Functions of Blood

Blood, the only fluid tissue in the body, circulates through blood vessels, driven by the heart's pumping action. It is categorized as one of the four fundamental tissue types, identified explicitly as connective tissue. Connective tissue's main role is connecting various body parts, providing structural support, enabling storage, and ensuring protection among the different tissue types-epithelial, muscle, nervous, and connective-connective tissue is most widely distributed across the body.

1.0Blood

Blood consists of both solid and liquid components. The solid parts, which include red blood cells, white blood cells, and platelets, are present in plasma, a straw-coloured liquid. Living blood cells constitute approximately 45 percent of blood volume, while non-living plasma accounts for the remaining 55 per cent.

2.0Components of Blood

Plasma

• Plasma is about 92% water and contains over 100 different dissolved substances, such as nutrients, respiratory gases, hormones, salts, and waste products. The most prevalent of these substances are plasma proteins. The liver mainly produces these proteins, which perform various roles. Among them, fibrinogen is crucial in blood clotting, albumins maintain water within the bloodstream, and gamma globulins function as antibodies to protect the body against foreign invaders.
• The plasma contains various salts such as sodium, potassium, calcium, magnesium, chloride, and bicarbonate. These salts are crucial for several vital bodily functions, including facilitating muscle contractions, transmitting nerve impulses, and maintaining the body's pH (acid-base) balance.

Constituents of Blood Plasma

Blood plasma consists of various protein components, including:

• Serum globulin
• Serum albumin
• Fibrinogen

The serum contains globulin and albumin, while fibrinogen is absent due to its conversion into fibrin during blood clotting.

Red Blood Cells

Red blood cells (RBCs), also known as erythrocytes, comprise the most abundant cell type in the bloodstream. In adults, red bone marrow located in the ribs, vertebrae, sternum, and pelvis is responsible for producing these cells. Red blood cells primarily consist of haemoglobin, a protein pigment that accounts for about one-third of their weight. An iron atom is at the core of each haemoglobin molecule, responsible for the cells' red colour. In the lungs, these iron atoms bind with oxygen to form oxyhemoglobin. The main function of red blood cells is to transport oxygen to cells across the body utilizing haemoglobin. Once the oxygen is delivered, haemoglobin then binds to the carbon dioxide produced by the cells and carries it back to the lungs. There, the carbon dioxide is partially released during exhalation.

Quantity

• In healthy adult males, the RBC count ranges from 5 million to 5.5 million cells per cubic millimetre of blood.
• The RBC count typically falls between 4.5 million and 5 million cells per cubic millimetre of blood for healthy adult females.
• Newborn babies generally have a higher RBC count, averaging around 6.8 million cells per cubic millimetre of blood.
• Polycythemia is an elevated RBC count, often observed in individuals residing at high altitudes, such as hill stations.
• Anaemia, conversely, denotes a decreased RBC count, leading to insufficient oxygen transport and related symptoms.

Shape and size

In most mammals, mature red blood cells (RBCs) exhibit a biconcave, disc-like shape, appearing circular in cross-section. This biconcave shape enhances the surface area of RBCs. However, there are exceptions to this norm. For instance, camels and llamas are mammals with biconvex and oval-shaped RBCs.

Structural Details of RBC 

• Mature red blood cells are enucleated, that is, they don't contain a nucleus. Their plasma membrane is referred to as Donnan's membrane.
• Endoplasmic reticulum (ER), Golgi complex (GC), ribosomes, and mitochondria are absent in mature mammalian red blood cells.
• A network of structural proteins called stomatin is present, forming a spongy cytoskeleton.
• Carbonic anhydrase, a zinc-containing enzyme and one of the fastest enzymes facilitates carbon dioxide transportation.

Function of Red Blood Cells

Conveys oxygen from the lungs to tissues across the body.

Assists in the transportation of carbon dioxide

Serves as a buffer, regulating hydrogen ion concentration

Affects blood viscosity

Contains blood group antigens and the Rh factor

Haemoglobin (Hb):

RBCs have a red coloured, iron containing complex protein called haemoglobin, hence the colour and name of these cells.

Haemoglobin is made up of 4 units. Each unit has 1 Fe in +2 state.

