Circulatory and Excretory Systems

Blood is a fluid connective tissue. Every other connective tissue (bone, cartilage, fat) is solid or semi-solid, but blood has a liquid matrix called plasma in which cells float. An adult has roughly 5 to 6 litres of blood. It is slightly alkaline, with a pH of about 7.4, and it is the body's delivery van, sewage line, police force and thermostat, all in one.

Blood has two parts. Plasma (about 55 per cent) is the straw-coloured liquid, and the formed elements (about 45 per cent) are the cells: RBC, WBC and platelets. Plasma is mostly water (about 90 per cent). Dissolved in it are plasma proteins, glucose, salts, hormones, urea and gases.

Blood cells are born in the red bone marrow (in adults, mainly in the flat bones and the ends of long bones). In a foetus, the liver and spleen also make blood. The spleen is called the 'graveyard of RBC' because old red cells are destroyed there.

Plasma proteins do three big jobs. Albumin (the most abundant) keeps the right water balance between blood and tissues. Globulins include the antibodies that defend us. Fibrinogen, made in the liver, is the raw material of the clot. Plasma from which fibrinogen and other clotting factors have been removed is called serum.

Why does blood clot? A cut vessel is a leak that must be sealed quickly. Platelets stick to the wound and release a chemical (thromboplastin). With calcium ions and Vitamin K-dependent factors made in the liver, it converts prothrombin into thrombin. Thrombin changes soluble fibrinogen into insoluble fibrin threads, which form a net that traps RBCs and makes the clot.

The red blood cell is built like a delivery bag with one job: carry oxygen. It is a biconcave disc (thin in the middle), which gives it a large surface area. In humans it has no nucleus (and no mitochondria), so there is more room for haemoglobin and the cell does not use up the oxygen it carries. It is made in the red bone marrow, lives about 120 days, and is destroyed in the spleen and liver. The hormone erythropoietin from the kidney tells the marrow to make more RBC.

Haemoglobin is an iron-containing protein (haem = iron part, globin = protein part). It is red and gives blood its colour. In the lungs, where oxygen is plentiful, it binds oxygen and becomes bright red oxyhaemoglobin. In the tissues, where oxygen is low, it lets go. Normal haemoglobin is about 14 to 18 g per 100 mL of blood in men and 12 to 16 g in women. Carbon monoxide binds haemoglobin over 200 times more strongly than oxygen, forming carboxyhaemoglobin, which is why CO is a silent killer.

The respiratory pigment is different in different animals. Humans and all vertebrates: haemoglobin (iron, red). Cockroach, prawn, snail, octopus: haemocyanin (copper, blue). Earthworm: haemoglobin, but dissolved in plasma and not inside cells.

Blood groups exist because red cells carry different markers (antigens) on their surface. Karl Landsteiner discovered the ABO system in 1900-01 and won the Nobel Prize in 1930. Your plasma carries the antibodies against the antigens you do not have. If the wrong blood is given, these antibodies attack the donor cells, which clump together (agglutination) and block small vessels.

The Rh factor is another antigen, first found in the Rhesus monkey (Landsteiner and Wiener, 1940). People who have it are Rh-positive; those who lack it are Rh-negative. An Rh-negative person must receive only Rh-negative blood. The danger comes in pregnancy: if an Rh-negative mother carries an Rh-positive baby, her body may make anti-Rh antibodies during the first delivery. In a later pregnancy these cross the placenta and destroy the baby's RBC, a condition called erythroblastosis foetalis. Doctors prevent it by giving the mother an anti-Rh injection after the first delivery.

The heart is a muscular pump about the size of your closed fist. It lies in the chest between the two lungs, tilted slightly to the left, and is wrapped in a double-layered bag called the pericardium, which holds a little fluid so the beating heart does not rub against its surroundings. Its wall is made of cardiac muscle, which is involuntary, striped and branched, and which never tires because it makes its own beat (it is myogenic).

