Kardiovaskulárny systém

The vascular system

Blood is circulated by the heart through a closed circuit of vessels.

The blood vessels of the pulmonary and systemic circulations consist of

1. arteries,
2. capillaries, and
3. veins.

The mechanical properties of blood vessels are defined by the ratio of the elastic and non-elastic structures and smooth muscles in the vessel wall. Vessel walls have viscoelastic properties: passive mechanical changes of the wall (vasodilation, changes in stretch) and reduced pressure (stress), which are normalised not immediately but with a delay by the pressure changes inside the vessel.  

In addition, the structure of the vessel wall determines how much blood can flow through the vessel at a specific internal pressure. Compliance is the capacitance or distensibility of a ship to react to an increase in pressure by distending or swelling and increasing the volume of blood it can hold, or with decreased pressure, a decrease in volume. 

The primary function of arteries is to deliver blood to the body's peripheral tissues and organs. Arteries are flexible and have a pulse. The various types of arteries are described in the following section. 

Elastic arteries are the ones closest to the heart (large arteries, i.e., the aorta). They are known as windkessel vessels (i.e., air chambers). The term describes the recoiling effect of large arteries.

The windkessel effect helps dampen blood pressure (pulse pressure) fluctuations in blood pressure (pulse pressure) throughout the cardiac cycle and maintains organ perfusion during diastole when cardiac ejection ceases. During systole, the arteries expand as blood pressure rises, and they recoil during diastole as the pressure falls.  

Changes in diameter cause the large arteries to contain more blood during systole than during diastole; thus, blood is discharged peripherally during the next diastole. The windkessel effect prevents excessive rises in blood pressure during systole.

Windkessel effect

Muscular arteries (or distributing arteries) are medium-sized vessels that arise from an elastic artery and branch into small arteries and arterioles. They contain more smooth muscle cells and a few elastic fibres. Hormones, local metabolites, and nervous system activation can influence their diameter, thus widening muscular arteries. Their key function is to control blood flow throughout the body during stress or muscle activity, meaning muscular arteries can alter blood flow to organs.

Small arteries, or arterioles, connect to capillaries. Precapillary arterioles are considered resistance vessels as they are found before the capillaries. Precapillary sphincters (the last portion of resistance vessels), the terminal segments of smooth muscle, control and help blood flow into capillaries. Small resistance arteries are the primary sites of vascular resistance.

Arterioles have diameters ranging from 20 to 200 µm, continuing as terminal arterioles measuring 8–20 µm. They possess a thick layer of smooth muscle, and their diameter varies considerably. 

The mean arterial pressure decreases significantly, to approximately 35 mmHg, in the arterioles, where the pulse diminishes. The primary function of the arterioles is to control blood flow from arteries to capillaries.   

The Poiseuille–Hagen law is a fluid dynamics principle used to calculate the pressure drop or flow rate through a long, cylindrical tube. The Poiseuille-Hagen formula considers stationary laminar flow in a pipe with viscosity µ, flow rate Q, average velocity U, pipe length L, pipe radius r, and pressure difference ΔP.  According to the formula, the flow rate is directly proportional to the fourth power of the radius (r⁴). This means that even small changes in the radius of a blood vessel can significantly affect blood flow rate.

Capillaries are the smallest and most numerous blood vessels. The term capillaris, from Latin, means “of or resembling hair” and refers to their hairlike diameter. Nutrients and wastes are exchanged through their thin walls. Capillaries are 4–7 µm in diameter and 500–1000 µm long. They lack smooth muscle in their walls; therefore, any change in their width is passive. There are three types of capillaries: continuous, fenestrated, and sinusoidal. 

Continuous capillaries have an uninterrupted lining consisting of endothelial cells that allows only small molecules, such as water, ions, and lipid-soluble substances to pass through. They only allow smaller molecules, such as water, ions, and lipid-soluble hormones, to pass through. This capillary type is primarily found in skeletal muscles, the lungs, the blood-brain barrier, and the skin. Within the microcirculation capillary walls are sites where oxygen and nutrients are delivered to tissues and metabolic wastes are removed. 

The tight junctions between the endothelial cellsrestrict blood-borne substances from entering the brain. The blood–brain barrier and the blood–testis barrier both prevent certain materials and agents from leaving the bloodstream and entering the surrounding tissues. 

Fenestrated capillaries belong to the second type of capillaries. In Latin, the word “fenestrae” means windows. These capillaries contain small openings or pores and small gaps between cells in their walls that allow the exchange of larger molecules. Fenestrated capillaries can be found in the small intestine, where nutrients are absorbed from food, and the kidneys, where waste products are filtered out of the blood. 

Sinusoid or discontinuous capillaries are the third type of capillaries. They are the “leakiest,” as they have many more significant gaps in their capillary walls and pores, as well as small gaps. This type of capillary is in the liver, spleen, and bone marrow.

Blood gases, nutrients, lipophilic compounds, and wastes cross capillary membranes by passive transport. 

Arterio-venous anastomoses (AVAs) are direct connections between small arteries and veins. Adrenergic axons densely innervate them, and their wall is composed of circular contractile smooth muscle cells, which means that if sympathetic impulses are lacking, these vessels are open. AVAs can be found in the glabrous skin of the hands and feet, which play an essential role in temperature regulation.  

Postcapillary venules (~20 µm in diameter) drain blood from the capillaries. Their walls consist of a single layer of endothelial cells, which are taller and have specialised functions. Smooth muscle is also present in the walls of venules. Together, venules and small veins form postcapillary resistance vessels. 

The two essential functions of veins are: a) to act as conduit vessels, transporting blood back to the heart from the body's organs and tissues (i.e., the venous return), and b) to act as capacitance vessels, accommodating large volumes of blood. The walls of veins are thinner and have larger diameters than those of arteries, with less muscle and less elastic tissue. 

Blood in the veins flows in one direction only due to venous valves in the vessel wall. Valves usually have elliptical cross-sections at low venous pressure, whereas their circumference becomes circular as pressure increases.  

Veins have high vascular compliance, meaning that their volume changes significantly with variations in pressure. They are highly distensible and can expand to accommodate large volumes of blood. Acting as capacitance vessels, veins contain roughly two-thirds (about 54%) of the total blood volume and therefore serve as an important blood reservoir. 

In most parts of the body, two veins usually run alongside an artery (the umbilical cord being an exception). Venous blood flow is aided by the arterial pulse wave, which rhythmically compresses the veins from the outside. In addition, larger veins contain valves that prevent blood from flowing back. When these valves fail to close properly, venous insufficiency develops, leading to chronic venous distension known as varicose veins.

Venous valves and the skeletal muscle pump facilitate the return of blood to the heart. Several other factors, including inspiration, also aid venous return, such as increased total blood volume, elevated venous tone, the cardiac suction effect, and the negative pressure within the thoracic cavity.​​​​​​​