Capillaries are the smallest of the body’s blood vessels that make up the microcirculation. They connect the arterioles and venules, forming an extensive network throughout the body’s tissues. Capillaries enable the exchange of water, gases, nutrients, and waste products between the blood and the interstitial fluid surrounding the cells. Though capillaries have the smallest diameter of all blood vessels, their abundant number provides a large surface area for these vital exchange processes to occur. Approximately 5% of the total blood volume is contained within the capillaries at any given time.
Anatomy of Capillaries
Capillaries consist of little more than a layer of endothelial cells and a basement membrane that wraps around them. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary. But in capillaries, the endothelial cells are extremely thin and flat to allow for rapid diffusion. There is no smooth muscle in the capillary walls or surrounding them. This lack of muscle control means that blood flow through the capillaries is entirely dependent on blood pressure generated by the heart.
The basement membrane provides structural support to the capillaries. It is comprised of proteins including laminin, collagen IV, nidogen, and perlecan. These proteins provide adhesion sites for endothelial cells and pericytes, another cell type found wrapped around capillaries. The basement membrane regulates passage of molecules and leukocytes between the endothelial cells, allowing only specific sizes to pass through.
Types of Capillaries
There are three main types of capillaries categorized by the structure of their endothelial cells:
- Continuous capillaries – Found in muscle, skin, lungs and connective tissues. Endothelial cells form a continuous barrier only permeable to small molecules.
- Fenestrated capillaries – Common in exocrine and endocrine glands. Endothelial cells have pores or fenestrations that allow larger molecules through.
- Sinusoidal capillaries – Present in the liver, bone marrow and spleen. Very large gaps exist between endothelial cells, allowing blood cells and proteins to pass.
The specific structure of capillaries is tailored to meet the functional needs of the tissues they supply. For example, kidney glomeruli need high pressure filtration so they have continuous capillaries. Intestinal villi require absorption of lipids so they have fenestrated capillaries. The liver sinusoidal capillaries process plasma proteins due to their open gaps.
Distribution of Blood Volume in the Body
The average adult has approximately 5 liters of blood circulating in their vascular system. This 5 liter total blood volume is distributed among the different types of blood vessels in proportions adapted for their distinct functions:
- Arteries: Approximately 15-20% of the blood is present in systemic arteries at any given time.
- Arterioles: These small arteries receive blood from arteries and comprise 5-10% of the blood volume.
- Capillaries: The extensive capillary beds contain only 5% of the circulating blood volume.
- Venules: Collect blood from capillaries and account for 5-10% of volume.
- Veins: Larger vessels returning blood to the heart contain 15-20% of the volume.
The pulmonary circulation through the lungs follows a similar pattern, with 5% of blood in the pulmonary capillaries. Though capillaries have the smallest volume percentage, the vital exchange processes that occur due to their structure impact the overall blood composition.
Factors Contributing to Capillary Blood Volume
Several key factors determine the volume of blood flowing through the capillaries at any given moment:
1. Total Blood Volume
The overall amount of blood in the body influences capillary volume. Loss of blood due to bleeding or dehydration reduces total volume. Conditions like polycythemia increase blood cell concentration.
2. Capillary Surface Area
Humans have an extensive capillary surface area of around 1000 square meters. This requires about 5% of blood volume to fill all capillary beds. Increased capillarization, such as in athlete’s muscles, necessitates greater capillary blood volume.
3. Arterial Blood Pressure
Arterial pressure generated by the pumping heart pushes blood into the capillaries. Higher pressures increase capillary volume. Narrowed arteries from atherosclerosis can lower pressure and capillary flow.
4. Venous Blood Pressure
Backpressure from blood accumulating in veins can impede capillary inflow if venous pressures are elevated. Conditions like right-sided heart failure can cause venous congestion.
5. Local Metabolic Factors
Metabolically active tissues like exercising muscle release chemicals that dilate local capillaries to demand increased blood flow. This expands the volume within that tissue’s capillary network.
Capillary Exchange Processes
The key functions of capillaries – exchange of gases, nutrients, wastes, and fluid regulation – are made possible by their unique structural adaptations:
Gas Exchange
The thin capillary endothelial cells and basement membrane allow for rapid diffusion of oxygen and carbon dioxide between red blood cells and surrounding tissues.
Nutrient Delivery
Substances like electrolytes, glucose, and amino acids are transported across capillary walls through facilitated diffusion, channels, and active transport.
Waste Removal
Metabolic waste products like urea freely diffuse into the blood from interstitial fluid for elimination from the body.
Fluid Balance
Net fluid movement between blood and interstitial space is regulated by hydrostatic and osmotic pressures across the capillary membrane.
Capillary Exchange and Tissue Function
The capillary network delivers oxygen and nutrients tailored to the specific needs of body tissues:
Skeletal Muscle
During exercise, vasodilation increases capillary blood volume. More oxygen is delivered and carbon dioxide removed.
Kidneys
High pressure filtration across glomerular capillaries forms urine and removes wastes.
Intestines
Lipids are absorbed by capillaries with increased permeability in intestinal villi.
Liver
Hepatocytes process proteins and toxins thanks to exposure to the large sinusoidal capillaries.
Capillary Exchange Disorders
Abnormal capillary function can arise due to:
- Microvascular disease damaging endothelial cells
- Anemia reducing oxygen delivery
- Diabetes increasing permeability
- Sepsis causing fluid leakage
- Cancer promoting angiogenesis
These disorders impair gas, nutrient, and waste exchange and can threaten organ function.
The Importance of Capillary Health
Though only 5% of blood is present in capillaries at a given moment, these microscopic vessels play a crucial role. Their thin walls and extensive surface area allow for the exchange processes vital for normal tissue function. Capillaries link the arterial and venous systems, delivering essential oxygen and nutrients and removing metabolic wastes. Maintaining healthy capillaries through good nutrition and lifestyle protects our vital organs from disruption of these critical exchange processes.
References
- Fung, Y.C. Biomechanics: circulation. New York: Springer-Verlag, 1997.
- Hall, John E., and Arthur C. Guyton. Guyton and Hall Textbook of Medical Physiology. Philadelphia, PA: Elsevier, 2020.
- Klabunde, Richard E. Cardiovascular Physiology Concepts. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2011.
- Mulroney, S.E. and Myers, A.K. Netter’s Essential Physiology. Philadelphia, PA: Elsevier, 2018.
- Roemeling-van Rhijn, Marieke et al. “Structural and functional changes in capillaries during normal aging.” Aging 10.10 (2018): 2726-2737.