The human heart has four chambers: two upper, thin-walled atria that receive blood, and two lower, thick-walled ventricles that pump it out. A muscular wall, the septum, separates the right and left sides, so oxygen-rich and oxygen-poor blood never mix. The left ventricle has the thickest wall because it must push blood to the whole body, while the right ventricle only pushes it to the nearby lungs.

Valves are one-way doors that stop backflow. Between the right atrium and right ventricle is the tricuspid valve (three flaps); between the left atrium and left ventricle is the bicuspid or mitral valve (two flaps). At the exits of the ventricles are the semilunar valves: the pulmonary valve and the aortic valve. The familiar 'lub-dub' heart sound comes from valves closing: 'lub' when the tricuspid and bicuspid valves shut, 'dub' when the semilunar valves shut.

The cardiac cycle. One heartbeat means one full cycle of contraction (systole) and relaxation (diastole), and takes about 0.8 second. In a resting adult the heart beats about 72 times a minute. In each beat the atria contract first (about 0.1 s), then the ventricles (about 0.3 s), and then the whole heart rests (about 0.4 s). Each beat pumps roughly 70 mL (the stroke volume), so the heart pumps about 5 litres every minute, the whole blood volume.

Who sets the beat? A small patch of special muscle in the wall of the right atrium, the SA node (sinoatrial node), fires first and so is the natural pacemaker. Its signal spreads over the atria, reaches the AV node, travels down the bundle of His, and spreads through the Purkinje fibres to make the ventricles contract. If the SA node fails, doctors fit an artificial pacemaker.

Blood travels in closed pipes called blood vessels. Think of a city's water system: big mains leave the pumping station (arteries), a mesh of tiny lanes reaches each house (capillaries), and return pipes bring the used water back (veins). Each pipe is built to match its job.

Why is the artery wall thick and the vein wall thin? Right after the ventricle contracts, blood hits the artery at high pressure in strong jerks; the elastic wall stretches and then recoils, which smooths the jerks into a steady flow. By the time blood reaches the veins the pressure has dropped, so the wall can be thin, but then the blood needs valves (and the squeezing of nearby muscles) to push it uphill towards the heart.

Double circulation. In humans, blood passes through the heart twice in one complete round. The pulmonary circulation takes blood from the right ventricle to the lungs and back to the left atrium. The systemic circulation takes it from the left ventricle to the whole body and back to the right atrium. Because of this, the body always gets fully oxygenated blood at high pressure.

Blood pressure (BP) is the force of blood on the artery walls. It is measured with a sphygmomanometer and written as systolic over diastolic. Systolic pressure is the pressure when the ventricles contract; diastolic pressure is the pressure when they relax. Normal BP of a healthy adult is about 120/80 mm Hg. A persistent reading of about 140/90 or above is hypertension (high BP); a reading much below normal is hypotension. High BP is dangerous because it can burst a brain vessel (stroke) or damage the heart and kidneys.

The pulse is the wave of expansion that runs along an artery every time the heart pumps. Pulse rate is therefore the same as heart rate (about 72 per minute). It is usually felt at the wrist (radial artery) or the neck (carotid artery).

The lymphatic system is the body's drainage and defence network. Blood plasma constantly leaks out of the capillaries and bathes the cells as tissue fluid. The extra fluid is collected by tiny lymph vessels and returned to the blood as lymph. Lymph is a colourless fluid like plasma but with no RBC and fewer proteins; it has many lymphocytes. It finally drains into large veins near the collarbone (the subclavian veins).

Open versus closed circulation. In a closed system (humans, all vertebrates, earthworm, octopus and squid) blood stays inside vessels. In an open system (insects like the cockroach, prawn, most molluscs such as snails and mussels) the heart pumps blood-like fluid into body cavities (the haemocoel) where it bathes the organs directly. Insect 'blood' (haemolymph) does not carry oxygen, because insects breathe through tracheae.