Each haemoglobin molecule can carry a maximum of four molecules of O₂. O₂ can bind with Hb in a reversible manner to form oxyhaemoglobin

• In each RBC 26.5 crores molecules of Hb are present.
• Red-coloured respiratory pigment, which helps in the transportation of O₂ & CO₂.
• A healthy individual has 12-16 gms of haemoglobin in every 100 ml of blood.
• Molecular weight of each molecule of haemoglobin – 67,200 Dalton

Formation of RBC (Erythropoiesis)

• Organs which produce RBCs are called Erythropoietic organs.
• Hormone which stimulate Erythropoiesis is called Erythropoietin, synthesized by Kidney.
• 1st RBC is produced by yolk sac.
• During embryonic life RBC are produced by Liver, Spleen, Placenta, Thymus gland.
• In adult stage RBCs are produced by RBM (Red Bone marrow).

White Blood Cells

White blood cells, also known as leukocytes, are significantly fewer in number than red blood cells. They range from 4,000 to 11,000 per cubic millimetre of blood and comprise less than 1 per cent of the total blood volume. White blood cells are essential for the human immune system despite their relatively small numbers. They protect the body against threats like bacteria, viruses, parasites, and tumour cells. The blood cells are produced primarily in the red bone marrow, but some are also formed in lymphatic tissue.

Unlike red blood cells, which remain within the bloodstream, white blood cells can move out of blood vessels and travel to areas of infection or damage in the body.

• Total leukocyte count (TLC): 6000 – 8000 / mm3 of blood
• Differential leukocyte count (DLC): (%) of different types of leukocytes in the blood:
• Acidophils: 2 – 3% of TLC
• Basophils: 0.5% – 1% of TLC (minimum)
• Neutrophils: 60 – 65% of TLC (maximum)
• Monocytes: 6 – 8% of TLC
• Lymphocytes: 20 – 25% of TLC
• Leukocytosis: Increased TLC
• Leukopenia: Decreased TLC
• Leukemia: Abnormally elevated TLC (more than 100,000), also known as blood cancer.

Types of White Blood Cells

There are five distinct types of white blood cells, primarily classified based on the presence or absence of granules:

• Granulocytes
• Agranulocytes

Granulocytes 

Granular leukocytes are identified by their prominent, lobed nuclei and distinct granules within their cytoplasm. There are three types: neutrophils, eosinophils, and basophils.

Eosinophils

• Eosinophils are a type of white blood cell that contains prominent granules that appear bright red when stained with eosin, an acidic dye.
• The lysosomes within these cells have enzymes like oxidases and peroxidases, indicating their role in neutralizing foreign proteins and other substances.
• Typically, eosinophils rise in response to allergic reactions and parasitic infestations, such as those caused by tapeworms.

Basophils 

• When stained with basic dyes, basophils display deep blue granules. These cells are similar to eosinophils in their involvement in allergic reactions.
• Unlike other cells, basophils do not have lysosomes. Their cytoplasmic granules include histamine, which widens blood vessels and increases the permeability of capillaries.
• In the context of tissue injuries and allergic reactions, basophils release this histamine.
• Other granules within basophils also contain heparin, an anticoagulant that prevents blood from clotting unnecessarily inside the blood vessels.

Neutrophils 

• Neutrophils are the primary phagocytic cells in the bloodstream and are highly skilled in locating and engulfing bacteria.
• They also play a crucial role in cleaning up dead cells, which becomes particularly significant following an injury or infection.
• Most granules within neutrophils are filled with enzymes that break down the materials they ingest.

Agranulocytes

Agranular leukocytes are characterized by their absence of prominent granules and typically have rounded or kidney-shaped nuclei. This category includes lymphocytes and monocytes. Lymphocytes can be differentiated into those that produce antibodies and those that directly combat foreign entities like bacteria and viruses. Due to their unlobed nuclei, these cells are often referred to as mononuclear white blood cells (WBC).

Monocyte

• Monocytes, the most significant type of white blood cells, can be up to 20 µm in diameter.
• They circulate in the bloodstream for approximately 24 hours before exiting to mature in the body's tissues.
• Once there, monocytes undergo significant enlargement and transform into macrophages, which are large cells that act as the body's cleanup crew.
• All tissue macrophages originate from monocytes. These macrophages are particularly effective at devouring bacteria, dead cells, and other debris.

Platelets

• Shape: Platelets exhibit a disc-like, oval-shaped, or biconvex morphology.
• Size: They measure approximately 2 - 3 μm in diameter.
• Lifespan: Platelets have a lifespan ranging from 2 to 5 days.
• Normal Platelet Count: The typical platelet count ranges from 1.5 to 3.5 lakh/mm3 of blood.
• Thrombocytopenia: A decrease in the number of blood platelets, known as thrombocytopenia, can cause clotting disorders leading to excessive blood loss from the body.
• Non-nucleated: Platelets lack nuclei.
• Exclusive to Mammals: Platelets are unique to mammals.
• Critical Count: The critical count of thrombocytes is 40,000/mm3. A count below this critical threshold can result in the appearance of red spots or rashes on the skin, a condition known as Purpura disease.
• Cytoplasmic Granules: Basophilic granules are present in their cytoplasm, which can be stained by methylene blue.

