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
3. veins

1. The inner layer is the tunica intima, which consists of a single sheet of endothelial cells, including the basal lamina. This layer anchors a fibril matrix and carries negative charges. 
2. The middle layer is tunica media, composed of smooth muscle cells and elastic fibers ordered in roughly spiral layers. This layer is thicker in the arteries.
3. The external layer is tunica adventitia, consisting of collagen and elastic fibers in loose connective tissue. In this layer, the vasa vasorum, which has tiny blood vessels, comprises a vascular network supplying the walls of large blood vessels. 

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) are normalized not immediately but with 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 peripheral structures and the body's tissues. Arteries are flexible and have a pulse. There are various types of arteries:  

Elastic arteries are the nearest to the heart (large arteries i.e. aorta). They are known as windkessel vessels (it means air chamber). The term describes the recoiling effect of large arteries. 

The windkessel effect helps dampen blood pressure (pulse pressure) fluctuation over the cardiac cycle and maintains organ perfusion during diastole when cardiac ejection ceases. The arteries expand their diameters when blood pressure rises during systole and recoil when it falls during diastole. 

The diameter changes 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 draw blood from an elastic artery and branch into small arteries and arterioles. They contain more smooth muscle cells and a few elastic fibers. Hormones, local metabolites, and nervous system activation can influence their diameter, thus widening muscular arteries. Their key function is to control the distribution of blood throughout the body during stress or muscle activity, which means that muscular arteries can change the distribution of blood to the organs.   

Small arteries or arterioles link up 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 act as the primary site of vascular resistance. 

The diameter of arterioles is between 20 and 200 µm, which continues in terminal arterioles with a diameter of 8–20 µm. The arterioles have a thick, smooth muscle layer, and their diameter changes between wide ranges.  

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 formula is the fluidic law to calculate flow pressure drop in a long cylindrical pipe. 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 the flow rate of blood. 

Capillaries are the smallest and most numerous of the blood vessels. The Latin capillaris means "of or resembling hair” and refers to the hairlike diameter of a capillary. Nutrients and wastes are exchanged through the walls of capillaries. They are 4–7 µm in diameter and 500–1000 µm long tubes. They do not have smooth muscle in their wall; thus, any change in their width is passive. 
There are three types of capillaries: continuous, fenestrated, and sinusoid capillaries.

Capillaries can be continuous in that the endothelial cells provide an uninterrupted lining. They only allow smaller molecules, such as water, ions, and lipid-soluble hormones, to pass through. This capillary type is present primarily in the skeletal muscles, lungs, blood-brain barrier, and skin. Nutrients and waste are exchanged through the walls of capillaries. This is the location (microcirculation) where oxygen and nutrients are delivered to tissues.  

The tight junctions between the endothelial cells restrict blood-borne substances from entering the brain. The blood-brain barrier or the blood-testis barrier prevents specific material or agents from leaving the blood and entering the tissue.   

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 wall and pores and small gaps. This type of capillaries is in the liver, spleen, and bone marrow.  

Blood gases, nutrients, lipophilic compounds, and waste cross the membrane of capillaries via 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 wall consists of one layer of endothelial cells, which are higher and have specific functions. Smooth muscle is also found in the wall of venules. 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 arteries with less muscle and elastic tissue.  

Blood in veins flows in one direction only due to venous valves in the vessel wall. Valves usually have elliptical cross-sections under low venous pressure, while their circumference is round if the pressure increases.  

Veins have high vascular compliance, so the volume change rate with changing pressure is high. This means that veins are highly distensible and expand to accommodate large volumes of blood. Veins also act as capacitance vessels, accommodating large volumes of blood. The venous structures contain roughly two-thirds of the total blood volume (54%) and thus act as a blood reservoir. 

Two veins usually run alongside an artery in the body (the umbilical cord is an exception), so blood flow in the veins is maintained by the arterial pulse wave, which rhythmically squeezes the veins from the outside. In addition, more prominent veins possess valves to prevent blood backflow. Abnormal closing of valves results in venous insufficiency and leads to chronic distension of veins, known as varicose veins.    

Venous valves and the skeletal muscle pump facilitate blood return to the heart. Venous return is also facilitated by several other factors, including inspiration, increased total blood volume, increased venous tone, the cardiac suction effect, and negative pressure within the thoracic cavity.