systems. They branch extensively to supply every cell in the body with oxygen and nutrients. Capillaries have very thin walls that are only one cell thick. This allows diffusive exchange between the blood and tissue fluid.
Some key features of capillaries include:
- Very narrow lumen – Typically 3-5 μm wide, allowing only single file passage of red blood cells.
- Extremely thin walls – Made of a single squamous epithelial cell, allowing passive diffusion.
- Abundant numbers – Tens of billions in the human body.
- Slow flow – Red blood cells take 1-3 seconds to pass through.
The slow flow and thin walls facilitate exchange by allowing time for nutrients, gases, and wastes to diffuse in and out of the capillary. Plasma and small solutes flux through pores or spaces between cells while larger molecules cross through transport vesicles.
After collecting waste products and releasing nutrients, the deoxygenated blood exits the capillary beds into postcapillary venules that converge into veins returning blood to the heart.
Types of Capillaries
There are three main types of capillaries designed to facilitate nutrient and waste exchange:
- Continuous capillaries – Found in abundance throughout the body. Tiny slit-like pores allow small molecules to pass through the endothelial wall.
- Fenestrated capillaries – Contain fenestrations (pores) that allow larger molecules like proteins to pass through. Found in areas like the small intestine, endocrine glands and kidneys.
- Sinusoidal capillaries – Irregularly shaped and very leaky with widened endothelial gaps. Allow cells and large proteins to pass through intact. Found in areas like the liver, spleen and bone marrow.
Blood Volume Distribution
Now that we’ve looked at the major types of blood vessels, we can consider how blood volume is distributed between them:
| Blood Vessel | Volume of Blood (L) | % Total Blood Volume |
|---|---|---|
| Arteries | 2.0 | 7% |
| Capillaries | 4.0 | 14% |
| Veins | 21.0 | 75% |
| Microcirculation | 4.0 | 14% |
| Total | 28.0 | 100% |
As the table shows, the majority of blood in the body is found within the veins, accounting for about 75% of blood volume. Arteries and capillaries each hold only 7% and 14% respectively. The microcirculation, which includes vessels smaller than capillaries like arterioles and venules, also accounts for 14%.
This distribution correlates with the roles of each vessel type. Arteries need to be elastic and withstand high pressures but do not require large volumes. Capillaries have small volumes but vast numbers to facilitate diffusion. Veins can accommodate large volumes at low pressure as they return blood to the heart.
Summary of Key Points
- The circulatory system contains three main types of blood vessels: arteries, veins and capillaries.
- Arteries carry oxygenated blood away from the heart. They have elastic, pulsating walls to cope with surges of blood.
- Veins return deoxygenated blood from the tissues back to the heart. They have valves to prevent backflow.
- Capillaries link the arterial and venous systems, allowing diffusion of gases and nutrients.
- The majority of blood volume (75%) is contained within the veins.
- Arteries contain only 7% of blood volume but withstand high pressures.
- Capillaries make up a small fraction of volume but provide the key site for diffusive exchange.
Conclusion
In conclusion, while arteries, veins and capillaries work together to circulate blood, the majority of blood volume resides within the veins. Approximately 75% of the 5-6 liters of blood in the average human body is contained within the veins at any given time. The structure and function of each vessel correlates well with the blood volume distribution. Veins contain over 10 times the volume of similarly sized arteries due to the differences in their roles. Understanding the distribution of blood across the circulatory network provides insight into the underlying physiology.