Formation

Platelets are cell fragments derived from megakaryocyte cells in the bone marrow.

Function of Blood Platelets

• Coagulation of blood clotting
• Repair the endothelium of the blood vascular system by forming a platelet plug.
• Synthesis of thromboplastin to aid in blood clotting.
• Synthesis of serotonin.

3.0Blood Vessels

Blood vessels are categorized into five classes: arteries, arterioles, veins, venules, and capillaries.

Structure of Blood Vessels

Blood vessels comprise three layers except capillaries: the tunica interna, tunica media, and tunica externa.

Tunica Interna

The innermost layer of the blood vessel, known as the tunica interna, consists of a single layer of flat epithelial cells termed the endothelium. This smooth lining, which is in direct contact with the blood, offers minimal resistance to blood flow. 

Tunica Media

The tunica media is the arterial wall's middle layer, predominantly consisting of smooth muscle fibers and elastin.

Tunica Externa

The tunica externa, the outermost layer of the blood vessel, is mainly made up of connective tissue fibers. These fibers serve to safeguard the blood vessels and secure them to surrounding tissues.

Types and Functions of Blood Vessels

Three categories of blood vessels include:

• Arteries
• Veins
• Capillaries

Arteries

• Arteries are thick walls and narrower lumens compared to veins, which aids in preserving blood pressure as it circulates through the body. An artery is a blood vessel designed to transport blood away from the heart. In the systemic circuit, arteries feature thick walls capable of withstanding the high pressure of blood pumped from the heart. The arteries nearest to the heart possess the thickest walls, including two layers of elastic fibers. 

Arterioles

• Blood flows from arteries into arterioles, so these are smaller arteries. These arterioles transport blood to the capillaries. Structurally, arterioles comprise two primary layers: an inner endothelium and a layer of vascular smooth muscle (VSM). The diameter of arterioles, also called lumens, can vary significantly due to the contraction or relaxation of the VSM. Contraction leads to vasoconstriction, which decreases blood flow and increases blood pressure, whereas relaxation leads to vasodilation, which increases and reduces blood pressure. 

Veins

• Veins, as defined, are blood vessels responsible for transporting blood back to the heart and are the largest in the body by diameter. Both veins and arteries consist of three layers. However, veins distinguish themselves by having thinner walls but larger lumens than arteries. The lumens of veins are often described as 'irregular.' This characteristic is generally observed when veins are sectioned and viewed in an empty state, causing them to collapse due to their thinner walls.

Venules

• A venule is a small vein, ranging from 8 to 100 µm in diameter, that gathers blood from various capillaries, leaving a capillary bed. As they progress toward the heart, venules merge to form more prominent veins. The structure of venules includes an inner lining of endothelium and an external layer of connective tissue. 

Capillaries

• A capillary, the smallest and most delicate blood vessel in the body, primarily functions as an exchange vessel throughout all tissues. It enables the movement of gasses, nutrients, and waste products between the blood and surrounding cells and interstitial fluid via perfusion. Capillaries typically have a diameter of 5–10 micrometres.

4.0Functions of Blood

Blood serves several essential functions.

1. Transportation -Blood circulates various substances throughout the body, ensuring the delivery of essential elements such as oxygen and nutrients to the required locations while facilitating the removal of waste products like carbon dioxide and urea for elimination. Specifically, it transports:

• Oxygen from the lungs to various tissues.
• Carbon dioxide is transported from body tissues to the lungs to be exhaled.
• Nutrients from the digestive tract to body cells.
• Hormones from endocrine glands to target organs.
• Urea from the liver to the kidneys for removal through urine.

2. Protection -   Blood plays a crucial role in the immune system. White blood cells within the blood seek out and destroy invading pathogens, thereby preventing and fighting infections.
3. Regulation - Another critical function of blood is maintaining a stable internal body temperature. Through the skin's capillaries, blood helps manage and distribute heat across the body, maintaining a constant temperature of around 37°C (98.6°F).
4. These functions highlight the essential nature of blood in supporting and maintaining overall health and homeostasis in the human body.