Excretion is the removal of harmful metabolic wastes from the body. When cells break down proteins they leave nitrogenous wastes (ammonia, urea, uric acid), and CO2 and water are produced in respiration. If these wastes pile up, they poison the body, so we need an excretory system. Do not confuse excretion with egestion (removal of undigested food as faeces) or secretion (release of useful substances like enzymes and hormones).

The human urinary system has a pair of kidneys, a pair of ureters, one urinary bladder and one urethra. The kidneys are bean-shaped, reddish-brown organs lying on either side of the backbone at the back of the abdomen, protected by the lowest ribs. The right kidney sits slightly lower because the liver takes up space above it. Blood comes to each kidney by the renal artery and leaves by the renal vein.

Cut a kidney lengthwise and you see an outer cortex, an inner medulla (which forms cone-shaped renal pyramids) and a funnel-shaped space, the renal pelvis, that leads into the ureter. The working unit of the kidney is the nephron: each kidney has about one million of them. A nephron is a long, thin, coiled tube with a ball of capillaries at its head.

How urine is formed is a three-step story: filter everything small, take back what the body needs, and add a few extra wastes. About 180 litres of filtrate is formed per day, yet only about 1 to 1.5 litres leaves as urine, which shows that more than 99 per cent of the filtrate is taken back.

Why is the filter at the glomerulus so powerful? The blood enters the glomerulus through the afferent arteriole, which is wider than the efferent arteriole that carries it away. Like squeezing water through a narrowing hose, this raises pressure inside the capillary ball and forces the fluid out. Blood cells and big proteins are too large to leave, so normal urine has no blood and no protein.

Hormones that control the kidney. (1) ADH (antidiuretic hormone, vasopressin), released from the posterior pituitary, makes the collecting duct reabsorb more water. Too little ADH gives a huge amount of dilute urine, a disease called diabetes insipidus. (2) Aldosterone from the adrenal cortex makes the kidney keep sodium (and so water). (3) Renin, from the juxtaglomerular cells of the kidney, starts a chain that raises blood pressure when it falls. (4) Erythropoietin from the kidney raises RBC production.

Nitrogenous wastes come from the breakdown of proteins and nucleic acids. Animals deal with them in three ways, and the choice depends on how much water the animal can spare. Ammonia is very toxic and needs a lot of water to flush out, so animals living in water can afford it (ammonotelic). Urea is less toxic and needs moderate water (ureotelic). Uric acid is almost non-toxic and leaves as a paste with very little water, so it suits animals that must save water (uricotelic).

Organs of excretion in humans are not only the kidneys. The lungs remove CO2 and water vapour. The skin removes water, salts and a little urea in sweat (its main job is cooling), and sebaceous glands release oily sebum. The liver converts toxic ammonia into urea (the ornithine cycle), breaks down haemoglobin into bile pigments (bilirubin and biliverdin) which leave with the bile in the faeces, and detoxifies drugs and poisons. The kidneys remain the main organ because they excrete most of the nitrogenous waste.

Kidney failure and dialysis. If the kidneys stop working, urea and other wastes build up in the blood (uraemia), and the patient can die. Two treatments exist. In haemodialysis (the artificial kidney), the patient's blood is taken from an artery, mixed with an anticoagulant (heparin) and passed through coiled tubes of a thin semi-permeable membrane (cellophane) kept in a dialysing fluid that has the same salts as normal plasma but no urea. Wastes diffuse out into the fluid, the cleaned blood goes back through a vein (with an anti-heparin to restore clotting). The idea came from the Dutch doctor Willem Kolff in the 1940s. The lasting cure is a kidney transplant, best from a close relative to limit rejection by the immune system.

Organ donation in India is governed by the Transplantation of Human Organs Act, 1994. A living person can donate one kidney (the other one is enough) or a part of the liver (which regrows). Other organs such as the heart, lungs, pancreas and corneas are donated after death, when brain-stem death is certified. Selling organs is illegal.