You are currently viewing CHAPTER THREE: BLOOD AND BLOOD PRESSURE: what constitutes blood?
Centrifuged blood to illustrate the components of blood


Part One.


Blood is a fluid connective tissue that the heart pumps around the body through blood vessels. Blood transports oxygen and nutrients to tissues and carbon dioxide and metabolic wastes away from the tissues. Within blood are suspended cells, hormones, and signaling molecules that help the body maintain homeostasis, immunity, and repair.

If you haven’t read the first two chapters, click on the links below.



Components characteristics of blood

8% of the adult human body is blood, at about 5 to 6 liters for males: and 4 to 5 liters for females.

Blood-centrifugation-scheme. Blood has two parts: cells and plasma

Blood has two components: cellular and fluid components. We observe this when blood is put in a test tube and spun in a centrifuge. The cellular portion has cells: the liquid part is called plasma.

Cells that form the cellular component of blood include red blood cells (erythrocytes), white blood cells (leukocytes), and platelets. The cellular portion is 45% of blood.

Plasma is straw-colored. It makes up 55% of blood. It has 92% water: the rest has electrolytes, metabolites, hormones, proteins, lipids, and other molecules.

Among the components of plasma are proteins and clotting factors. Plasma without clotting factors is known as serum. Proteins make up 7% of the total plasma volume. They include albumin, globulins, fibrinogen, and other regulatory proteins like hormones.

Albumin, manufactured by the liver, is the most abundant plasma protein at 60%. Albumin transports insoluble and hydrophobic compounds like fatty acids, and cholesterol. It also transports hormones and drugs. It significantly contributes to the blood’s osmotic pressure (oncotic pressure) that drives the exchange of nutrients at the capillary beds. The maintenance of blood volume and pressure depend on it.

35% of plasma proteins are alpha, beta, and gamma globulins. The liver makes the alpha and beta globulins that transport iron, lipids, and fat-soluble vitamins. Gamma globulins are also known as immunoglobulins. They are antibodies that mediate immunity. They are the only plasma proteins not made by the liver but by immune cells.

Two types of alpha and beta globulins exist – alpha 1 and 2: beta 1 and 2. They all transport hormones, lipids, and minerals; however, only alpha globulins facilitate or inhibit the actions of other enzymes.

5% of the plasma proteins are fibrinogen, essential for blood clotting. Fibrinogen is a soluble protein, cleaved into an insoluble protein – fibrin that wounds around an injured blood vessel or skin as part of the clot to stop bleeding, thus establishing hemostasis.

Other proteins are regulatory, constituting less than 1% of the plasma proteins. They include proenzymes, enzymes, hormones, and other molecules, whose levels constantly change with the current body’s status.

Plasma also contains other solutes like glucose, amino acids, vitamins, fatty acids, triglycerides, electrolytes, urea, creatinine, cholesterol, uric acid, bilirubin, oxygen, and carbon dioxide.

Blood is homogenized – a mixture of uniformly and evenly distributed elements. The color of blood indicates the level of oxygen in it (oxygenation). Bright red blood is highly oxygenated. It is due to the binding of oxygen molecules onto the hemoglobin component called heme. Blood assumes a dusky red (purple) or blue color when there’s less than optimal oxygen content.

Blood is a viscous fluid – thick and dense. Its viscosity is about five- to six-fold that of water. Blood viscosity is a function of plasma proteins and cells. Blood viscosity plays a pivotal role in resistance to blood flow and pressure.

Arterial blood gases (ABG) and pH.

Blood has a pH of about 7.35 to 7.45. Arterial blood is more basic at a pH of 7.41: venous blood is more acidic at a pH of 7.35. By convention, venous blood has more dissolved carbon dioxide than arterial blood.

In hospitals, blood pH is measured accurately by an ABG machine. Arterial blood gases assess the patient’s clinical status by measuring their pH, oxygen, and carbon dioxide levels by sampling blood from an artery. The test evaluates the functional capacity of the heart and lungs to deliver oxygen and remove carbon dioxide.

Venous and arterial blood

Warmth around the body is transported by blood whose temperature (380C) is subtly higher than the average normal body temperature (37.10C). The heat generated in the blood may be due to various metabolic processes and blood vessel resistance as blood flows through them.

We shall talk about how blood cells are formed in our next article.



MBChB (MUK), Graduate Fellow, Department of Physiology, Makerere University Founder and Content Creator Peer reviewer, Associate Editor

Leave a Reply